Dyes adsorption using clay and modified clay: A review

Accepted Manuscript Dyes adsorption using clay and modified clay: A review Abida Kausar, Munawar Iqbal, Anum Javed, Kiran Aftab, Zill-iHuma Nazli, Haq Nawaz Bhatti, Shazia Nouren PII: DOI: Reference: S0167-7322(17)34636-6 https://doi.org/10.1016/j.molliq.2018.02.034 MOLLIQ 8676 To appear in: Journal of Molecular Liquids Received date: Revised date: Accepted date: 2 October 2017 10 January 2018 8 February 2018 Please cite this article as: Abida Kausar, Munawar Iqbal, Anum Javed, Kiran Aftab, Zill-iHuma Nazli, Haq Nawaz Bhatti, Shazia Nouren , Dyes adsorption using clay and modified clay: A review. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), https://doi.org/10.1016/ j.molliq.2018.02.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Dyes adsorption using clay and modified clay: A review Abida Kausara,*, Munawar Iqbalb,*, Anum Javeda, Kiran Aftaba, Zill-i-Huma Nazlia, Haq Nawaz Bhattic and Shazia Nourend a CR IP T Department of Chemistry, Government College Women University Faisalabad b Department of Chemistry, University of Lahore, Lahore, Faisalabad c Department of Chemistry, University of Agriculture Faisalabad d Department of Chemistry, Government College Women University Sialkot Corresponding author’s E-mail’s. [email protected] (A. K), [email protected], [email protected] (M. I) Abstract The effective use of the sorption properties of different clays as sorbents for the removal of dyes US from wastewater has currently received much attention because of the eco-friendly nature of clay materials. Dyes are complex class of organic compound having wide range of applications in AN textile and food industries and a large amount of dyes are wasted, which get mixed in natural water resources. Mixing of dyes in water resources must be prohibited for the safety of natural ecosystem. The adsorbents (natural and modified) have been successfully for the adsorption of M dyes form wastewater. This review article highlights the importance of clay (simple and ED modified) as an adsorbent for the adsorption of dyes from textile wastewater. Appropriate conditions for clay-dye system and adsorption capacities of a variety of clays are presented and PT sorption process is critically analyzed in this study. Studies reported the clays as an adsorbent from 2004–2016 are included and different properties for the utilization of clay and clay-based CE adsorbents are discussed for effective removal of dyes. Based on studies, it was found that the clays (natural and modified) are affective adsorbents for the purification of wastewater AC containing dyes. Keywords: Textile wastewater; Dyes-clay interaction; Adsorption; modified clay; Characterization; Kinetic; Equilibrium 1. Introduction Dyes are colored organic compounds based on functional groups such as chromophoric group (NR2, NHR, NH2, COOH and OH) and auxochromes (N2, NO and NO2) [1]. There are different classes of dyes used for the dyeing of different substrates (Table 1), i.e., acid dyes are generally used for silk, wool, modified acrylics and nylon dyeing. These are also used in cosmetics, paper, food, ink-jet printing and leather dyeing. The major classes of acid dyes are azine, xanthene, 1 ACCEPTED MANUSCRIPT anthraquinone, triphenylmethane, nitroso, nitro and azo dyes [2]. Acid blue 2, acid red 57, methyl orange, orange (I, II) are common acid dyes. Basic dyes are used for modified polyesters, modified nylons, polyacrylonitrile dyeing as well as in paper industry and medicines. These are also used for tannin mordant cotton, silk and wool [2]. This class of dyes is soluble in water and yields colored cations and are also called cationic dyes [3]. The major classes are cyanine, thiazine, acridine, oxazine, hemicyanine and diazahemicyanine, i.e., basic red 46, malachite CR IP T green, basic yellow 28, crystal violet, methylene blue, basic brown and basic red 9 are the basic dyes. Disperse dyes are employed on cellulose acetate, nylon, acrylic fibers and cellulose fiber. These are non-ionic dyes and are insoluble in water and from aqueous solutions, used for acrylic fibers also. Main classes are benzodifuranone, nitro, styryl, azo and anthraquinone group [4] and US some common examples are disperse yellow, disperse blue, disperse orange and disperse red. Direct dyes are used for leather, cotton, rayon dyeing and in paper industry. These dyes have AN affinity for cellulosic fiber if dying process occurs in aqueous solution containing electrolytes. The main classes are oxazine, stilbenes and poly azo compounds. Examples are direct orange 34, direct black, direct violet and direct blue etc. [5]. Reactive dyes are used on nylon, wool as well M as cellulose and cotton fiber. The chromophores in these dyes are phthalocyanine, azo, triaryl ED methane and a covalent bond is formed between the dye and fiber [6]. Common examples are reactive yellow 2, reactive red, remazol and reactive black 5 etc. Vat dyes are used for wool, PT rayon fibers, flax wool and cotton (on cellulosic fibers mainly) dyeing i.e., indigoids and anthraquinone etc; vat dyes are insoluble in water. The common examples of vat dyes are vat CE green 6, vat blue and indigo [7]. The use of adsorption using clay sorbent (Bentonite – 0.05–0.2; Red mud 0.025; Clinoptilolite – AC 0.14–0.29) U$/kg for effluent treatment will be cost effective by saving the money that is used in the import of commercial active carbon (0.8–1.1 U$/kg) , natural zeolite (0.08 U$/kg), Chitin (15–20 U$/kg), Chitosan (16.5–10 U$/kg) Cross-linked-chitosan (5–10 U$/kg) [8, 9]. At the same time, comparatively, the adsorption process is technically also considered a better alternative in water and wastewater treatment because of convenience, ease of operation and simplicity of design, moreover be helpful to overcome on the problem of high energy input (used in reverse osmosis and UV sterilization) as the most developing countries lacking adequate electricity [10-12]. 2 ACCEPTED MANUSCRIPT Table 1: Different classes textile dyes and their health effects [13, 14] Dyes Examples Textile industrial products Acid dyes Acid blue 25, acid red 57, methyl orange, Congo red Basic red 46, malachite green, basic yellow 28, methylene blue, basic brown, basic red 9 direct orange 34, direct black, direct violet, direct blue. disperse yellow, disperse blue, disperse orange, disperse red Polyamide, polyurethane, acrylics, nylon reactive yellow 2, reactive red, remazol, reactive black 5 vat green 6, vat blue, indigo Nylon, wool, cotton. Reactive dyes CR IP T rayon, Bladder cancer carcinogen US cotton, polyacrylonitrile, polyamide, nylon, acrylic fibers, cellulose. DNA damage, induction of bladder cancer in humans, splenic sarcomas cellulose, Allergic respiratory problem wool, rayon fibers, flax Severe burns, skin and mucous wool cotton membrane irritation PT Vat dyes Leather, paper. Carcinogens, allergic skin reactions, allergic dermatitis, skin Irritation, mutations, cancer. AN Disperse dyes Silk, moderant acrylic, polyester, modified polyesters, modified nylons, paper. membrane M Direct dyes fibers, Skin and mucous modified irritation and burns ED Basic dyes Health effects CE A large variety of organic (residual dyes) pollutants are introduced into different water bodies from different sources like pharmaceutical industries, paper and pulp industries, tannery, bleaching industries, textile industries and other anthropogenic activities [15-37]. Currently, AC there are more than 100,000 dyes commercially existing (azo dyes, are about 70% on weight basis from these dyes) and over 1 million tons dyes are manufactured per year, of which 50% are textile dyes [38]. According to one estimate, the dyes produced are 2% that are directly discharged in aqueous waste matters, 10% is lost in the coloration procedure subsequently, 20% of these colored compounds are entered in the environment through waste matters [39]. The reported percentage fixation of different dyes on different substrates and discharge in effluent is shown in Table 2 [40]. 3 ACCEPTED MANUSCRIPT Table 2: Fixation degree of different classes of dyes on different substrates and percentage loss of dyes in textile effluent [40]. Fiber type Fixation degree (%) Loss in effluent (%) Direct Cellulose 70-95 5-30 Acid Polyamide 80-95 5-20 Base Acrylic 95-100 0-5 Reactive Cellulose 50-90 10-50 Disperse Polyester 90-100 0-10 CR IP T Dye class 2. Dye Separation Techniques US Dyes effluent mixed with wastewater may cause potential hazard to environment [41-43]. Different physico-chemical techniques have been developed for the remediation of waste of AN environmental concern [44-69]. The chemical treatment which includes photolysis and photocatalytic processes, whereas biological methods include anaerobic and aerobic degradation M and physiochemical methods include electro kinetic coagulation, ion exchange, adsorption and membrane filtration. All methods have their own limitation on the basis of cost, design and dye ED separation efficiency [13, 70, 71]. But adsorption is the most suitable method in comparison with others in different respects. A summary of advantages and disadvantages of these methods are PT presented in Table 3. CE Table 3: Separation techniques and their advantages and disadvantages [2, 13] Ozonation AC Separation Techniques Photo catalyst Advantages Disadvantages Chemical Methods No sludge generation Operational cost is very high, half life is short (20 min) Operational cost is Some photo catalyst degrades into toxic by-products. low and economically feasible Fenton reagent low-priced and reagent Disposal efficient production 4 issues and sludge ACCEPTED MANUSCRIPT procedure Biological Methods Anaerobic degradation By-products can be Under aerobic conditions require used as energy more treatment and yield of resources methane and hydrogen sulphide Provide suitable environment for growth of microorganisms and low and effective in very slow process removal of azo dyes Operational cost is CR IP T Aerobic degradation Physicochemical Methods High adsorption Low surface area for capacity for all dyes. some adsorbents, high cost of adsorbents. Need to dispose of adsorbents. Ion exchange No loss of sorbents Electro kinetic Coagulation Economically feasible Need further treatments by flocculation and filtration and production of sludge. Membrane Filtration Effective for all dyes Suitable for treating low with high quality volume and production of sludge. effluents For disperse dyes not effective CE PT ED M AN US Adsorption /Sorption 2.1. Adsorption for the removal of dyes from textile effluents AC Adsorption is one of the most important industrial separation processes for the treatment of waste. It is a mass transfer process through which the solid substance (adsorbent) can selectively remove dissolved constituents from an aqueous solution by attracting the dissolved solute toward its surface. The accumulation of concentrated matter at surface or at the inter phase is involved in this process. The adsorbent can exist in liquid, solid, gas or dissolved solute phase. Adsorption can be classified as chemical or physical; the former procedure is due to the exchange of electrons, adsorbate is chemically bounded to the surface. Whereas, in latter procedure waste matter is attached with adsorbent surface by physical forces as for instance, hydrogen bonding, polarity, Vander Waals forces and dipole-dipole interactions etc. However, the extent of 5 ACCEPTED MANUSCRIPT adsorption depends upon adsorbent nature like molecular size, molecular structure, molecular weight, solution concentration, polarity and also on adsorbent surface properties like surface area and particle size etc. [2]. This separation process discovers extensive use in the removal of dyes from aqueous medium and is considered as best method in comparison with others. The efficiency of this method depends on chemical and physical properties of adsorbent and adsorbate, their cost, availability, ease of operation, surface area and less toxicity [72]. Numerous CR IP T biomass types including bacteria, fungi, algae and yeast have proposed excellent sorbents for waste water treatment [73-75] intended the biocatalysts such as enzymes for degradation and mineralization of textile dyes from aqueous solution. Gisi et al. [75] classified the lowcost adsorbents into following five groups: (i) agricultural and household wastes, (ii) industrial by- US products, (iii) sludge, (iv) sea materials, (v) soil and ore materials and (vi) novel low-cost adsorbents and compared their efficacy for different pollutants like dyes and heavy metals. AN Furthermore Gisi et al., 2016 after comprehensive study of previous literature reported that dyes among different pollutants have maximum affinity for textile dyes. The affinity of sorbents in removing various pollutants, their applications on real wastewater, costs and considerations on M their reuse after adsorption processes are available, clay has been proved economically and ED technically suited as an adsorbents [57, 76]. In spite of a large amount of studies on clay adsorbents, there is little data containing a full study comparing the economic viability between PT these clay adsorbents. Even though a lot has been done on studies dealing with clay material sorbents, a great deal of work still needs to be done to predict the performance of the adsorption 3. Clays CE processes for dye adsorption in real world industrial effluents under various operating conditions. AC Clays are defined as fine grained minerals, which may be plastic in nature, clays can be hardened when dried or fired and they contain appropriate water contents. Clays generally contain phyllosilicates, however the other contents present may impart either plasticity or harden when fried or dried [77]. Clays can be differentiated from other fine grained soils by their difference in minerology and size. Adeyemo et al. [78] reported montmorillonite- sematic, kaolinite, Illite, bentonite and chlorite the main classes of clays. Kaolinite group includes the mineral kaolinite, dickite, halloysite and nacrite. The smectite group includes pyrophyllite, talc, vermiculite, sauconite, saponite, nontronite and montmorillonite. The illite group includes the clay micas. 6 ACCEPTED MANUSCRIPT Illite is the only common mineral [79]. Chlorites are not always considered clay; sometimes they are classified as a separate group within the phyllosilicates. There are approximately 30 different types of “pure” clays in these categories, but most “natural” clays are mixtures o f these different types, along with other weathered minerals. These differences among the clays provide an opportunity to investigate the impact of structure and layer charge on metal ion coordination to permanent charge sites. In addition, this choice of minerals provided various CR IP T arrangements of aluminol and silanol surface hydroxyl sites. The adsorption capabilities of clay are related to the negative charge on the structure of fine grain silicate minerals. This negative charge can be neutralized by the adsorption of positively charged cations such as dyes. Besides, the clays possessed large surface area, ranging up to 800 m2/g, which contributes to its high US adsorption capacity. There are many types of clays but montmorillonite clays are expected to have the highest sorptive capacity in comparison to other [80]. Further, clay could be modified to AN enhance its efficiency for the removal of pollutants from water and wastewaters. Zeolites are naturally occurring silicate minerals, which can also be synthesized at commercial level. Probably clinoptilolite is the most abundant of > 40 natural zeolite species. The adsorption ED M properties of zeolites depend upon their ion-exchange capabilities [5]. 3.1. Kaolinite PT This group contains trioctahedral minerals like chrysotile, cronstedite, antigorite, chamosite and dioctahedral minerals for example kaolinite, halloysite, dickite and nacrite. It is white and soft CE plastic clay, composed of the hydrated aluminium silicate, a mineral kaolinite. The general structure of the kaolinite group is composed of silicate sheets (Si 2O5) bonded to aluminium AC oxide/hydroxide layers (Al 2(OH)4) called gibbsite layers [81], which is a layered silicate mineral with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra. The primary structural unit of this group is a layer composed of one octahedral sheet condensed with one tetrahedral sheet. In the dioctahedral minerals, the octahedral sites are occupied by aluminium, while in the trioctahedral minerals, these sites are occupied by magnesium and iron. Kaolinite and halloysite are single-layer structures. Kaolinite, dickite and nacrite occur as plates; halloysite, which can have a single layer of water between its sheets, occurs in a tubular form. It is formed by the alteration of feldspar and muscovite [81, 82]. The common structure of kaolinite contains silicate sheets, bounded to 7 ACCEPTED MANUSCRIPT aluminium oxide/hydroxide layers, called gibbsite layers, which is a layered silicate mineral composed of one tetrahedral sheet, linked through the oxygen atoms to one octahedral sheet of alumina octahedra. Nacrite, kaolinite and dickite exist as plates, halloysite occurs in a tubular form, have a single layer of water between its sheets. Rocks having large amount of kaolinite are referred to as kaolin or china clay [77]. Kaolinite contains heterogeneous surface charge is a well-known fact. It is believed that its basal surface has a constant structural charge which is CR IP T attributed to isomorphs substitutions of Si4+ by Al3+. The charge on the edges is due to protonation or deprotonation of surface hydroxyl groups and so it depends on pH of solution. Adsorption can occur on flat exposed planes of silica and alumina sheets. It is least reactive clay. Kaolin has no side effects, no health problems till the fine dust particle is controlled, so it is safe US environmentally [83, 84]. AN 3.2. Bentonite Bentonite is aluminium phyllosilicate adsorbent, important impure clay, generally contains M montmorillonite. The structure of montmorillonite is a gibbsite layer sandwiched between two silica sheets to form the structural unit [85]. The substitutions are mainly within the octahedral ED layer (Mg2+, Fe2+) and to a much less extent within the silicate layer (Al 3+/Si4+). The chemical composition of the clay mineral is based on a hydroxyl-aluminosilicate framework. The crystal PT structures of the clay minerals are formed by a combination of sheets of silica tetrahedral and alumino octahedral. Part of the trivalent Al is substituted by divalent Mg or Fe in some cases. CE In such cases, substitution is accompanied by the addition of alkaline metals such as Na and K or alkaline earth metals such as Mg and Ca to provide charge balance. The investigated AC organoclays vary in the degree of their total cation exchange capacity (CEC) exchanged by organic counter ions [78, 85]. There are different types of bentonite which are named with respect to the presence of dominant element in them such as calcium, potassium, aluminium and sodium. Bentonite is commonly produced due to the weathering of volcanic ash mostly in the presence of water. Two main classes of bentonite are calcium and sodium bentonite, mainly used for industrial applications. Calcium bentonite is an effective adsorbent of ions not only in solution but also in oils and fats. Sodium bentonite when added in water; absorb many times as its dry mass present in water and 8 ACCEPTED MANUSCRIPT expand when it is wetted, it is very significant because of its wonderful colloidal properties. It is used for geotechnical and environmental investigation by drilling mud for oil and gas wells [86, 87]. By ion exchange process, calcium bentonite can be converted into sodium bentonite, to attain a number of properties of sodium bentonite. Bentonite is environmentally safe and is beneficial in wine making process to remove extra quantity of protein from white wines. Due to its adsorption properties, it is also used as desiccant, they have been used to protect nutraceutical, CR IP T pharmaceutical and diagnostic products from extend shelf life and moisture degradation. According to the adsorption properties of bentonite, it has overall neutral charge on its lattice excessive negative charge is present which is characterized by a three layer structure with two silicate layers, enveloped by an aluminate layer as opposite charges attract, negative charge US surfaces have affinity for cationic dye. A number of cationic dyes was absorbed by bentonite clay [88]. So far, clay could possibly be used for the removal of dyes due to its efficiency, AN availability, abundance and economically beneficial. 3.3. Sepiolite/palygorskite M Sepiolite/palygorskite are natural clay minerals, composed of magnesium hydrosilicates and ED belonged to silicate group [89]. Both groups are Mg silicates but palygorskite have more diversity in structure and has more Al and less Mg than sepiolite. They are non-swelling and PT inert clays. When dispersed in water they produce irregular lattice that is capable of liquid trapping and give best suspending gelling and thickening properties [77, 78]. CE 3.4. Montmorillonite/smectite AC It is a very soft phyllosilicate mineral belongs to smectite. Montmorillonite (a member of the smectite family) has 2:1 expanding crystal lattice. The smectite group refers to a family of nonmetallic clays primarily composed of hydrated sodium calcium aluminium silicate, a group of monoclinic clay-like minerals with general formula (Ca, Na, H)(Al, Mg, Fe, Zn) 2(Si, Al)4O10(OH)2·nH2O [78, 90]. Chemically, it is hydrated sodium calcium aluminium magnesium silicate hydroxide (Na,Ca) x(Al,Mg)2(Si4O10)(OH)2·nH2O. Potassium, iron and other cations are common substitutes and the exact ratio of cations varies with source [91]. The basic structural unit is a layer consisting of two inward-pointing tetrahedral sheets with a central alumina octahedral sheet. The layers are continuous in the length and width directions, 9 ACCEPTED MANUSCRIPT but the bonds between these layers are weak and have excellent cleavage, allowing water and other molecules to enter between the layers causing expansion in the highness direction. Isomorphous substitution gives the various types of smectite and causes a net permanent charge balanced by cations in such a manner that water may move between the sheets of the crystal lattice, giving a reversible cation exchange and very plastic properties [78, 90].The smectite clay is a family of nonmetallic clays primarily consisted of hydrated sodium calcium CR IP T aluminium silicate, which is a group of monoclinic clay like minerals, iron, potassium and other cations are common substitutes and the exact ratio of these cationic species varies with source. In these clays the main structural unit is a layer composed of two inward pointing tetrahedral sheets with a central alumina octahedral sheet [86]. Various types of smectite clays are formed due to US isomorphous substitution due to which a net charge is balanced by cations. During this process water can move between sheets, gives plastic properties and reversible cation exchange [5]. In AN comparison with other clays, montmorillonite expand largely due to penetration of water in the interlayer molecular space. This expansion is due to the exchangeable cations. There are many uses of sodium montmorillonite; it is a main constituent in non-explosive reagents for rock M splitting in natural stone quarries to limit the amount of waste [91]. ED 4. Effect of modifications on adsorption capacity of dye Clays are one of the most widely used low-cost adsorbent due to high flexibility. Current studies PT have established that the adsorption capability can be improved by modification of adsorbents via physical, thermal and chemical processes [92, 93]. Clay can be modified with different like manganese CE materials Oxide (Eren, 2009a), cationic surfactant (hexadecyltrimenthylammonium chloride) [71] and gemini surfactants [94] etc. to enhance the AC adsorption capacity and mechanical strength of clay [91, 95]. Compared to raw bentonite, efficiency of adsorption capacity of MB with cold plasma was increased from 168 to 231 mg/g. The active species such as high-energy electrons and reactive radicals generated in the plasma can activate the upper molecular layers of the interface [96]. From the results obtained it was observed that organoclay is 1.6 times effective than Nabentonite for the removal of MB. This is due to the fact that after modification, the surface area and its porosity also increases. Increasing dose increases the partitioning of per gram of adsorbent, which leads to an increase in adsorption. As a result, more adsorbate molecules can be bound to adsorbent surface through chemical bonding [71]. 10 ACCEPTED MANUSCRIPT According to Srinivasan [97], clay adsorption capacities are usually dependent on the net charges, large pore sizes and surface area. Generally, clays have exchangeable ions that play crucial role in the environment by being natural pollutant scavengers by way of both cations and anions take up through adsorption and ion exchange. Ions that are usually found on surfaces of the clay include H+, K+, Na+, Ca2+, Mg2+ , NH4+ and Cl-, SO4 2-, PO4 3-, and NO3-. Cation exchange readily without affecting the clay mineral's structure [98]. Clay minerals display a CR IP T strong attraction to cationic and anionic dyes [99]. Nevertheless, the adsorption capability for basic dyes (cationic dyes) is comparable greater than that of acid dyes (anionc dyes). But this generalization is not always the case, because different physico-chemical variables like pH, temperature, pulp density, point to zero charge etc., also effect significantly on sorption capacity US of clays [100, 101]. The objective of the present review were; to investigate the effect of modification to evaluate the adsorption efficiency of adsorbent for the removal of dye from AN aqueous solution, to examine the effect of contact time, initial dye concentration, pH and temperature on the adsorption process, to determine the fitting of adsorption kinetics and isotherm data with various models, to calculate the thermodynamic parameters and to understand AC CE PT ED M the interactions between adsorbent and dye molecule. 11 ACCEPTED MANUSCRIPT Table 4: Application of clays (raw and modified) as an adsorbents for different types of dyes Sr. No . 1 Clay (adsorbents) Modification Dye pH Efficiency Equilibri Kinetic um study study Montmorillonite Methylene blue 7.3 99.47% 1 2 Raw clay Fe3O4/activated montmorillonite (Fe3O4/Mt) nanocomposite Modifications were carried out by calcination at different temperature (S2, S3), acidic activation, and acetylation. Basic Red 46 (BR46) and Reactive Yellow 181 (RY181 - 2.805, 4.232, 1.968, and 2.756 mmol/g for CI Basic Red 46 0.031, 0.030, 0.046, and 0.050 mmol/g for CI Reactive Yellow 181 1 - Methyl green and methyl blue 5 68.35% for methyl green 95.95% for methyl blue Basic Yellow 2 (BY2 Methylene blue 11 12 3 Montmorillonite 4 Montmorillonite - 5 Bentonite clay The cold plasma treatment C S - Ref. FTIR, XRD [102] - FTIR, XRD [3] - 2 2 XRD [103] 434.196 mg g-1 1 2 2 XRD, FTIR [104] 303 mg/g 1 2 FT-IR, XRD, SEM [96] U N A T P C A T P I R 2 Characteriz ation 2 D E E C TDS* M 12 ACCEPTED MANUSCRIPT 6 7 Tunisian raw clay Vermiculite 8 Attapulgite 9 Bentonite y 10 Montmorillonite 11 Montmorillonite 12 Bentonite 13 Montmorillonite - Direct orange 34 Basic Red 9 - 7.75 mg/g. 1 2 1 6.8 2 2 1 Methylene Blue 10.0 7.66 ×10−5 mol·g−1 215.73 mg g−1 1 2 Methylene blue (MB), Crystal violet (CV) and Rhodamine B (RB) 9.0 399.74 µmol/g for MB 2 Gemini surfactants Montmorillonite supported porous carbon nanospheres (MMT-PCN) Bentonite (Bt) modification using (ortho, meta and para) bis-imidazolium cations Methyl orange Methylene blue (MB) - TelonOrange, Telon-Red and TelonBlue - Novel kappacarrageenan/poly Crystal violet - Expanded vermiculite Aminofunctionalized attapulgite clay nanoparticle Cationic surfactant (Hexadecyltrime nthylammonium chloride) /organoclay C A 1 XRD, FTIR, SEM [105] - XRD, FTIR, [71] [72] C S 365.11 µmol/g for CV U N 324.36 µmol/g for RB A - M 161.03 to 271.74 mg g−1 686.94 mg g-1 1 2 2 XRD, FTIR [94] 1 2 - XRD, FTIR, SEM [106] 108.3 mg/g for Telon-Orange 2 1 1 FTIR, XRD TGA [107] 1 2 - FTIR, SEM, XRD, and [108] D E T P E C [15] T P I R 2 XRD, XRF, FT-IR XRD, FTIR 437 96.7 mg/g for Telon-Red 82.4 mg/g for Telon-Blue 151 mg g−1 13 ACCEPTED MANUSCRIPT 14 Moroccan Illitic 15 Bentonite 16 17 Bentonite Zeolite (vinyl alcohol) nanocomposite hydrogels - TEM Hexadecyltrimet hylammonium chloride (HDTMA)intercalated bentonite clay New adsorbents derived from tragacanth gumgraftpoly(methyl methacrylate) and bentonite (TG-gPMMA/B) Zeolite-reduced graphene oxide (zeolite-rGO) (reduced graphene oxide) Methylene Blue(MB) Acid red (AR) 3 - 19 Natural Illitic clay mineral Sodium Montmorillonite, - C A Nano clay filled composite 1 1 1 T P 900 mg/g for CR XRD [109] 2 FTIR, SEM, [110] XRD, TGA and potentiometri c titrations 2 - FTIR, SEM, and TG/DTA 1 - - [112] [113] 1 U N C S [111] A D E T P Methylene blue (MB) Congo red and methyl 2 750 mg/g for MO 8.5 mg g-1 for AB-113 - E C 18 1 I R Congo red (CR), methyl orange (MO), and acid blue 113 (AB113) methylene blue and malachite green 13,698 mg/g 140.84 μmol/g M 53.3 mg g-1 for methylene blue 48.6 mg g-1 for malachite green - 24.87 mg g-1 1 2 2 7 110 mg/g for CR 4 1 1 14 FTIR, SEM, XRD, [114] ACCEPTED MANUSCRIPT bentonite hydrogels of poly acrylic acid and polyethylene glycol Starchmontmorillonite/ polyaniline (StMMT/PANI) nanocomposite Polymer-clay composite violet 1 5.895mg/g 2 Montmorillonite 21 Kaolinite 22 Natural untreated clay Basic Yellow 2 5.12 833.33 mg/g 23 Montmorillonite Methylene blue 5 150.2 mg/g 24 Smectite - Bromophen ol blue 2 - FT-IR, XRD, SEM, TGA and TEM [115] 1 - [116] 1 2 1 1 2 2 FTIR, SEM, XRD and Thermal analysis. FTIR, PSD, TEM, XRD and BET XRD, FTIR, TG A M T P I R C S U N D E T P DTA–TGA 91.74 mg g-1 20 Dodecyl sulfobetaine. surfactantmodified montmorillonite An alginatebased nanocomposite hydrogel enhanced by organoillite/smectite clay Reactive dye 111mg/g for MV [117] [118] Methylene blue 10 1843.46 mg/g - 2 - FTIR, XRD and SEM [119] Methyl orange (MO) <7 13.624–16.779 mg/g 1 2 1 - [120] E C C A 25 Activated clay 15 ACCEPTED MANUSCRIPT 26 Bentonite Modified with iron chloride Brilliant blue FCF - 6.16 mg/g for natural clay 14.22 mg/g for iron-modified clay 1 27 Montmorillonite Lignocelluloseg-poly(acrylic acid)/montmorill onite (LNC-gPAA/MMT) hydrogel nanocomposites N-vinyl-2pyrrolidone/itaco nic acid/organo clay nanocomposite hydrogels Paper-like composites of cellulose acetate–organomontmorillonite Novel carrgeenan-based hydrogel nanocomposites containing laponite RD Modified by Methylene blue (MB - 1994.38 mg/g 1 28 Montmorillonite 29 30 Montmorillonite Montmorillonite 31 Laponite RD Clay 32 Bentonite 1 [121] - FTIR, XRD, SEM, and TEM [122] T P I R 2 XRD and SEM C S Safranine-T 6 550.0mg g A D E T P U N −1 1 2 - XRD, FTIR, SEM [123] M 4 5.5 530.645 mg g-1 85.7 mg/g 2 1 2 1 2 2 XRD, FTIR, TGA and SEM [124] [125] Crystal violet (CV) - 79.8 mg g-1 1 2 - XRD, SEM TEM [126] Congo red 5.5 95% 2 2 1 SEM [92] Basic red 18 Acid Scarlet G E C C A 1,2 16 ACCEPTED MANUSCRIPT 33 Ghassoul 34 Bentonite 35 36 37 38 39 thermal and acid activation - Methyl violet Glycol bis-N- Methyl cetylnicotinat orange e dibromide, Cetyltrimethy lammonium bromide Palygorskite clay Heat-treated Palygorskite Activated Clay Modified by iron oxide (Fe-clay) Biotite clay Chitosan-gpoly(acrylic acid)/biotite (CTS-gPAA/BT) hydrogels with unique clay biotite (BT) Ball Clay Calcined and uncalcined ball clay Methylene blue Alizarin red s (ARS) Methylene blue (MB Clay minerals of Halloysite Methyl violet Halloysite nanotube 625 mg/g 1 2 2 XRD [127] - 99.02%for GNBt 80.12%for CBT. - - - FTIR and XRD [128] - - [129] 1 2 - XRD and BET - 1 2 1 78.11mg/g - 32.7 mg g-1 - 2,125.70 mg/g A D E I R C S U N 7 M T P 1 [130] [131] - T P E C C A 10. Crystal violet (CV) - 11 1.6 × 10-4 mol g-1 for calcined 1.9 × 10-4 mol g-1 for uncalcined ball clay 113.64 mg/g 17 1,3 - - XRD, FTIR , GTA [132] 1 2 1 TEM [133] ACCEPTED MANUSCRIPT 40 Bentonite 41 Smectite clay 42 Montmorillonite 43 Montmorillonite 44 Natural clay 45 Pillared clays 46 48 Clay materials (bentonite, kaolin and zeolite) Granular inorgano-organo pillared clays Halloysite 49 Purified 47 Iron-pillared bentonite Organofunctional ized Amazon smectite (SMC and SMCAMP.) Rhodamine B Brilliant Orange 3R 5.0 98.62mg/g 1 2 - XRD, FTIR [134] 4 1.26 mmolg−1 for 1 SMC 2 1 XRD, FTIR [135] Montmorillonite/ CoFe2O4 composite Diphosphoniumintercalated montmorillonite - Methylene blue 9 97.75 mg g−1 2 XRD, SEM, VSM [136] Telon dyes (Red, blue and orange) Acid Red 88 (AR88) 2 11-26 to 110-160 2 mg.g-1 1 XRD [137] SEM and zeta potential analysis FTIR and thermal analysis XRD, SEM ,BET [138] Alginate encapsulated pillared clays Lime Safranine 2.07 mmolg−1for SMCAMP 2 1 2 C S U N −1 1133.10 mgg A D E I R T P 1 1 2 M 6.9 963 µmol.g-1 1 2 - 12 94% 2 2 - Basic Yellow 28 6 514 mg/g 1 1 - XRD , FTIR , SEM [141] Halloysite nanotubes Neutral Red 7 2 2 XRD, FT-IR, TEM and BET [142] Acid activation Methylene 54.85, 59.24 and 1,2 65.45 mg/g at 298, 308 and 318 K 500 mg/g. 5 - - X-ray [143] E C C A High-shear wet granulation T P Congo Red 18 [139] [140] ACCEPTED MANUSCRIPT 50 51 52 53 Moroccan Clay mineral Smectite-rich clayey rock (AYD) Natural clay Moroccan crude clay Bentonite blue Activated clay (sulphuric acidactivated products) (AYDS) diffraction Indanthrene Blue RS (C.I. Vat Blue 4) 6.0 for AY DS and 7.3 for AY D 13.92 mg/g for AYD 17.85 mg/g for AYDS Nile Blue (NB) and Brilliant Cresyl Blue (BCB) Basic Red 9.5 46 (BR46) Humic acidCationic 8.0. immobilized dyes amine modified (Malachite polyacrylamide/b Green entonite (MG), composite Methylene Blue (MB) and Crystal Violet (CV)) MagnesiumCrystal 6.5 oxide coated violet (CV+) bentonite (MCB) Congo Red 7 25 mg/g for NB - 1 - 2 XRD, FT IR [144] T P I R 2 C S 1 - XRF [145] 1 2 1 XRD [146] 99.0% 2 2 - SEM [147] 496 mg/g 1 2 - XRD [148] 19.9 mgg−1 by Sodium For kaolin 1, 2 1 SEM, XRD [149] U N 42 mg/g for BCB A M 54 mg/g D E T P E C C A 54 Bentonite 55 Clay minerals of bentonite, kaolin 19 ACCEPTED MANUSCRIPT and zeolite bentonite 56 Montmorillonite - 57 Raw bentonite (RB) Manganese oxide-modified (MMB) bentonite Bentonite Humic acid immobilized polymer/bentonit e composite 58 59 60 Bentonite Kaolin Ca-bentonite - Methylene blue Crystal violet (CV+) Montmorillonite 62 Montmorillonite Basic dyes (Malachite Green, Methylene Blue and Crystal Violet) Congo red Crystal violet and brilliant green Sodium montmorillonite clay Chitosan-gpoly(acrylic 8.0. 1 2 1 2 2 XRD [150] - XRD [151] C S U N 99.0% I R T P 2 2 - XRD, conducto [152] metric and potentiometri c titrations 2 1 2 2 1 1 BET , XRD [153] [154] A D E T P E C C A 61 5.6 mgg−1 by kaolin 4.3mgg−1 by zeolite 11 292.15 mgg−1, 97.38%. 11.0. 0.32 m mol/g for RB 1.12 m mol/g for MMB Bentonite and zeolite by the 2l 10 7 M 95.92% 47.27 mg/g for Crystal violet 65.42 mg/g for Brilliant green RhodamineB 7 42.19 mg/g 1 2 1 BET [155] Methylene blue (MB) 6.5 1859 mg/g 1 2 - IR [156] 20 ACCEPTED MANUSCRIPT 63 Attapulgite clay 64 Spent activated clay (SAC) Bentonite 65 66 Bentonite 67 Montmorillonite 68 Bentonite 69 70 Natural mesoporous Sepiolite Bentonite acid)/montmorill onite (CTS-gPAA/MMT) nanocomposites Sonicationsurfactantmodified attapulgite clay Intercalation of bentonite with a layered double hydroxide, Mg– Al–Cl LDH - Red MF-3B 1 81.96, 82.64, and 2 85.47 mg/g at 30, 50, and 70 ◦C Methylene blue Reactive Yellow 2 (RY2) 5.5 2.44×10−4 mol/g 2 - 64.1 mg/g Malachite green 9 Cr(III)intercalated montmorillonite Organobentonite (modified using cationic surfactants) Supranol Yellow 4GL, Acid Red 151 - Acid red 57 C A Dodecyltrimethyl Acid Blue ammonium 193 bromide- 2 XRD, FTIR, surface area analysis. [157] 2. SEM [158] - - XRD, SEM [159] 1,2 and 6 - - UV–Vis spectrophoto meter. XRD [160] 1 C S I R T P U N A M 91% D E T P E C 2 2.8 58.47 mg/g 1 2 1 3 357.14 mg g−1 for CDBA-bent 1 2 - XRD and FT-IR [88] 1 - - XRD [89] 2 2 1 EDX, XRD [162] 1.5 416.66 mg/g for CP-bent 1.35× 105 mol g−1 740.5 mg g−1for DTMA– bentonite 21 [161] ACCEPTED MANUSCRIPT modified bentonite (DTMA– bentonite AND Na–bentonite. *Kinetic Studies 1-Pseudo-first order Kinetic Model, 2-Pseudo-Second Order Kinetic Model *Equilibrium studies 1- Langmuir, 2-Freundlich, 3-Redlich-Peterson isotherm, 4-Fritz–Schlunder (FS) model, 5-Toth model and 6-D–R adsorption isotherms *Thermodynamic studies (TDS) 1-Exothermic, 2-Endothermic T P I R C S U N A D E M T P E C C A 22 ACCEPTED MANUSCRIPT 5. Effect of pH Solution pH plays an important role in the sorption process. It appears to interrupt the solution chemistry of dyes and functional groups of the adsorbents. Adsorption capacity of dye depends on pH of the solution [163]. Usually, at low pH the percentage of anionic dye removal from CR IP T solution increases due to electrostatic attraction between the positive surface charge of adsorbent and anionic dye. There is an electrostatic attraction between the negatively charged adsorbent and positively charged dye molecule when solution has high pH (basic), causing decrease in the percentage removal of anionic dye [58, 59, 63, 164]. Whereas, when solution has high pH the adsorption capacity and removal of cationic dyes will increase because positive charges on the US dye ensured that they are attracted by anionic adsorbent so there are electrostatic attractions AN between positive charges of dye and negative surface of adsorbent [64, 65, 165, 166]. It is revealed that the pH of solution is optimized for maximum adsorption of dyes (Table 4). The previously reported literature indicates that optimized pH depends upon nature of dye, type of M clay used and modification of the clay. Sharma et al. [167] investigated the effect of pH on the ED removal of methyl green, a cationic dye and methyl blue, an anionic dye, from aqueous system onto the montmorillonite clay. It was observed that the adsorption capacity of methyl green onto PT the clay was increased and that of methyl blue onto the clay was decreased with increase of initial pH of the suspension. At low pH, there was abundance of H+ ions on the clay–water CE suspension, imparting a repulsive force towards the positively charged methyl green dye molecule. As the pH increased the amount of OH− ions was also increased. As a consequence, AC the amount of negative charge on the suspension also increased; facilitating the adsorption of the methyl green dye molecule onto the montmorillonite clay. While the adsorption of anionic methyl blue dye molecule was more at low pH due to the high abundance of positive charge in the suspension [103]. The influence of pH on the adsorption capacity of the adsorbent was monitored in dye solutions of initial pH values varying from 2.0 to 9.25. The results showed that the maximum adsorption capacity was observed at around pH 6 for AYDS and 7.3 for AYD, it was due to the fact that acid activation increases the number of sites responsible for dye adsorption and at any pH; the amount of dye adsorbed per unit mass (qe) of acid-activated clay had a higher adsorption capacity compared to the non-activated clay. Under the experimental 23 ACCEPTED MANUSCRIPT conditions, acid-activated clay could remove as much as 89.90%, while the untreated clay could remove 84.67% (pH 6.0) [144]. 6. Kinetics studies The adsorption kinetics is a significant factor for scheming adsorption process and is essential for choosing the optimum operating conditions for adsorbent-adsorbate interaction [117]. In order to CR IP T comprehend the behavior of the adsorbent and to investigate the controlling mechanism of the adsorption procedure, the pseudo first-order, pseudo second order and intraparticle diffusion models are useful to check the kinetic information [127], i.e., Zhou et al [105] fitted the experimental data to pseudo-first-order and pseudo second- order kinetic models in order to investigate the mechanism of adsorption for the adsorptive removal of MB by using amino- US functionalized attapulgite clay nanoparticle as adsorbent. Correlation coefficients of the pseudo second-order kinetic model was relatively greater than that of the pseudo first-order kinetic AN model, implying that the MB adsorption can be described more appropriately by the pseudosecond-order model. It may be explained by the reason that a large number of vacant surface M sites were available for adsorption during initial stage. Though, with an interval of adsorption time, the remaining vacant surface sites were difficult to be occupied due to steric hindrance ED between MB dye adsorbed on the surface of ATP@CCS and solution phase. Based on the assumption of the pseudo-second-order model it can be concluded that the adsorption of MB PT onto ATP@CCS was chemical adsorption [105]. Hernández-Hernández et al [168] applied three models i.e. pseudo-first-order model, second-order model (Elovich) and pseudo-second-order CE model to determine the adsorption mechanism brilliant blue FCF dye by natural clay and modified with iron chloride. The best adjustment was found with the pseudo-second-order AC model. For unmodified clay, the sorption constant of Lagergren was higher at 20 and 30 °C and then decreased. For the iron-modified clay, the adsorption capacities were 1.0 mg/g at the different temperatures and the pseudo-second-order rate constant increased as the temperature increased [121]. The results of kinetic studies reviewed research articles (Table 4) shows that pseudo-second order kinetic model is more fitted to the experimental data as compared to other models, however; depending upon the reaction other kinetic models also show correlation to the data. Literature (Table 4) revealed that pseudo second order model is mostly found but in some papers pseudo first order is also observe [107] and based on present study, 24 it is suggested that ACCEPTED MANUSCRIPT adsorption of textile dyes was quite rapid initially, the rate of adsorption became slower with the time and reached a constant value (equilibrium time). The initial faster rate may be due to the availability of the uncovered surface area of various sorbents. The intra-particle diffusion model proposed the involvement of diffusion mechanism. According to this theory, the adsorbate uptake qt varies almost proportionally with the square root of the contact time, t½ rather than t. CR IP T 7. Equilibrium studies Equilibrium data, generally known as sorption isotherms are elementary necessity to comprehend the mechanism of the sorption. Adsorption isotherms are used to describe the sorption procedure and for assessing sorption capability. Adsorption isotherm can be describes as the equilibrium correlation between the concentration in the adsorbent phase on the adsorbent elements and the US concentration in the liquid phase at a certain temperature [92]. In order to describe the adsorption of dye onto the clay, the experimental data is examined by adsorption isotherms at specific AN temperature as equilibrium concentrations are temperature dependent. A number of isotherms are used like Freundlich, Langmuir, Dubinin-Radushkevich, Redlich-Paterson, Halsey and Sips. M Different characteristics of adsorption process are described by each equilibrium model, but most suitable methods are Langmuir and Freundlich [163]. A number of linear forms of these ED isotherms having different axis have been used. Linear analysis is not so much precise and reliable. Nonlinear statistical functions are more effective and precise than linear analysis in such PT conditions. In recent years, a developing interest in the utilization of non-linear optimization modelling has been noted as reported in recent literature of dyes adsorption [169]. CE Vimonses et al. [170] removed congo red by using clay minerals of bentonite, kaolin and zeolite. The Freundlich and Langmuir models were applied to the experimental data. The results indicate AC that zeolite and bentonite were best described by the Freundlich model (R2 = 0.90, 0.97) however Langmuir model provided a better fit on the experimental data of kaolin with high R 2 value (R2 = 0.98). The experimental data followed Freundlich adsorption isotherm for zeolite and bentonite shows that the adsorption occurs on a heterogeneous surface, due to the presence of various active sites on sodium bentonite and zeolite having different attractions to CR molecules. The complete adsorptive process is dominated as a physical adsorption procedure. Kaolin may be described at the molecular level by the applicability of single layer treatment of dye molecules on the surface. There is very slight or no substitution taking place between layers. So, it is a fact that kaolinite crystal is balanced due to the structure charge of kaolin. The surface where 25 ACCEPTED MANUSCRIPT adsorption of kaolinite occurs, only equates the exterior surface area and the edge surface area. It can be seen that the adsorption with bentonite can take place between interlayer spaces, in contrast. Adsorption isotherm is probably depended on the affinity and surface properties of the adsorbent [149]. In the present review, the equilibrium study of available literature of dyes adsorption proves that Langmuir is better as compared to Freundlich isotherm in prediction of textile dyes adsorption processes. Langmuir adsorption qm is an important Langmuir constant, CR IP T representing the maximum capacity at equilibrium. The KL values of refers to the different in binding strength and capacity of the dyes with the surface of sorbents in general values of K L decreased with the rise of temperature [3, 94, 96, 102, 104, 105, 108, 117, 118] etc. However, freundlich model did not provide any information about the saturation adsorption capacity; US however the parameters of KF and 1/n exhibited intense change at higher temperatures. The values of 1/n (0.1 < 1/n < 1) indicated favorable adsorption of dyes at experimental conditions AN [72, 92, 107, 116, 124]. 8. Thermodynamic studies M Textile industries discharge their wastes at moderately high temperatures; therefore, temperature ED can be a significant parameter in dye removal process. Thermodynamic study of the adsorption process is helpful in establishing the nature and possibility of reaction. Different thermodynamic PT factors which comprise standard enthalpy change (ΔHo), standard entropy change (ΔSo) and standard Gibbs free energy change (ΔGo) of adsorption can be calculated from temperature and CE sorption procedure [163]. A thermodynamic study of dye adsorption onto clay shows endothermic and exothermic nature in reported literature. Ozturk and Malkoc [117] conducted experiments to check out the effect of temperature on the removal of BY2 by adsorbing it on AC natural untreated clay (NUC), at different temperatures 25, 35 and 45 ◦C. The results showed that by increasing the temperature of the adsorption process there was a decrease in adsorption efficiency and adsorption capacity, indicating that the process was exothermic. When the temperature of the process was raised, thermal energy of adsorption system was also increased, by increasing the mobility of the adsorbate causing desorption, as a result, there was decrease in adsorption capacity. The values of ΔG◦ at all studied temperatures showed that the process was spontaneous [117]. Fan et al [118] investigated the effect of temperature on the removal of MB and Cu2+ onto Mt-SB12.The thermodynamic features provided in-depth information about the 26 ACCEPTED MANUSCRIPT energetic changes related to adsorption process. ΔGo values at different temperatures were negative and ΔHo values were positive, indicating that the adsorption process was spontaneous and endothermic. The positive values of ΔSo reflected an increase in randomness at the solid/solution interface during the adsorption of MB and Cu2+ onto Mt-SB12 [118]. In order to evaluate the feasibility and the effect of temperature better, for MO adsorption onto activated clay, the thermodynamic parameters such as standard free energy change (ΔG°), CR IP T standard enthalpy change (ΔH°), and standard entropy change (ΔS°) were also studied. The negative value of ΔG° at different temperature indicated the spontaneous nature of MO adsorption onto activated clay, and the absolute term of ΔG° revealed that the adsorption trend decreases with the increase in temperature, which was consistent with the result that the US adsorption lessens with increasing in temperature. Enthalpy change (ΔH° =-12.289 kJ mol-1) was negative, and it was implied that adsorption process for MO was exothermic [120]. Summary of AN the thermodynamic studies shows that the sorption process may be exothermic or endothermic for dyes absorption on to clays. 9. Characterization studies M There are a number of techniques which have been used to describe the adsorbent and ED characterization study helps to understand adsorption process. Different techniques like XRD, FT-IR, SEM, TGA, BET (Tables 5-9) [171-177] and TEM. FT-IR have been used for PT characterization and clay adsorbents. FT-IR gives information about the presence of functional group in the adsorbent. This technique also shows difference in functional group before and after CE treatment. There are some functional groups which are common in different clays. For an instance, Si–O functional in montmorillonite clay [103], Fe3O4/activated montmorillonite AC nanocomposite [102], bentonite clay [96], smectite raw and modified [135] and HNTs [142] have been observed commonly. Adsorption peak of Si–O for following clays appears at 1,057 and 792 cm−1 for montmorillonite clay, 1030, 791 and 528 cm−1 for Fe3O4/activated montmorillonite nanocomposite, 1100–950 cm−1 for bentonite clay, 900cm−1for raw and modified Smectite and 1105 cm-1 for HNTs [96, 167, 178, 179]. Similarly, Al-Al-OH functional group is common in Tunisian raw and bentonite clay and its adsorption peak appears at 3694 cm−1 and 918 cm−1 [15, 96]. Table 5 presents the most prominent adsorption peaks for different functional groups for clays involved in adsorption process. SEM analysis is used for the evaluation of structural ordering determination, morphologies (surface) and cracks, cavities and 27 ACCEPTED MANUSCRIPT fine particles attached to the surface of adsorbent, the SEM analysis revealed a significant difference before and after treatment of clays. It was revealed that a significant changes have been occurred on the clay surface after adsorption of dyes [149]. Similarly, TGA gives information about the weight loss of adsorbent on specific temperatures and their stability at specific temperatures. In TGA analysis, a sample at specific rate is heated and the change in mass as a function of temperature and time are measured. From TGA, it was observed that the CR IP T difference in weight loss between raw and modified clay before and after treatment was also changed significantly [125]. The average pore size and specific surface area of the adsorbent can be analyzed by BET technique. According to the definition stated by the International Union of Pure and Applied Chemistry (IUPAC), adsorbent pores are categorized into three groups: (1) US micropores (diameter <2 nm), (2) mesopores (2–50 nm), and (3) macropores (>50 nm) [117]. Based on pore size, the nature of clay can be estimated. Studies performed using AN montmorillonite, it was observed that clay has mesopores since pore size was 38.34 nm [103]. Surface area of clay may change after modification; an increase in surface area indicates the adsorption capacity of clay enhanced and vice versa. XRD is used for determination of chemical M composition of adsorbent before and after adsorption, also changes in the structure of adsorbent ED can be predicted [71]. For an instance, the results of XRD analysis for expanded vermiculite showed that sample contained primarily vermiculite and minor amounts of mica, illite and AC CE PT hydrobiotite [72]. 28 ACCEPTED MANUSCRIPT Table 5: FT-IR analysis of modified and raw clay used for the adsorption of dyes Clay (adsorbents) Raw Modified Functional group Al–Al-OH, Si–O Peaks 918 cm−1, 1100–950 cm−1 regions Raw clay Al-Al-OH- stretching and bending 3694 and 917 cm_ 1 Bentonite stretching vibrations bands of Si—O—MVI (M¼Al, Mg, and Fe), sharing of the OH group between Fe and Al in octahedral sheets 400–550 cm-1 Bentonite Montmorillon ite Clay Natural bentonite (GZ Ben) and FeBen Halloysite nanotubes Al–O–Si deformation, Si–O–Si deformation D E Weak band of Fe-Ben can be ascribed to the stretching of NO3−. C A Stretching vibrations of inner-surface –OH, stretching mode of apical Si–O Peaks 1600 cm−1 T P [96] C S I R - U N aromatic C—H bonds of bis-imidazolium 3035 cm-1 molecule A [15] [107] M Stretching vibrations of aromatic ring double bond, 1450 and 1600 cm-1 913 cm 527 cm−1 - - [103] 467 cm−1 1384cm−1 - - [134] - - [142] T P E C Functional group -OH deformation band of water to observe raw and modified bentonite - Reference _1 3701 and 3626 cm_1 1105 cm_1 29 ACCEPTED MANUSCRIPT Bentonite Attapulgite clay - - -CH3, -CH2 for modified bentonite, ammonium ion 1031 cm−1 asymmetric stretching modes of Si– O–S for attapulgite clay, OH bending band, C–N vibrations in quaternary amines asymmetric stretching modes of Si–O–S for organoclay, OH bending band, C–N vibrations in quaternary amines 2921 and 2854 cm−1, I R C S 985 cm U N −1 absent D E A M T P E C C A 30 T P 1469 cm−1 1032 cm−1 991 cm−1 1402 and 1466 cm−1 [88] [157] ACCEPTED MANUSCRIPT Table 6: Scanning electron microscopic (SEM) analysis of modified and raw clay used for the adsorption of dyes Clay (adsorbents) Bentonite Raw Structure Regular layered structure of bentonite Particle size 1 µm Spent activated clay (SAC) Irregular in shape and porous 6 µm Modified Structure The small flakes of LDH nanoparticles seem to be bound onto the bentonite surface - Untreated clay Rough and porous structure of the clay 5 µm - Montmorillonite/CoFe2O4 composite with magnetic separation It shows an aggregated morphology with the irregular plate-like shapes Smooth due to closely packed flakes 1 µm Natural bentonite T P I R M 10 µm T P E C [158] - [138] Magnetic nanoparticles slightly Modified the flaky structure of MMT clay. 2 µm [136] Ragged appearance of the TA bentonite, Clay surface more porous for ATA 10 µm [92] C S 10 µm C A 31 References [159] - U N A D E Particle size 1µm ACCEPTED MANUSCRIPT Table 7: Thermogravimetric (TGA) analysis analysis of modified and raw clay used for the adsorption of dyes Clay (adsorbents) Raw Weight loss at temperature 150°C Montmorillonite clay Reason Due to evaporation of water, Due to loss of structural hydroxyl groups 670°C Bis-imidazolium modified bentonite Polymer-clay composite [P(AAm-AA)-Kao] Modified Weight loss at temperature - 9.5% weight-loss at the temperature range of 30–200 0C for organo-Bt - Due to the 245 evaporation of the desorbed water molecules - D E Reason C S U N 90% weight loss at the temperature range of 350–400 0C for bisimidazolium cations weight loss of 70.3% at 550 °C. T P Montmorillonite E C C A Weight loss of Mt at Due to 600–700 °C. dehydration of the structural OH units of Mt T P - [103] It is related to the thermal decomposition of the products Weight loss started at 505 °C, it may be due to the introduction of kaolinite to polymer network results in an increase in thermal stability - [107] I R A M Reference SB12 completely decomposed at about 350 °C. Weight loss of MtSB12 at 350–500 °C Due to the decomposition of dodecyl 32 [116] [118] ACCEPTED MANUSCRIPT Alginate encapsulated pillared clays - Mass loss at 800 ◦C was respectively 40%, 50% and 70% for AlMont-EnPILC, CTAB-Al-Mont-PILC and alginate. - sulfobetaine surfactant on Mt - I R T P C S U N A D E M T P E C C A 33 [139] ACCEPTED MANUSCRIPT Table 8: Bruner Emmer and Teller (BET) analysis of modified and raw clay used for the adsorption of dyes Clay (adsorbents) Surface area Vermiculite Raw 7.8 m2·g−1 for vermiculite Modified 9.8 m2·g−1 for expanded vermiculite 2 2 Bentonite Natural clay References 64.2 m /g for raw Pore volume /pore size T P U N 0.0929 cm3/g for cold plasma treated bentonite A - Montmorillonite clay 249 m /g - Bentonite 75.5 m2/g untreated clay 99.6 m2/g iron-modified clay D E M T P E C C S [96] 0.0923 cm3/g for raw - 2 [72] I R 65.3 m /g for cold plasma treated bentonite 20 m2/g. 0.0043 cm3·g−1 with mesoporous character C A 34 [15] 38.34 nm [103] - [121] ACCEPTED MANUSCRIPT Table 9: X-Ray diffraction (XRD) analysis of modified and raw clay used for the adsorption of dyes Clay (adsorbents) Vermiculite Chemical composition Raw Si , Mg , Al Fe3+ and K+ 4+ 2+ 3+, Si Raw clay Halloysite (H) (61%) and kaolinite (K) (39%) - Montmorillonite Illite–montmorillonite-type structure - Natural untreated clay 2Ө= 27.9◦, 29.4◦,40.5◦, 43.2◦, 47.5◦, 48.6◦and 57.3◦ - Halloysite nanotubes Dehydrated halloysite Al2Si2O5(OH)4 D E E C C A 35 Modified Mg , Al , Fe3+ and K+ 2+ T P 3+ I R U N C S [72] [15] [103] [117] A M T P 4+, Reference - [142] ACCEPTED MANUSCRIPT 10. Conclusions The clays (raw and modified) are low-cost sorbents, which have been successfully used for the adsorption of dyes from wastewater since last three decades on laboratory scale. Attempts have not been made to use clay as sorbent in scale-up process specially using real effluents. The CR IP T performance of different types of clays (raw and modified) was compared for the adsorption of dyes belong to different classes based on experimental conditions including pH, temperature, particle size and initial dye concentration. Modified clays offered more efficiency for the adsorption of dyes. The techniques FTIR, XRD, SEM and BET have been used for the US characterization of clays before and after the adsorption of dyes, however, most of the studies lack information about the complete characterization of clays used as an adsorbent, which needs AN to be investigated in future studies. 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Liu, Study on the adsorption of Neutral Red from aqueous solution onto halloysite nanotubes, Water Res. 44 (2010) 1489-1497. 46 ACCEPTED MANUSCRIPT Highlights CE PT ED M AN US CR IP T In recent years, clay have attracted much attention as an adsorbent Appropriate conditions for dyes adsorption for particular clay-dye system are presented Sorption process is critically analyzed for > 10 year published studies Clay adsorbents proved to be promising for dyes removal from wastewater AC     47