Sulphur functionalized materials for Hg(II) adsorption: A review

Journal of Environmental Chemical Engineering 7 (2019) 103350

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Sulphur functionalized materials for Hg(II) adsorption: A review Tarisai Velempini, Kriveshini Pillay

⁎

T

Department of Chemical Sciences, University of Johannesburg, Doornfontein, 2028, Johannesburg, South Africa

A R T I C LE I N FO

A B S T R A C T

Keywords: Mercury Thiol Sulphur Thiourea Adsorption Adsorbents

Water is a scarce commodity and access to safe drinking water is a challenge due to increased release of inadequately treated effluent containing toxic heavy metals into the water systems. Due to widespread mercury contamination via anthropogenic activities such as artisanal gold mining and its high toxicity, it is necessary to remove mercury from water to protect both human and aquatic life. Currently, significant efforts are being made to develop materials with large and tailorable surfaces capable of selective and efficient adsorption of Hg(II). The high affinity of mercury towards sulphur is crucial in developing methods for the functionalization of various materials with sulphur for Hg(II) abatement. The technique of adsorption is favourable for mercury removal from water over other conventional techniques such as ion exchange, solvent extraction, membrane separation, precipitation, etc. The adsorption process offers design and operational dexterity whereas the latter techniques have several disadvantages such as huge energy consumption, high initial capital costs, sludge generation etc. In this review, recent trends in Hg(II) uptake by sulphur functionalized materials via adsorption are discussed. A variety of materials such as metal oxides, carbon nanotubes, metal organic frameworks, mesoporous silica, that provide a platform for sulphurization via various innovative methods are discussed. The physical adsorbent characteristics, adsorbent capabilities, role of sulphur in Hg(II) uptake and the adsorption process mechanisms are comprehensively highlighted. Conclusively, this review paper outlines the commercial viability and future perspectives and trends of sulphur functionalized adsorbents for Hg(II) uptake.

1. Introduction The focus on mercury as a pollutant of concern has increased over the past decades. Mercury is regarded as one of the most toxic heavy metals even at very low concentrations. Mercury is non-biodegradable but can be transformed from one toxic form to the other in the environment by chemical and biological processes [1]. Transportation and transformation of mercury are complex processes, which require dissolution processes, biological and chemical breakdown of organometallic compounds. For example, certain methylating microorganisms bio transform inorganic mercury (Hg2+) to the more toxic organic mercury form (methylmercury) [2]. Inherently, mammals are eventually exposed to and ingest mercury. Therefore, regulatory bodies such as the US Environmental Protection Agency (USEPA) and World Health Organisation (WHO) have set maximum acceptable concentrations in drinking water at 0.002 mg/L and 0.001 mg/L respectively, while the maximum mercury concentrations in industrial wastewater set by the USEPA is 0.01 mg/L [3,4]. Immediate symptoms related to high levels of Hg contamination and exposure in humans include ataxia, dysarthria, and constriction of the visual fields, auditory disturbances and tremor. Mercury poisoning can eventually lead to problems such as ⁎

neurological damage. Consequently, it is important to remove mercury from water. Sources of mercury contamination are both natural and anthropogenic. Natural mercury contamination includes emissions from volcanic eruptions and volatilization from ocean surfaces. Natural activities annually account for a smaller amount, about 643 tonnes out of the total 24 Tera-grams Hg(0) emitted, compared to human activities such as improper disposal of mercury containing devices and wastes. Since the Minamata incidence (from 1932 to 1968), where mercury containing industrial wastewater was discharged directly into Minamata Bay resulting in serious mercury poisoning [5], mercury and mercury compounds are still being used in devices such as telescopes, fluorescent lighting, batteries and also in dermatological therapy [6]. Thimerosal which contains 50% (by weight) mercury is still being used as vaccine preservatives particularly in developing countries [7,8] despite its known toxicological effects [9]. Human activities have a high impact on the amount of mercury in the environment. The majority of mercury is released into the atmosphere during artisanal gold mining (about 37%) [10]. It has been established that artisanal gold mining contributes about a third of the world mercury contamination in water, soil and air. On the other hand coal combustion contributes about 33% of

Corresponding author. E-mail address: [email protected] (K. Pillay).

https://doi.org/10.1016/j.jece.2019.103350 Received 8 June 2019; Received in revised form 3 August 2019; Accepted 8 August 2019 Available online 10 August 2019 2213-3437/ © 2019 Published by Elsevier Ltd.

Journal of Environmental Chemical Engineering 7 (2019) 103350

T. Velempini and K. Pillay

temperature [18] and 3) adsorption process factors such as adsorbent dose, adsorbent-adsorbate contact time [29]. Generally, adsorption of metal ions in solutions is carried out in batch or fixed bed column containing a specific mass of adsorbent [41]. The manner in which metal ions adsorb to the surfaces of an adsorbent can be classified as either physisorption or chemisorption. The physisorption process involves weak interactions (usually Van Der Waals forces) between the adsorbate and adsorbent whereas chemisorption implies formation of a chemical bond (mainly covalent bond) between the adsorbate and adsorbent. Predictions on whether an adsorption process is chemisorption or physisorption are discussed in details in the latter sections of this review. Although the adsorption technique is preferred over other previously mentioned techniques (section 1.1 and Table 1), it has the demerit of absence of adsorbent selectivity for a particular metal ion of concern (e.g. mercury). However, this lack of selectivity can be overcome by material functionalization. Functionalizing certain materials ensures improvement in certain characteristics and adsorption capacity of the adsorbents towards a targeted metal ion. Herein, the targeted metal ion for discussion is Hg(II). Consequently, after functionalization, the materials are applied as adsorbents in the adsorption of Hg(II) with good adsorbent selectivity.

the total gaseous mercury emitted into the atmosphere [11]. Coal which contains mercury in the form of cinnabar, is mainly used in electrical power generation and some countries are currently building new coal generated power stations (world energy resources, 2013 Survey) [12]. Although the use of mercury in the chlor-alkali industry has decreased due to other alternative methods of chlorine production, mercury contamination from this industry is still occurring. This is because most of the chlor-alkali plants have not been decommissioned or “cleaned -up” and the non-biodegradable mercury is lying idle or slowly vapourizing into the atmosphere in most sites [5]. A recent report showed high levels of present-day mercury in animals (0.4 to 10.5 μg/g) and surface sediments (350–1100 ng/g) despite mercury discharge from a chlor-alkali plant occurring in the late 1960s and early 1970s [13]. A wide range of conventional water treatment techniques such as chemical precipitation, ion exchange and membrane separation have been applied for the removal of mercury from water [14]. However, these techniques are operationally expensive and inefficient in removing heavy metal ions [15]. On the other hand, the adsorption technique is convenient and practical, offers an extensive selection of adsorbents and offers an adsorbent regeneration option [16]. Various activated carbons, polymers, celluloses, clays, zeolites, silicas have been applied for Hg(II) adsorption [17]. Recently specialized and sophisticated materials such as metal organic frameworks (MOFs) [18], graphene/graphene oxides, metal oxides [19], carbon nanotubes [20], quantum dots [21] are emerging as adsorbents of choice in the abatement of Hg(II). Functionalization of both past and recent materials have been tailored towards specific Hg(II) removal. These materials are functionalized with sulphur or sulphur containing groups to impart certain surface functional groups and to alter specific surface area, surface chemistry, pore volume, pore diameter etc. thus producing adsorbents appropriate for Hg(II) adsorption [22–27]. The functionalization of these materials with sulphur has been explored because Hg (II) has an affinity for sulphur, based on the Hg behaving as a typical soft base and sulphur as a soft acid. Recent reviews have focused on Hg(II) removal using particular materials [28–30]; or methods [31]. This review aims to explore the use of sulphur and sulphur compounds in the development of composite adsorbent/materials for the removal of Hg(II) in water, thus gaining knowledge on the importance of sulphur in Hg(II) adsorption.

1.3. Materials for Hg(II) adsorption Various conventional (activated carbon, zeolites etc.) [44] and nonconventional (MOFs, nanoparticles etc.) [28,45,46] adsorbents have been reported for the uptake of Hg(II). Although unmodified adsorbents have been shown to remove Hg(II) from aqueous solutions, some have been reported to show poor adsorption capacities, poor selectivity, inefficiency and are mechanically weak [47]. To address these challenges, much effort has been focused on adsorbent modification/functionalization. The type of modifying agent plays a crucial role in producing highly efficient, selective and robust adsorbents. Some agents which have been identified for the modification of various materials for Hg(II) uptake are metal sulphides, dithiocarbamates/ thiocarbamates, thiosemicarbazones, thioureas, thiadizoles, thiazoles and their use in materials functionalization is discussed in more detail (Sections 2–5 of this review). The main constituents of these agents are carbon and sulphur groups. Sulphur is highly selective towards mercury hence the use of sulphur in material modification/functionalization is highly desirable.

1.1. Techniques used for Hg(II) treatment 1.4. Mercury and sulphur binding chemistry Over the years, technologies such as ion exchange, chemical precipitation and solvent extraction have been used for the effective removal of low mercury levels from various effluent waters. The various techniques for removing heavy metals (including Hg) in water have been detailed elsewhere [32,33] and shall not be covered in this review. However, a summary of some of the disadvantages and advantages associated with the various technologies are discussed in Table 1. As summarized in Table 1, most of the methods for the removal of Hg(II) require high-energy, require large quantities of chemicals or result in incomplete removal of low concentrated heavy metals [34]. The adsorption process is the most favoured method of metal ion removal because it is cost-effective and efficient even at trace quantities of heavy metal ions (especially after adsorbent modifications) from environmental samples [35,36].

Due to the need to fill the vacant 6 s orbital, Hg(II) forms stable compounds with electron donating elements such as S to form HgS. However, at elevated temperatures (beyond 400 °C) HgS is unstable and decomposes to the metallic form according to Eq. 1 [48].

HgS + O2 → Hg + SO2

(1)

Hg(II) also binds strongly with ligands containing phosphorus, nitrogen and oxygen to form stable complexes. The theory of hard-soft acid and base (HSAB) states that soft bases form stable complexes with soft acids, likewise hard bases preferably bond with hard acids. In the case of sulphur and mercury; sulphur is a soft base and covalently binds to the soft acid Hg [49,50]. Mercury has relatively large ionic size, low electronegativity, and high polarizability which are features of a soft acid [51]. The reaction thermodynamics also proves this, for which Hg has high affinity to sulfur (i.e., log K≈52.7–53.3) compared to that for Hg to organic matter (i.e., log K = 22–28) [52]. The Eh-pH diagram of Hg is shown in Fig. 1. Generally, an Eh-pH diagram depends on the concentration of the metal species [53]. Fig. 1 indicates that mercury becomes more soluble under reducing conditions. Usually, solid HgS is formed at low pH, whereas soluble Hg-S complexes occur at high pH [54] and from the diagram, the affinity of mercury for sulphur is apparent. Mercury shows a strong affinity to sulphur thus a large proportion of

1.2. Adsorption The process of adsorption is described as the binding of the adsorbate (metal ions) onto surfaces of a solid (adsorbent). The ability of an adsorbent to remove metal ions from solution is referred to as the adsorption capacity. Adsorption capacity often depends on: 1) adsorbent characteristics, that is, presence and types of surface functional groups, porosity, specific surface area, pore volume, and 2) the physiochemical state of the water, that is, pH, metal ion concentration, 2

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T. Velempini and K. Pillay

Table 1 Removal technologies of Hg(II) from water. Method

Advantages

Disadvantages

References

Chemical precipitation

-simple and convenient method -not energy intensive

[37,38,39]

Reverse osmosis

-simple and compact equipment -removal of wide range of other contaminants (e.g. organics, bacteria)

-large amounts of chemicals are needed -usually uses corrosive chemical -generation of sludge causing secondary contamination -in-efficient in lowly concentrated Hg ion wastewater -requires pre-treatment step -high pressure and energy needed for operation

Membrane separation

-controlled selectivity depending on membrane type -requires simple and non-toxic materials

Electrolytic

-highly selective

Ion exchange

-tends to be cheap when natural zeolites are used -efficient when thio based resins are used

Phytoremediation bio- remediation

-formation of less harmful by-products

-not specific to Hg - high cost associated with membrane fouling -cannot be used with certain solvents and at high temperatures, membrane stability and integrity usually poor -sometimes eliminates essential mineral like Mg and Ca -high costs associated with electrodes -may consume large amounts of energy -requires a pretreatment step -high cost of resins -resins used during process require chemical regeneration producing secondary pollution -for live microorganism, method ineffective when metal concentration is high - may affect plant growth and photosynthesis ability -sensitive to operational environment -low selectivity

-low cost Adsorption

Solvent extraction

- cost effective -easy to operate -highly efficient -high adsorption rates -availability of a wide selection of adsorbents -highly efficient -high Hg selectivity -low detention periods -high Hg(II) selectivity

Photocatalytic

-inexpensive depending on catalyst used

Flotation

[37]

[32,33]

[32,37] [33,39]

[40]

[33,37,41]

-high initial capital costs

[42]

-requires post-treatment step because of low separation efficiency -generation of secondary wastes -time consuming -formation of volatile Hg(0), which is also toxic and requires trapping

[33,40]

[43]

2. Sulphur functionalized adsorbents for Hg removal Sulphur atoms are introduced to some materials mainly via pyrolysis of the materials in the presence of compounds such as H2S or SO2 gas or CS2 solution K2S [50], P2S5 [59], sodium thiosulfate (Na2S2O3) [60], S2 [52,61–64], dibenzyl disulfide [65]. The sulphurization method and agent amount impact different physical properties to the adsorbent, such as increase or decrease in the surface area of the adsorbent and different amounts of the sulphur on the surfaces of the adsorbent. Regardless of the outcome of the sulphurization process, the most crucial aspect of the modified adsorbent is the amount of sulphur available on its surfaces because of the importance of the Hg(II) and S bond formed during adsorption. For example, Saha et al. [60] sulphurized ordered mesoporous carbon (MC) using different carbon to Na2S2O3 ratios (4:1; 2.5:1and 1:1) to obtain 3 adsorbents (MC1.MC2,MC3 respectively). The unfunctionalized sample MC indicated a surface area of 605 g/m2 whereas functionalized samples MC1, MC2 and MC3 showed higher surface areas of 837, 1228 and 2865 g/m2 respectively. However, it was noted that for sample MC1, although it had the lowest surface area, the sulphur content was higher (12.9%) and thus selected for adsorption studies. Remarkably, the adsorbent displayed an adsorption capacity of 75.8 mg/g for Hg(II) in a mixture of equally concentrated Pb, Cd and Ni cations. While a decrease in surface area has been noted from 828 to 721 m2/g after sulfurization of some activated carbon (AC) with sulphur dioxide by Asasian et al. [66], sulphurized activated carbon showed higher removal capacities compared to unsulphurized activated carbon. This increase in adsorption was attributed to the organic (83%) and oxidized (17%) sulphur forms which provided adsorption sites for Hg(II). The sulphurization of AC was carried out at various temperatures, duration and SO2 concentrations to determine the optimal

Fig. 1. Mercury potential/pH (Eh-pH) diagram for an Hg-OH-S-Cl system [53].

mercury occurs in a concentrated form as sulfide minerals (HgS), as shown previously. Liu T. et al. [55] and Li et al. [56] demonstrated the formation of HgS using X-ray absorption near-edge structure spectra (XANES) analysis in Hg contaminated soils. Liu P. et al. [57] also applied XANES in addition to micro-X-ray fluorescence (μ-XRF) mapping, confocal X-ray micro-fluorescence imaging (CXMFI) analyses, extended X-ray absorption fine structure (EXAFS) analyses to prove formation of strong Hg-S bonds in Hg(II) adsorption using sulphurized biochar. The -S-HgOH complex has been evidenced by XANES [58]. Because of the favorable Hg and sulphur interaction much effort has been directed in designing sulphur incorporated adsorbents for the abatement of Hg(II) from water. 3

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Table 2 Sulphur functionalized adsorbent characteristics and performances for Hg(II) uptake. Adsorbent

Surfacearea m2/g

Functionalizing agent

Sulphur % (wt)

[Hg] mg/L

pH

Qmax(mg/g)

Reference

sulfur-impregnated coal Sulfur- ordered mesoporous Carbon sulfur-doped reduced graphene oxide Sulfurized AC Thio-Polyacrylonitrile Fiber Sulphur microporous polymer (MoS2)-reduced graphene oxide (rGO) hydrogel MoS2 Aerogels Thio layered double hydroxide organosolv lignin dithiocarbamate (OLDTC)

50 837 449.43 751.3 – 517 118 – – 4.5

Sulphur atoms Sulphur atoms Benzyl disulphide S atoms, SO2 S/thio sulphur atoms MoS2 metal sulphide metal sulphide CS2

7.5 12.9 9.96 11.0 17.3 31.4 – – – 16.1

– 100 100-800

– 5 7 7 7 1 2 5.4 2-10 6

248.6 70.8 multi metal 833.33 523 322.8 595 340 1527 594 257.1

[50] [60] [65] [66] [67] [68] [71] [79] [80] [82]

conditions. The adsorption capacity increased with time from 30 to 60 min but considerably decreased from 60 to 180 min. Increasing SO2 concentration from 2 to 4% (vol/vol) led to higher Hg adsorption capacity; however, beyond 4% the adsorption capacity did not increase significantly. Variation in sulphurization temperature from 400 to 900 °C at a specific time and SO2 concentration resulted in increases in adsorption capacity. However, according to the authors, at higher temperatures, above 700 °C, lower yields after sulphurization were noticed. The authors suggested that at higher temperature because of readily available heat and oxygen, carbon weight loss occurs as a result of reactions between the carbon in the AC and SO2/S2 yielding carbon sulphides and carbon oxides. Ultimately, 60 min, 700 °C and 4% was considered as the optimal time, temperature and SO2 concentration respectively. Likewise, a recent report by Ting et al. [52] noted a drop in the surface area of activated carbon from 818 to 728 m2/g after modification with sulphur powder. The authors reported a 4% S content in the sulphur activated carbon (SAC). Although this percentage was lower than most reports (Table 2), the amount of sulphur was adequate in adsorbing Hg(II) and expectedly the SAC adsorption was higher than pristine AC. Conversely, Wagima and Suguwara [50] reported an increase in surface area with sulphurization temperature. In the study, K2S was used as a source of sulphur for the modification of raw coal. Analysis of the raw coal showed a 2.7% sulphur content and upon sulphurization the content increased to 7.5%. Although raw coal was activated at various temperatures, minimum differences in the amount of sulphur were observed among the adsorbents. On the other hand, the adsorbents surface area increased from 10 to 50 m2/g with increase in temperature (800–1000 °C). Coal activated at 900 °C showed the highest adsorption of Hg(II) at 28.6 mg/g (Table 2). However, the competitive adsorption of the adsorbent was not tested hence its selectivity for Hg(II) was not determined. Previous studies have suggested that the method of sulphur introduction has a significant effect on the characteristics of the final adsorbent. Deng et al. [67] introduced sulphur (using NaS2) to polyacrylonitrile fibers by the microwave (MW) assisted method and the conventional (CV) heating method to obtain PANMW-thio and PANCVthio fibers respectively. Elemental analysis results showed that the sulfur content in the PANMW-thio fiber was nearly twice that of the PANCV-thio fibers. Furthermore, PANMW-thio fibers were crystalline and showed higher strength compared to PANCV-thio fibers. Xu et al. [68] have synthesized a novel sulfur rich microporous polymer with sulfur content of 31.4% using cheaper metal catalyst for the uptake of Hg. The polymer chemical structure depicted in Fig. 2 shows densely populated sulphur atoms which were effective in Hg(II) adsorption. Moreover, this adsorbent can be applied over a wide pH range (pH 2–12) and can still efficiently remove Hg after 5 cycles of adsorptiondesorption. In a recent report by O'Connor et al. [69], a biochar produced from a waste rice husk was modified using elemental S and subsequently showed a sulphur content of 13.04%. The adsorbent was applied for the remediation of Hg(II) extracted from contaminated soil and the adsorption capacity was 67.11 mg/g.

20 − 500 25 to 500 – – 50-1800 –

Fig. 2. Chemical structure of sulphurized polymer a) and effect of pH on Hg(II) removal b) [68].

The general trend for sulphur functionalized adsorbents is that the sulphur carrying compound/ agents are used to introduce the sulphur component only to the adsorbent materials. However, the sulphur carrying compounds can be combined with some materials, for instance, graphene oxide/graphene [70–72], polymers, Fe3O4 [73,74], organic frameworks [23,75] and/or other agents to form composite adsorbents for Hg(II) adsorption. Highly stable and dispersible FeS nanoparticles were prepared with carboxymethyl cellulose [75–77] and chitosan [78] before application in the uptake of Hg(II). Meanwhile, a different approach was used for sulphur introduction to aerogel-supported Au nanoparticles [79]. Composite aerogels were fabricated by doping MoS2 with graphene oxide (GO) via a hydrothermal method and then Au and Fe3O4 nanoparticles (NPs) were embedded between the GO-doped MoS2 sheets through coordination. The synthesized three-dimensional nanocomposite exhibits a prominent 4

Journal of Environmental Chemical Engineering 7 (2019) 103350

T. Velempini and K. Pillay

Fig. 3. Thiols commonly used in adsorbent functionalization 3-mercaptopropyltriethoxysilane a), cysteamine b) and cysteine c).

3.1. Adsorbents based on carbon

desorption and recycling performance for Hg(II) adsorption even after 10 adsorption − desorption periods, rendering it an excellent adsorbent for removal of Hg(II). Ali et al incorporated MoS42− into double layered hydroxides (LDH) of Mg, Mn and Al to produce Mn-MoS4 [80]. It was found that Mn-MoS4 performed well over a broad pH range (from 2.0–11.0) and in the presence of organic matters due to the strong coordination between thio groups as soft Lewis acid and Hg(II) as a soft Lewis base. Recently, two dimensional MoS2 alone has been applied for Hg(II) uptake and it was found that the high MoS2 surface reactivity for Hg(II) was due to sulphur vacancy defects [81].

Carbonaceous materials have many features such as large surface area, well-developed porosity, non-polar and modifiable surface functional groups that enhance their adsorption efficiency for Hg(II) [83,84]. 3.1.1. Activated carbon Commercially activated carbon has traditionally been applied for the uptake of metal ions from contaminated solutions. While it is commercially viable to use activated carbon as an adsorbent for highly concentrated (higher mg/L) metal ions, the challenge is encountered with Hg adsorption because Hg contamination is often lowly concentrated. Despite their high surface areas (typically 1300 m2/g [85]), activated carbons poorly adsorb low concentrated Hg ions [59]. Moreover, commercial activated carbon is expensive, therefore, other low-cost materials have been explored as carbon sources for the production of activated carbon. There have been many strides in incorporating thiols in various activated carbons obtained from various sources. For example, Shang et al. [86] modified low cost coal gangue (CG) by first acidifying the materials by sequentially subjecting it to 20% H2SO4, sulfuric acid-water solution (v/v = 1:1, 100 mL) at 80 °C and then adding excess KMnO4. The acidified product, oxidized CG (CG-O), was then thiolated with 3-mercaptoproyl trimethoxy silane to produce CG-SH which had a higher sulphur content of 26.06% compared to 10.32% of the starting material CG. Table 3 shows the surface

3. Thiol and thio functionalized adsorbents for Hg(II) removal Thiols are chemical compounds that contain a sulphur group attached to hydrogen whereas thio compounds contain the sulphur group not attached to any hydrogen. Fig. 3 shows some of the commonly used thiols in functionalization of materials for adsorption purposes. Thiols are incorporated into different materials to improve the material’s adsorptive characteristics for Hg(II). These materials offer support on which thiols can be anchored and they range from carbon based (e.g. CNTs, activated carbon), biopolymers (e.g. chitosan, cellulose, lignin, cyclodextrin, alginate), polymers, metal organic frameworks to clays. Various adsorption studies on thiol-based adsorbents for the removal of Hg(II) from water have been reported in Table 3 and described in the following subsections.

Table 3 Thiol/thio functionalized adsorbent characteristics and performances for Hg(II) removal. Adsorbent

Surface Area m2/g

Sulphur % (wt)

[Hg] mg/L

pH

Qmax(mg/g)

Ref.

Aluminum–silicate oxides Nanocellulose Wood sawdust Coal gangue Activated coke Multi-walled carbon nanotube-SH Multi-walled carbon nanotube-SH Thiol-magnetic graphene oxide Carbon dots Polypyrrole-zeolite polysilsesquioxane coated poly(p-phenylenetherephthal amide) 3D-porous hyper- crosslinked polymer Polypyrrole/mercaptoacetic acid (PPy/MAA) Homopolymer maghemite nanoparticles (MAMNPs) Ion imprinted Ion-imprinted Sugar cane bagasse Thio-modified cellulose resins Chitosan–thioglyceraldehye Mercapto-grafted rice straw UiO-66-SH UiO-66-(SH)2 Metal-organic framework Fe3O4@MOF Si/Al@CN@CME PAF-1-SH Magnetic/ CoFe2O4@SiO2-SH Fe3O4@SiO2-SH nanoparticles Polyhedral oligomeric silsesquioxane Silica-gel-supported sulfur dendrimers TPB-DMTP-COF-SH MAF-SCMNPs MMSP

243 – 1162 47.2 193.7 – 181.6 88.04 321.43 38 0.39- 38.40 333 12.86 92.41 – – – – 70.5 11.46 1574 499 800 258 77 3,274 48.90 – – 140.99 291 71.9 21.78

8.75 21.31

– 25–250 10-110 100-3500 0-100 5-100 10-80 – 30-1000

6 5.6 6 5.5 5 6 6 7 3 8 5 4 5.5 6 7.5 6 6 4 5 6.5 4 3-5 – 6 6 6.8 8 6 6 6 5.6 5-6 4

3232 240 ̴ 129 179.2 694.9 84.66 205 208.78 625 163.9 3.52– 10.64 – 1736.8 237.5 28 92.22 192.31 23 98 ± 2 161.30 785 236.4 714.3 348.43 3079 1 014 641.0 132 12.9 336.8 4395 355 243.83

[47] [49] [61] [86] [87] [90] [91] [95] [102] [114] [117] [118] [119] [121] [126] [127] [14] [133] [134] [139] [167] [168] [169] [25] [170] [177] [182] [183] [184] [185] [178] [186] [187]

26.06 1.94 – – 1.54 – 3.6 1.41- 3.51 mmol/g SH 10.3 (3.4 mmol/g SH) – – – – 19.04 – – 5.69 3.45 mmol/g SH 24.51 14.27 6.84 17.6 (5.5 mmol/g SH) – – 8.3 2.77 6.32 – –

5

– 4 100-800 – 20-100 – – – 10-400 10-300 2-351 – 0.1-1400 0-1000 100-2000 – 20 -200 5-100 – – – 10-100 50-300

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Fig. 4. Functionalization of MWCNTs a) and effects of adsorbent type on Hg(II) adsorption [91].

nanotoxicity, CNTs use in water treatment can be harmful in the long run as CNTs are assumed to interact with animals at cellular levels, putting both human and animal health at risk [41].

area and adsorption capacities of the adsorbents. Activated coke was thiol functionalized using mercapto-acetic acid to produce SH-AC adsorbent for Hg(II) uptake. The adsorption was characterized by high adsorption capacities (Table 3) and short equilibrium adsorption time of 15 min [87].

3.1.3. Graphene/graphene oxide Several studies have explored the use of thiol/graphene oxide [93–97], thiol-graphene [98] and thiol-reduced graphene oxide [99] for the adsorption of Hg(II). Graphene is typically a two-dimensional structure consisting of a single layer of sp2 hybridized carbon atoms arranged in a hexagonal pattern. Graphene tends to aggregate in aqueous solutions, thus limiting its use in adsorption. However, the introduction of functional groups such as oxygen-containing functional group improves the dispersion properties of graphene in solution, thereby increasing the adsorption capabilities of graphene. Graphene oxide (GO) is obtained when graphene is oxidized using H2SO4, H3PO4, KMnO4 and H2O2 via the modified Hummers method [99]. Reacting GO with hydrazine hydrate produces reduced GO (rGO) [100]. Recently, a thiol-functionalized graphene oxide/Fe-Mn composite (SGO/Fe-Mn) was reported for Hg(II) uptake [97]. The authors investigated the uptake of Hg(II) in low concentrations (0.1–11 mg/L) because they argued that mercury concentrations in the environment are typically less than 3 mg/L. The adsorbent offered positive attributes in that it performed well over a wider pH range of 4.5–8.0 and thus adsorption was carried out at a pH of 7 ± 0.2, which again simulated the typical pH of environmental water. However, the disadvantage was that it took 72 h to reach adsorption equilibrium. In another study, exfoliated graphene oxide (EGO) was chemically combined with L-cysteine in a simple reaction to yield a Lcysteine–EGO adsorbent [101]. When the L-cysteine–EGO adsorbent was applied for the adsorption of Hg(II), a maximum adsorption capacity was 79.36 mg/g and competitive experiments showed that the adsorbent is highly selective towards Hg(II). Furthermore, the practicability of upscaling Hg(II) adsorption using L-cysteine-EGO was tested through column studies. It was revealed in this study that 1 g of adsorbent significantly extracted 1 mg/L Hg(II), at a flow rate of 8 mL/ min, and a sample volume of 400 mL with adsorption exhaustion rate of 2.5 g/L. Thus, the authors concluded that on an industrial scale, an increase in the amount of the adsorbent in the column will result in an increase in the upper limit for the sample volume. Asiabi et al. [102] reported hydrothermal treatment of a sodium citrate and L-cysteine mixture to obtain carbon nanodots (CDs) decorated with N2 and S2 groups (N,S-CDs). The S,S-CDs were subsequently

3.1.2. Carbon nanotubes Carbon nanotubes (CNTs) have since emerged as one of the great 20th century scientific discoveries. The unique physical and chemical robustness attributes of CNTs have attracted their potential application in various areas including Hg(II) adsorption [88,89]. Hadavifar et al. [90] compared the adsorption of Hg(II) by aminated multiwalled carbon nanotubes (MWCNTs-EDA), carboxylated multiwalled carbon nanotubes (MWCNTs-COOH) and thiolated multiwalled carbon nanotubes (MWCNTs-SH). MWCNTs-SH showed the greatest capability for Hg uptake at 84.66 mg/g due to the strong covalent bonds formed between the sulphur groups and Hg(II). A latter report by Hadavifar et al. [91] (Fig. 4), using the same adsorbents synthesis route as the previous study [90], revealed that Hg(II) adsorption was 205 mg/g. This figure was higher than the former report despite the same adsorbent being used and minor differences in the adsorption conditions. That is, in the former study a pH of 6, adsorbent dose of 400 mg/L, temperature of 25 °C, initial Hg(II) concentration of 5–100 mg/L and agitation time and speed of 60 min and 200 rpm, respectively were used; while a pH of 6, adsorbent dose of 200 mg/L, room temperature, initial Hg(II) concentration of 10–80 mg/L and agitation time and speed of 60 min and 200 rpm respectively were applied for the latter study. However, the latter study [91] showed that in a binary system containing Hg(II) and Cd(II), adsorption of Hg(II) by MWCNTs-SH was satisfactory, with an adsorption capacity of 35.89 mg/g. In another report thiol-derivatized single walled carbon nanotubes (SWCNT-SH) powders were synthesized via a condensation reaction between carboxylated- SWCNT (SWCNT-COOH) powders and cysteamine using the popular 1-ethyl-(3-3-dimethylaminopropyl) carbodiimide (EDC) and N- hydroxy succinimide (NHS) coupling agents in a study conducted by Bandura et al. [92]. The surface areas and sulphur content were not determined in the study. Although, comparatively SWCNT-SH showed better adsorption than SWCNT-COOH, the role played by the thiol in Hg(II) adsorption was not intricately probed. However, despite their good performance in Hg(II) adsorption CNTs have the limitations of practical application at large commercial scales because of high production costs. Furthermore, because of their 6

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Fig. 5. SEM image a), TEM image b) of N,S-CDs-LDH and proposed Hg adsorption c) [102].

functionalized polysilsesquioxane coated poly(p-phenylenetherephthal amide) fibers for the adsorption of Hg and achieved a Langmuir theoretical adsorption capacity of 1489–2049 mg/g. A thiol concentrated 3D polymer network, with the SH groups protruding out of the polymer network, was synthesized by Abadast et al. [118] using calix [4]resorcinarene and functionalized by 3-(trimethoxysilyl)-1-propanthiol. The resulting polymer was stable under hydrolysis and could potentially be used in various solvents without loss of integrity. Das and coworkers prepared a polypyprrole/mercaptoacetic acid (PPY/MAA) composite adsorbent via chemical oxidative polymerization of the pyrrole monomer in the presence of MAA and ammonium persulfate (APS) as the initiator for the polymerization reaction [119]. The Hg(II) adsorption to the PPY/MAA occurred best at pH 5.5 and adsorption kinetics were well fitted to a pseudo- secondorder model. A nylon-based membrane was coated with some polyamine prior to addition of a thiol (3-mercaptopropionic). A synergic reaction was allowed to occur between the amine groups and the carboxylic groups of the thiol through the coupling agent EDC/NHS [120]. The resulting membrane removed 90.4% of the initial 30 mg/L mercury ions in water containing oil. Usually, membrane-based adsorbents have a disadvantage of disintegrating or show instability in strongly basic, acidic and oxidizing environments but the prepared membrane was stable under 1 mol/L concentrations of HCl, NaCl and NaOH, which indicated promising use for Hg adsorption under naturally harsh conditions. Madrakian et al. [121] studied the adsorption capability of a thiolized homopolymer-modified with maghemite nanoparticles. The adsorption data fitted the Sips isotherm well which showed that at low adsorbate concentrations, the adsorption system is multilayer in nature whereas at high concentrations, a monolayer adsorption coverage is projected. Ballav et al. [122] recently demonstrated the application of

grafted on the surface of a layered double hydroxide (LDH) through a co-precipitation method to produce N,S-CDs-LDH. Fig. 5a–c depicts the surface morphologies and the proposed Hg(II) adsorption on the surfaces of the adsorbent. The N,S-CDs-LDH were highly selective towards the targeted Hg(II) and high adsorption capacities and selectivity for Hg (II) (as highlighted in Table 3) was credited to the soft acid/soft base bonding interactions of Hg2+/S2−. 3.2. Polymer based adsorbents Polymers usually include materials such as polyester, polypyrrole, and polyaniline. They have physical attributes such as high elasticity, great mechanical strength and large surface area-to-mass ratios, and on this account, thiol modified polymers have been reported as candidates for Hg(II) adsorption [103–113] (Table 3). Javadian and Taghavi [114] demonstrated a polypyrrole/thiol/ silica nanocomposite for wastewater treatment. The adsorbent effectively removed 42.8 g/L of 43 mg/L of Hg(II) in contaminated chlor-alkali effluent. Adsorbent reuse was also tested and adsorption efficiency decreased to 93% after the 5th cycle using H2SO4 as the desorbing agent. Interestingly, the sulphur content of the recycled adsorbent was higher (8.8%) compared to the unused adsorbent (3.6%), and this change was not explained. Moreover, a reduction in surface area was noticed with subsequent functionalization, specifically the starting material a zeolite (Beta/MCM-41) had a specific surface area of 585 m2/g. The surface area dramatically reduces to 89 m2/g after addition of the thiol and the subsequent introduction of polypyrrole further reduced the specific surface area to 38 m2/g. It is worthwhile to note that some researchers have reported similar changes in surface area after thiol functionalization [115,116]. Wang and coworkers [117] used 3-mercaptopropyltriethoxysilane (MPTES) 7

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Fig. 6. Representation of a zwitterionic polymer synthesis and Hg(II) removal [123].

bulk, suspension, precipitation or emulsion polymerization [125]. Mercury imprinted thiol-based have been reported as listed in Table 3. Firouzzare and Wang [126] prepared a ion imprinted polymer by radical copolymerization which was selective towards Hg ions. The first monomer was synthesized via the Schotten-Baumann reaction using 2mercaptoethylamine (MEA) to yield N-methacryloyl-2-mercaptoethylamine (MMEA) monomer. The MMEA monomer was then complexed with Hg(II) and, next radical copolymerization carried out using the Hg (II)-MMEA complex monomer in the presence of second monomer (methacrylic), ethylene glycol dimethacrylate as a crosslinker and the 2,2-azobisisobutyronitrile (AIBN) as the initiator. Thiourea was used to remove Hg(II) to create imprinted cavities that are highly selective to Hg(II). The selectivity co-efficient for Hg(II) against Cd, Zn and CH3HgCl were 64.8, 288.3 and 91.7 respectively. Earlier, a non-conventional ion-imprinted polymer based on a thiol and silica was synthesized for the adsorption of Hg(II) via surface imprinting [127]. Usually in surface imprinting, polymerization of a monomer does not occur; but the resulting surface imprinted polymer possesses accessible adsorption sites, high selectivity, and exhibits high mass transfer rates and rapid binding kinetics [4]. In the report by Wang et al. [127], the ligand, 3-mercaptopropyltrimethoxysilane (MPS) was complexed with Hg(II), and the silica gel (which was activated by methanesulfonic acid to provide surface silanol groups) was added as a support. The adsorbent was shown to effectively bind to Hg(II) in the presence of Cu(II) and Cd(II) ions which were twice the concentration of Hg. In a study by Monier and Abdel-Latif [128], ion imprinted polymers were strategically synthesized using the surface imprinting technique by graft copolymerization of acrylonitrile (AN) onto Poly (ethylene terephthalate) (PET) fibers followed by functionalization with thiosemicarbazide moieties (Fig. 7). Afterwards, the Hg template was

polypyrrole doped with L-cysteine for Hg(II) uptake, reporting a remarkable adsorption capacity of 2042.7 mg/g at 25 °C. Here, the only drawback was the long contact time of 24 h needed to achieve the adsorption capacity. However, characterization of the material showed a thermally stable polypyrrole doped L-cysteine material compared to pure polypyrrole for temperatures up to 150 °C. Furthermore, reapplication of the Hg(II) loaded spent adsorbent for the catalytic organic transformation was established. Removal of lowly or ultra-concentrated Hg(II) is often a challenge and there are a few reports in this area. Ali et al. [123] have thus synthesized a cross-linked polyzwitterion (CPZ) via cyclopolymerization of N,N-diallylmethionine hydrochloride, capable of adsorbing 99.1% and 98,9% of 200 μg/L and 400 μg/L of the metal ion respectively (Fig. 6). The CPZ has a sulphur content of 12.06%. The adsorption of Hg(II) followed a pseudo second-order process and the activation energy (Ea) of adsorption was 48.1 kJ/mol, both suggesting a chemisorption mode of interaction.

3.3. Adsorbents based on ion-imprinted polymers While most adsorbents are effective in pristine Hg(II) only contaminated water, it should be noted that most environmentally contaminated waters contain other contaminates which may adversely affect the adsorption of Hg(II). Usually for the contaminants to affect the Hg(II) adsorption, their chemistry should be similar (e.g. same ionic charge) to Hg(II). To avoid such challenges some researchers use ionimprinted polymers for Hg extraction [124]. Briefly, ion-imprinted are tailored for the uptake of a specified ions even in the presence of competing ions in solutions. Typically, preparations of ion-imprinted polymers are carried out via a specific polymerization technique, i.e.

Fig. 7. Schematic representation for synthesis of PET-TSC chelating fiber [128]. 8

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Fig. 8. Schematic representation of synthesis of nanoparticles (NPs) based on thiol-functionalized chitosan (CS) and Hg(II) adsorption, step 3: NPs preparation via capillary microfluidic device, step 4: dispersing NPs in a solution of Hg(II) [24].

displayed high selectivity (in a mixture of 14 other cations) and good adsorption capacity (23 mg/g, Table 3), for Hg(II) in acidic solutions (1.0 mol/L HNO3).

loaded onto PET-TSC and crosslinking carried out using formaldehyde as a cross-linker. 3.4. Natural biopolymers-based adsorbents

3.4.2. Chitosan Chitosan is a linear polymer of acetylamino-D-glucose obtained from chitin and is another biopolymer that is functionalized with thiol and thio groups and used in adsorption [134–136]. In the work of Morsi et al. [137], polythiophene modified chitosan/magnetite nanocomposites were prepared by first combining chitosan nanoparticles/magnetite nanocomposite and then coating the nanocomposites with polythiophene. The Langmuir maximum adsorption capacity was 117.7 mg/ g for the composite adsorbents, indicating good selectivity for Hg in the presence of Pb, Cu and Cd. However, the reuse of adsorbents was not determined. A chitosan thiobarbituric acid adsorbent was prepared by dissolving chitosan in glacial acetic acid and then incorporating 2-thiobarbituric acid (TBA) [138]. The mixture was heated and NaOH subsequently added to form a supramolecular assembly held together by intra hydrogen bonds between chitosan and TBA. The TBA role was that of crosslinker and sulphur source. Nuclear Magnetic Resonance (NMR) results showed the emergence of a new peak attributed to the binding of Hg(II) to sulphur. The adsorbent demonstrated a high adsorption capacity of 2505 mg/g toward Hg(II) within 3 h and at pH 4.0 [138]. In another report nanoparticles (NPs) based on thiol-functionalized chitosan (CS) were produced using capillary microfluidic (MF) device combined with ionic gelation method for Hg(II) uptake [24] (Fig. 8). The nanoparticles offered a removal capacity of 1192 mg/g with the ability of efficient removal of Hg(II) after four adsorption/desorption cycles using HCl as an eluent. Other low cost biopolymers which have been thiol modified for Hg adsorption with good selectivity and adsorption capabilities include rice straw [139], corn bract [140], biochar [141] and sugarcane bagasse [14]. It is worth mentioning that when working with biopolymerbased adsorbents, the stability of the biopolymers is a challenge in chemical environments, e.g. acidic media. The structural integrity of the adsorbent is destroyed considerably reducing the adsorption performance for Hg(II). Hence most researchers reported cross-linking of biopolymers before any further modifications and applications in adsorption. Some cross-linking agents which have been employed to impact stability to the biopolymers include epichlorohydrin [28,142], glutaraldehyde [143].

Biopolymers are low cost, environmentally sustainable and safe materials that can be utilized as adsorbents. Biopolymers are natural polymers; examples include chitosan, cyclodextrin, sodium alginate and cellulose. 3.4.1. Cellulose Cellulose is a natural occurring polysaccharide with repeating glucose rings (C6H12O6)n of the β-1,4 glusodic bond. Where n is the number of glucose units that can range from several hundred to over 10,000. High concentrations of hydroxyl functional groups on cellulose surface are available for modification or manipulation [129,130]. Cellulose derivatives are usually synthesized by modifying cellulose with acetic anhydride, methyl chloride, propylene oxide and chloroacetic acid to yield cellulose acetate, methyl cellulose, hydroxypropyl cellulose and carboxymethyl cellulose respectively through simple chemical reactions [129] and these steps widen the applicability of cellulose. Gupta et al. reported on the modification of a biopolymer with 2 mercapto ethanol for the remediation of Hg(II) contaminated water streams [14]. The adsorbent was low cost and small amounts of the adsorbents (40 mg) were needed to reduce the concentration of Hg to below the detection limit. Nowadays, nano forms of cellulose (nanocelluloses) have attracted much attention in research. For example, Ram and Chauhan [49] synthesized a thiol-spherical nanocellulose (SNC) adsorbent via acid hydrolysis of cellulose followed by lipase catalyzed esterification using 3-mercaptopropionic acid. In addition to retaining advantages of cellulose mentioned before, nanocelluloses are lightweight, have high mechanical strength and have a large specific surface area [131]. In this light, Bansal et al. [132] recently worked with cellulose nanofibers (CNFs) in the adsorption of Hg(II). Cellulose nanofibers were obtained from bagasse through a series of alkalization, bleaching with H2O2 and finally treatment with HCl. In order to selectively and effectively adsorb Hg(II), CNFs were functionalized with L-cysteine to impart thiols groups to the CNFs surfaces. The authors noted that because of ease of cellulose nanofiber synthesis and its low cost, it is a worthwhile contender for Hg(II) adsorption [132]. Thio modified resins were prepared for Hg(II) adsorption of Hg in highly acidic media [133]. The thio modified resins removed 2 mg/L of the Hg (II) concentrated in 0.1 mol/L HNO3. Moreover, the thio modified resin 9

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two discussed reports, the Fe3O4@thiol-MOFs [25] displayed highly selective and easy removal of Hg2+ ions from the mixed metal ions wastewater compared to the MOFs-thiol only [169]. While Fe3O4@ thiol-MOFs showed a lower adsorption capacity of 384 mg/g compared to thiol-MOFs which had the adsorption capacity of 714.29 mg/g [169], the Fe3O4@thiol-MOFs practical application on an industrial scale is plausible. Thiol functionalized metal oxide [47,170] magnetic metal oxyhydroxide [171] and metals [172] have also been applied for Hg(II) remediation with some success.

3.5. Clays based adsorbents Clay is another low-cost material which is readily available. Clays are naturally abundant, biocompatible, chemical and mechanically stable and often exhibit high surface areas. Clays have an overall negative charge and silanol or aluminol groups at the particle edges which are ideal for adsorption of heavy cationic metals such as Hg [144,145]. Clays are classified into three major classes; mica, kaolinite and smectites [146]. Smectites such as mantomentrollite and bentonite are generally favoured for adsorption applications because of their characteristically small crystals, high surface area and high ionic exchange capabilities. However, unmodified naturally occurring clays have limitations such as low adsorption capacity [147] due to their inaccessible layers [148]. Hence, much research has recently been directed on the manipulation of the clay layered structure to introduce other chemical components to improve their adsorption capacities. Various thiols have been presented for clay modification over the years [145,149,150]. Adraa and co-workers [151] synthesized a montmorillonite (Mt) and cysteine composite for the adsorption of Hg(II). Comparison with the pure montmorillonite showed superior adsorption capacities with the composite adsorbent.

3.7. Covalent organic frameworks Covalent organic frameworks (COFs), analogs to MOFs, are porouscrystalline materials constructed with organic building units. Their promising application in decontamination of Hg(II) in water has been realized [173–177] (Table 3). In most of the studies, the resulting COFs consists of hexagonally shaped individual frameworks interlinked with 6 other frameworks via covalent bonds as illustrated in Fig. 10. To produce this framework Huang et al. [175] reacted 1,3,5- Tris(4-aminophenyl)benzene (TAPB) and 2,5-Bis(methylthio) terephthalaldehyde (BMTTPA) under solvothermal conditions. Batchwise adsorption experiments indicated that the COFs exhibited a Hg(II) adsorption of 734 mg/g at a pH of 7. A remarkable adsorption capacity of 4395 mg/g was reported in one study because oxidation of the disulphide group was prevented during the incorporation of dithiothreitol ligand to the organic framework [178]. This resulted in production of large amounts of -SH groups which were in turn available for interaction with Hg(II).

3.6. Metal organic frameworks Metal organic frameworks (MOFs) consist of metal ions/clusters linked by organic bridging ligands. Typical surface areas of unfunctionalized MOFs are between 1000 to 10,000 m2/g [152]. MOFs have easily tailorable pore sizes and structures, numerous active sites, large surface area and facile charge-separation under ambient light [153–155], therefore recently much effort has been directed towards their use in Hg(II) adsorption [17,18,23,25,45,156–166]. A thiol, 3mercaptoisobutyric acid, was incorporated into UiO-66 using a facile method to obtain UiO-66-SH [167]. The study revealed a Hg(II) adsorption capacity of 785 mg/g in 20 min. Although the highest adsorption capacity was determined at pH 4, the UiO-66-SH still showed a high adsorption performance at pH 2, 3 and 5–8 range. Leus et al. [168] also functionalized the metal organic framework UiO-66 with 2,5-dimercaptoterephthalic acid to obtain UiO-66-(SH)2. The adsorption capacity of UiO-66-(SH)2 for Hg(II) was 236.4 mg/g which was lower than the previous mentioned study [167] despite the double thiol group in UiO-66-(SH)2 compared to UiO-66-SH, Table 2. Perhaps the higher adsorption capacity of Li et al. report was due to the UiO-66-SH higher surface area (1574 m2/g) compared UiO-66-(SH)2 (499 m2/g). However, although both studies used UiO-66 as a support and starting material for synthesis, it is difficult to compare the performance of the finished adsorbent because different functionalized reagents and synthetic routes were used. Moreover, different initial adsorption concentrations were applied which would give different adsorption capacities. A thiol-functionalization copper based metal organic framework (MOF); [Cu3(benzene-1,3,5-tricarboxylate)2(H2O)3]n, was applied for the uptake of Hg(II) [169]. The thiol ligand, dithioglycol was anchored to the MOF via unsaturated metal centers in MOF (Fig. 9). Compared to non-functionalized MOFs which showed no adsorption for Hg; thiolMOFs revealed an adsorption capacity of 714.29 mg/g as shown in Table 3. According to the authors, the great Hg(II) adsorption capacity was due to the high thiol group concentration and the large specific surface of the thiol functionalized MOFs. However, a latter similar report (using Fe3O4@thiol-MOFs) suggested that the presence of S plays a crucial role more than the surface area of the adsorbent in high adsorption capacity [25]. Although several reports have shown that thiolbased adsorbents are effective for the Hg uptake challenges are often encountered in separation of the adsorbent from the aqueous media. In recent years magnetic composites have been commonly adopted in adsorption because the magnetized adsorbents are easier to separate from the bulk aqueous solutions. For instance, looking at the previous

3.8. Silica based adsorbents Silicas have typical surface areas of between 175 to 225 m2/g whereas ordered mesoporous silicas have been reported to have surface areas in the range of 700–1500 m2/g [58]. Mesoporous silicas are wellknown for their large surface area, good chemical and mechanical stability, unique pore structure, uniform pore size, rapid adsorption rate and ease of surface functionalization. Thus, mesoporous silica is more favoured for use in adsorption. Periodic mesoporous organosilicas were thiol-functionalized to produce an ordered arrangement and uniform mesoporosity adsorbent with an excellent adsorption capacity of 1183 mg/g for Hg(II) [179]. Physical characterization of synthesized material showed that indeed the thiol groups were incorporated with SH concentration and total sulphur concentrations of 2.42 mmol/g and 4.3 mmol/g respectively. The adsorption capacity was attributed to accessibility of all the thiols for the mercury ions and the presence of disulfide species which can coordinate more than one mercury atom. Thus, stereo-coordination chemistry of S with Hg2+ was possible. On the other hand, cheap periodic mesoporous organosilicas with low thiol density of 0.22 mmol/g were synthesized [180]. Despite the low thiol density, the adsorbent displayed an adsorption capacity of 46.1 mg/g. Furthermore, rapid adsorption characteristics were shown when 49 μg/ L Hg (II) were extracted within 15 min. The authors suggested the good adsorption is not dependent on the number of the sulfur coordination sites, but rather on the possibility of multi-layer and physical adsorption of Hg(II) at high concentrations. The mesoporous silicas were also effective in removing Hg(II) spiked natural waters. However, the reuse of the adsorbent was not tested. Similarly Shen et al. [181] prepared an ordered mesoporous adsorbent (SBA-15) and functionalized it with thiol groups (-SH) by a simple hydrothermal method. The adsorbent (SBA-15-SH) had a moderately high adsorption capacity of 195.6 mg/g at pH of 8 despite a relatively small surface area of 50.9 m2/g. Mesoporous silica-coated magnetite nanoparticles functionalized with thiol groups have been explored for the removal of Hg from wastewater. Magnetite (Fe3O4) nanoparticles (NPs) were first synthesized by a coprecipitation method and the resulting Fe3O4 NPs were then coated by a mesoporous silica; finally the thiol groups were added 10

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Fig. 9. Illustration of the thiol-modification of MOFs a) and SEM images of b) the as-synthesized [Cu3(BTC)2(H2O)3]n, and c) thiol functionalized Cu-BTC-DTG-0.92 (where 0.92 is the S to Cu ratio [169].

the adsorption process. Similarly, polycondensation of alkoxysilane reagents with thiol groups have been investigated without a support platform for Hg adsorption [58]. In another study silica gel was used to support sulfur-capped dendrimers with the thiol as methyl isothiocyanate (MITC) to yield adsorbents with different sulphur percentages [185]. The adsorbent with the highest percentage of sulphur showed the highest adsorption capacity. Other thiol functionalized silicas have been effective in Hg removal [22,186–191] (Table 3). Although thiols are the common sulphur source other compounds such as dithiocarbamates, thiocarbamates, dithiocarbazide and thiourea can also provide a reliable source of sulphur.

by introduction of (3- mercaptopropyl) trimethoxysilane (3-MPTMS) [51]. The resulting adsorbent registered an adsorption capability of 207.7 mg/g, although the amount of thiols or sulphur incorporated was not determined. The authors attributed the good adsorption due to the -SH groups, and thus the theory of hard acid and soft base mentioned previously applied. The adsorption of Hg by the material was characterized by fast kinetics, equilibrium was reached within 15 min, and Hg could effectively be adsorbed over a wide pH range of 2–7. The practical application of the adsorbent was also realized when adsorbent was tested in Hg spiked bottled, tap water matrix and high adsorption capacities of Hg(II) were still reported. Zhu et al. [182] improved on the adsorption capacity of a silica/magnetic/thiol composite (Table 3) by introducing Co. Specifically CoFe2O4 were prepared prior to coating with silica and then functionalizing with 3-MPTMS. Wang et al. [183] have also reported SiO2 coated with Fe3O4 nanoparticles and functionalized with trimethoxysilyl- propanethiol (TMMPS). The adsorption kinetics followed the pseudo-second-order kinetic model and the maximum adsorption capacity was 132 mg/g. The norm in aforementioned reports is for silica to provide a support on which to anchor the thiol. An exception was the work Wang et al. [184] who developed a thiol-rich polyhedral oligomeric silsesquioxane (POSS-SH) framework via hydrolytic condensation of 3-mercaptopropyl trimethoxysilane (MPTS) in HCl and methanol. Fig. 11 shows the adsorbent preparation. The POOSSH were synthesized without a support platform and subsequently applied in Hg adsorption [184]. Adsorption of Hg(II) was effective within pH 2–7 and sorption efficiencies of 93% for 0.5 μg/L were reported. The adsorption isotherm was described well by the Langmuir isotherm which suggested that a monolayer adsorption coverage occurred during

4. Dithiocarbamate functionalized adsorbents Thiocarbamates are basically organosulphur compounds represented by either ROC(=S)NR2 or RSC(=O)NR2. Dithiocarbamates are formed by the replacement of O by S in thiocarbamates. Dithiocarbamate ligands display a variety of coordination patterns such as monodentate, bidentate and great diversity of molecular and supramolecular structures [192]. Dithiocarbamates functionalized adsorbents based on polymers [193], silica [194–197], magnetite [198–200], graphene oxide [201] and starch [202] have been used in Hg(II) uptake and have shown adsorption capacities of 555 mg/g [201], 22.05 mg/g [193], 206 mg/g [198] 181.82 mg/g [200] 172.61 mg/g [194] 163.3 mg/g [195], 113 mg/g [196] 109.5 mg/g [199], 4.83 mg/g [197] for Hg(II) adsorption. Iron oxide (Fe3O4)/silica particles functionalized with dithiocarbamate groups (Fe3O4/SiO2/NH/CS) were applied for Hg(II) removal 11

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Fig. 10. Schematic representation of TAPB-BMTTPA-COF synthesis via the condensation of TAPB (1,3,5- Tris(4-aminophenyl)benzene) and BMTTPA (2,5-Bis(methylthio)terephthalaldehyde) [175].

[198]. Mercury concentrations of 50 μg/L were reduced to acceptable regulatory limits by using only 6 mg/L of the adsorbent. Despite the adsorbent having a lower dithiocarbamate coverage (1.53 × 10−4 mol/ g) compared to a silica gel/dithiocarbamate adsorbent (3.7 × 10−4 mol/g) and a mesoporous silica/dithiocarbamate adsorbent (2.5 × 10–4 mol/g) have been reported by Venkatesan et al, 2007 [203] and Venkatesan et al., 2003 [204], respectively; where the adsorption capacity of the Fe3O4/SiO2/NH/CS particles was higher compared to the former two studies. Furthermore, due to the magnetic properties of the

adsorbent, easy separation of the sorbent from solution is possible. Thus Fe3O4/SiO2/NH/CS particles appeal for industrial applications for Hg (II) removal. Similarly, Tavares et al. [195] prepared silica coated magnetite particles functionalized with dithiocarbamate groups (Fe3O4@SiO2/SiDTC). The adsorbent displayed an adsorption capacity of 163 mg/g and this was credited to the presence of dithiocarbamate moieties. Congruently, Akintola et al. [193] reported Hg adsorption was independent of the surface area and was directly proportional to the dithiocarbamate concentration. Their adsorbent showed an adsorption

Fig. 11. Illustration for the preparation of thiol-rich polyhedral oligomeric silsesquioxane (POSSSH) [184]. 12

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5. Miscellaneous adsorbents There are a few reports on the application of thiazole based adsorbents for the uptake of Hg(II) [205–209]. Thiazole are heterocyclic compounds that contain both the sulphur and nitrogen. Adsorption of Hg(II) by thiazole based adsorbent is characterized by good selectivity and adsorption capabilities due to the synergetic N and S interaction with Hg [206]. The N atoms are also capable of complexation with cationic Hg through the lone pair of electrons. Elhami and Shafizadeh [210] reported the application of a clay-dithizone composite. The adsorbent removed 90% of 50 mg/L of the Hg(II) in 90 min. In addition, the clay-dithizone was tested against real environmental water samples with excellent efficiency. Thiourea has also been used in adsorbent modifications and in most of the reports polymers have been applied as supports [15,211–219]. Monier et al. [212] prepared an ion imprinted polymer using thiourea and cellulose fibers as supporting materials. The ion-imprinted polymer showed higher adsorption capacity (87.5 mg/g) compared to the nonimprinted fibers (45.3 mg/g) because of higher surface area of the imprinted which was 7.66 m2/g compared to 3.87 m2/g for the non-imprinted. Furthermore, the imprinting effect played a role in improved adsorptions. Table 4 summarizes the application of adsorbents form different sulphur sources for adsorption.

6. Adsorption mechanisms The sorption mechanism of any metal ion onto an adsorbent often involve a chemical reaction or ion exchange reaction between the functional groups on the adsorbent and the metal ions. Mechanisms involved in Hg(II) adsorption process are precipitation [224], surface complexation [141], ion exchange [186] and the widely reported complexation/chelation [47,99,138,142,145,171,220] (Fig. 13). Chemical interactions between mercury and sulfur frequently lead to the formation of HgS. HgS is a less toxic form of mercury due to its insolubility and very low volatility [27]. In sulphur functionalized adsorbents, many mechanisms of Hg(II) removal are possible including the reduction of S to give S2−, which makes it easier to combine with Hg2+ to form HgS. For thiol (-SH) functionalized adsorbents, the hydrogens of two adjacent thiols can be replaced by Hg to form S-Hg-S or the Hg(II) unfilled orbital can bond with free SH lone pair of electrons. The binding of Hg by thiol/thio based adsorbents has been probed using various techniques such as Ultraviolet Visible spectroscopy (UV-VIS), Raman spectroscopy, Fourier Transmission Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS).

Fig. 12. Schematic representation synthesized for sythesis of 2-hydro- xyacetophenone-4 N –pyrrolidine thiosemicarbazones (HAPT) Ligand a), absorbance spectra for the colour optimization of the [Hg+2-HAPT]n+ complexes at different Hg(II) ions concentrations b) and visible Hg(II) concentrations in solid and liquid state (inset) [194].

capacity of 22.05 mg/g despite a surface area of 3.93 m2/g. Recently a ligand, 2-hydro- xyacetophenone-4 N –pyrrolidine thiosemicarbazones (HAPT) bearing thiosemicarbazones was synthesized as shown in Fig. 12a, and applied for removal of Hg(II) from aqueous solutions [194]. Basically, adsorption of Hg(II) onto HAPT resulted in formation of a complex, [Hg-HAPT]n+. The absorbance spectra of the [Hg-HAPT]n+ increased linearly with increase in Hg(II) concentrations (Fig. 12b) and showed good Hg(II) adsorption at pH 12.5 maximum (Table 4). Furthermore, as the adsorbent absorbs more Hg(II) the colour changes are notable to the naked eye both in aqueous and solid phases (Fig. 12b. inserts) Table 4 Miscellaneous adsorbent characteristics and performances for Hg(II) removal. Adsorbent

Surface area m2/g

Functionalizing agent

Sulphur % (wt)

[Hg] mg/L

pH

Qmax (mg/g)

Ref.

Cross-linked polydithiocarbamates Ligand anchored conjugate Silica coated magnetite particles Magnetite Polyacrylonitrile-2-aminothiazole

3.93 593 33.9 14.6 27.82 – – 316 – 7.66 – 57.3 – – – –

Thiocarbamates Thiosemicarbazone Dithiocarbamate Thiocarbamate Thiazole Thiazole Thiol/thiadizole Thiourea Thiourea Thiourea Thiourea Thiourea Thiourea Thiazole Thiazole Thiazole

– – 2.27 0.98 12.63 – – – – 15.6 – 13.86 6.14 15.04 3.1 –

20-100 2.65–75 0.05–0.6 50 267-467 5-3000 20-200 200 0.1 -250 10-400 100-900 – 50-1000 100-500 – –

5 12.5 4-8 – 6.5 5 4.6 6 5 5 5 – 5 6.5 4 6

22.05 172.61 163 142-206 454.9 13.46 186.9 122 52.04 110.3 1106.72 – 290.8 @ 30 °C 307.7 204.08 2.24

[193] [194] [195] [198] [205] [206] [209] [211] [15] [212] [213] [214] [220] [221] [222] [223]

Poly-2-mercapto-1,3,4-thiadiazole Mesoporous silica Thiourea chelating fiber Ion-imprinted fibers Chitosan-based granular Thiol resin Magnetic bio-material Poly(2-aminothiazole) Biopolymer/ mercaptobenzothiazole (MBT) 1-(2-thiazolylazo)-2-naphthol (TAN) activated carbon

13

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Fig. 13. Possible adsorption mechanism of Hg(II): complexation by sulfur atoms of Cys-δ-FeOOH (L-Cystine functionalized d-FeOOH nanoparticles [171] a), chelation with IT-PRGO (2-imino-4-thiobiuret − partially reduced graphene oxide [99] b) and precipitation reaction process by FeS c) [224].

groups can also be speculated to interact with Hg(II) through cationic exchange reactions

6.1. Binding analyses for thiol functionalized adsorbents X-ray photoelectron spectroscopy (XPS) is a more popular and insightful technique for Hg-S binding analyses compared to other techniques such as UV-VIS spectroscopy and Raman spectroscopy as it provides information on the state of the mercury species on the adsorbent surface and the mercury sulphur interactions. XPS was employed to assess the interaction of Hg and sulphur [117,119]. The shift of binding energy of S2p in PPTA-PMPSQ-5 from 164.20 eV to 164.56 eV (to a higher binding energy) after adsorption of Hg to the adsorbents revealed that SH group largely contributed to the chelation Hg to the adsorbents [117]. Some studies have also reported the S2p binding energy shift to higher binding energies after Hg adsorption [97,122,225] The peak shifting of the S2p spin orbitals to higher binding energies observed by Das et al. [119], also confirmed the involvement of the thiols in the uptake of Hg(II). Moreover, Das and coworkers were able to show that Hg was adsorbed in the Hg(I) and Hg(II) states and not the Hg(0) state since peaks of the former two Hg species were present and that of the latter Hg(0) species was absent. On the other hand, XPS results of the Hg 4f region of one study showed two peaks at 101.95 and 104.07 eV for 4f7/2 and 4f5/2, which confirmed the existence of mercury in its metallic form. Thus indicating reduction of Hg(II) to Hg(0) at some point during the adsorption process [225]. Conversely, Tran and co-workers reported a downward binding energy shift of the S2p spectrum assigned to the CS bonds from 161.6 eV before Hg adsorption to 152.1 eV after adsorption due to the transference of electrons from S atoms of -CS bonds to the Hg(II) ions [145]. Also this downward shift of the S2p bond has been reported elsewhere after Hg(II) adsorption [122,138,178,181] Arshadi et al. [47] on the other hand used the UV–Vis diffuse reflectance spectra to probe the interaction of Hg(II) ions and the prepared adsorbent, before and after adsorption of Hg(II). The frequency synonymous with the stretching of the -SH group of cysteine methyl ester disappeared after the removal of mercury ions, thus suggesting the chelation of Hg to the sulphur of L-cysteine. As much as adsorption of Hg on sulphur based adsorbents is mainly attributed to the S element, it should be noted that for most adsorbents other groups present on the adsorbent surfaces can also interact with Hg. XPS also revealed the role played by oxygen containing groups (COO, OH) in Hg removal through the attraction of oppositely charged ions (catanionic Hg and anionic) [122]. The study revealed that the binding energy peak of O1s atom shifted to higher energy value from 531.3 to 532.7 eV. Positively charged functional

6.2. Binding analyses for sulphur functionalized adsorbents The interaction of FeS and Hg may involve precipitation, ion-exchange and surface adsorption [78]. According to Sun et al. [78] the XPS narrow scans of Fe, S and Hg further proved the Hg-S interaction. The scan of S after Hg adsorption showed a characteristic peak at 161.9 eV, which is typical of the HgS compound. This observation proved that one possible mechanism of Hg removal by the CTO-M FeS nanoparticles was the precipitation of Hg as HgS. Moreover, the scan of Hg revealed separations of peaks at 103.9 eV represented as HgS, and at binding energy of 99.9 eV which might be represent [Fe(1−x)Hgx]S. Meanwhile Raman spectroscopy was used to identify Hg mechanism of adsorption on sulphurized AC and MWCNTs in another study. The chemisorption mechanism of Hg removal was revealed by the appearance of a strong Hg-S bond on the Raman spectra [59]. The authors further clarified the chemisorption mechanism by the fact that the mercury was difficult to desorb from the surfaces of S-AC and SMWCNTs when HCl was used as the desorbing agent. Desorption only occurred when thiourea was present in the washing agent. 7. Sorption energy The adsorption of Hg(II) onto sulphur/thio etc. functionalized adsorbents can occur through some interactive forces or mechanisms such as precipitation, ion-exchange, Van der Waals forces electrostatic attraction. The magnitude of these forces determines the type of adsorption; specifically, physisorption or chemisorption. In the literature many mathematical models are applied to confirm chemisorption or physisorption attachments. 7.1. Adsorption isotherms Isotherms describe the relation between the quantity of adsorbate on the surfaces of an adsorbent and the equilibrium concentration of the adsorbate in solution at constant temperature. Most reports frequently adopt the Langmuir and Freundlich isotherms for data interpretation; although these isotherms cannot inherently draw conclusion on the nature of adsorption, that is, whether it is chemisorption or physisorption. On the other hand, the Dubinin − Radushkevich 14

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and ratios as an eluent for adsorbed mercury [51,98,167,184]. Other desorbing agents have been applied such as HClO4 [138], HCl [90,92,199], HNO3 [205], KI2 [171], and EDTA [94] for Hg desorption, with at least 90% desorption of Hg(II) in the first three cycles. Furthermore, for the stated studies the adsorption efficiency was still high after three adsorption and desorption cycles. More recently however, two reports have shown a different way to tackle the problem of secondary pollution after adsorption by using the Hg loaded adsorbents as catalysts [119,122]. In the first study the spent adsorbent, polypyrrole mercaptoacetic acid/Hg(II) was used in the catalytic conversion of toxic phenylacetylene to less toxic acetophenone (55% yield) [119]. In the latter report, phenylacetylene was transformed to acetophenone, with a 52% yield using L-cysteine doped polypyrrole/Hg(II) [122].

isotherm which is represented by Eq. 2, can be used to calculate adsorption energy (EDR) (Eq. 4) and thus predict type of adsorption.

Inqe = Inq′ − KDRε 2

ε = RTIn (1 +

(2)

1 ) Ce

EDR = (−2KDR)

(3)

−1/2

(4) 2

2

Where KDR (mol /KJ ) is the constant related to the adsorption energy, R is the gas constant, 8.314 J/ (mol K) and T is the temperature (K). The plot of ln qe versus ε2 gives a straight line with the slope KDR and the intercept ln qˋ. When the EDR value is in the range of 8–16 KJ/ mol, Hg adsorption is chemisorption [139] and EDR values of less than 8 KJ/mol indicate physisorption [95].

9. Challenges in use of sulphur/thio functionalized materials for Hg(II) removal

7.2. Adsorption kinetics In some studies pseudo first order and pseudo second kinetics are reported as indicative of physisorption, chemisorption [47,64,82,145,170,226,227] or a combination of both physical and chemical sorption [78]. However, in most these reports statistical data to support this assumption is lacking. Hence some reports calculate the activation energy (Ea) using the Arrhenius (represented by Eq. 5) to draw a comprehensive conclusion.

Ink2 = InA −

Ea RT

Various factors may limit the practical applications of sulphur functionalized materials in the adsorptive treatment of Hg(II) contaminated water. For example, the strong Hg and S interaction is advantageous in the uptake of Hg from multi metal elemental and/or organic contaminated water because of the higher selectivity factor of S for Hg. The challenge is then encountered in adsorbent regeneration whereby Hg is desorbed out of the spent adsorbent using strong desorbing reagents e.g. highly concentrated acids. These strong desorbing reagents may destroy the structural framework of the adsorbent or the chelating agents [228] in the adsorbents possibly reducing adsorbent effectiveness when reused. To address this practical limitation, mildly concentrated acids in conjunction with thiourea have been used as desorbing agents. The sulphur from thiourea readily bonds with mercury, withdrawing mercury from the adsorbent surfaces and cavities without adsorbent structural interferences [59]. Another problem in the use of thiol functionalized materials is that thiols are generally expensive, for example bulk buying of 100% purity cysteine costs approximately US $10-30 per kilogram and 99% purity 5Mercapto-1H-tetrazole-1-acetic acid cost in the range of US $10-50 per kilogram. Therefore, upscaling adsorbent synthesis can incur huge industrial costs associated with adsorbent production. In one study, operational costs (energy etc.) were taken into consideration in addition to materials costs when calculating the costs of removing Hg(II) in aqueous solutions [95]. An amount of US$32,000 was needed to remove 1 kg Hg(II) using thiol functionalized magnetized GO (MGO-NHSH) and US$40,000 when using thiol functionalized magnetic SiO2 (Fe3O4@SiO-NH-SH) as adsorbents respectively. Finally, some disulphides and thiols (aromatic and amino) have been shown to be toxic to mammals [229] and aquatic organisms [180]. To overcome this challenge low thiol density functionalization of a support material have been reported to produce an efficient and safe adsorbent for Hg(II) uptake under environmental conditions [180]. Therefore, in addition to careful selection of thiols for application in materials functionalization to avoid toxic thiols, the cost of the supporting/starting material has to be considered. Some low-cost materials have been functionalized with thiols, reducing the overall cost of the adsorbent and moving forward the practical applications of thiol functionalized materials in Hg(II) uptake.

(5)

Where K2 (g/(mg min)) represents the adsorption rate constant (at temperature T (K), A is the frequency factor g/(mg min), and R is the gas constant. An Ea value of 5–40 kJ/mol suggests chemisorption dominance [66,94,139,220] while Ea < 5 kJ/mol shows the likelihood of physisorption. 7.3. Adsorption thermodynamics Finally some reports have used Gibbs free energy (ΔG) to confirm the adsorption type [90,95]. The Gibbs free energy of adsorption is determined by the following equations:

InK d = −

ΔG =

ΔH o ΔS o + RT R

ΔH o

−

TΔS o

(6) (7)

where Kd is the distribution coefficient, R is the gas constant and T is the absolute temperature (K). A linear plot is obtained by plotting In Kd versus 1/T; where the slope and intercept are the enthalpy (ΔH) and entropy (ΔS) changes are respectively. The change in free energy for physisorption is often between -20 and 0 kJ/mol, while that for chemisorption is usually between -80 to -400 kJ/mol, whereas the physisorption and chemisorption occur together within the range of -20 to -80 kJ/mol [90]. The mechanism of adsorption is a complex process, limiting the choice of accurate isotherm, thermodynamic and kinetic models. However, the procedure of curve fitting based on statistical assessment has been a feasible option regardless of the fact that curve fitting criterion may not reflect the accurate mechanism of adsorption. Thus, while interpreting adsorption data, it is best to pursue the appropriate models through rigorous theoretical characteristics along with goodness of curve fitting.

10. Future perspectives It has been shown without doubt that sulphur is very important for the removal of the Hg(II) as reviewed in this article. The challenge then becomes the selection of the support materials. Over the past decade, there has been an increase in research on the application of low-cost adsorbents especially natural occurring materials like cellulose and chitosan for the removal of Hg(II). Moreover, these materials are nontoxic and biodegradable, have an abundance of functional groups on which sulphur ligand/material/chemicals can be anchored. However

8. Regeneration and reusability One of the crucial factors of adsorbents is their ability to be reused as this aspect is cost related and minimizes secondary pollution. The strong Hg-S interaction often makes it a challenge to remove adsorbed Hg on sulphur functionalized adsorbents. In this regard most reports have studied a mixture of HCl and thiourea in various concentrations 15

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low-cost materials (e.g. biopolymers) are not efficient in the adsorption of low concentrated Hg ions, show poor selectivity or specificity for the Hg ion and often disintegrate in very low pH environments (e.g. chitosan). However, with appropriate modifications of biopolymers (since these contain an abundance of functional groups) other materials can be used as reinforcements. On the other hand, adsorbents such as carbonaceous, synthetic polymers, that indicate robustness effective attachments of the sulphur ligand and high Hg(II) adsorption capacities, may be systematically used with low-cost materials to produce adsorbents suited for Hg(II) abatement. The use of nanomaterials as an option for Hg(II) adsorption may be expensive in terms of cost of production since is still in its infancy. For example, carbon nanotubes have been known to be expensive and produce poor yield production even at laboratory scale. However, nanomaterials have shown effectiveness in removal of low concentrated Hg(II) because unlike most heavy metals, Hg(II) is still toxic at low concentrations and lowly concentrated Hg(II) is often difficult to remove in aqueous solutions. Furthermore, interactions of nanomaterials in the mammalian body at cellular level and their potential toxicity has been raised [230]. Hence more studies need to be conducted to assess the risk and danger that these materials may cause to human health. However, regarding nanomaterials and thiol toxicity, magnetizing of adsorbents and perhaps in combination with membrane technology can be applied to ensure complete removal of adsorbent. Separation of polymers from aqueous solution by filtration may not be effective or efficient. Finally, for the sulphur functionalized adsorbents to be worthwhile in industrial commercialization, more work is needed in: 1) testing sulphur based adsorbents on reuse after loading with Hg(II), 2) testing on real wastewater containing a variety of contaminates, both inorganic and organic and 3) laboratory scale columns testing and pilot plant testing.

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11. Conclusions Despite all challenges with support material and adsorption of Hg (II), in general sulphur functionalized materials show an overall far better performance for Hg(II) uptake compared to pristine or non-sulphur functionalized materials. Therefore, certain factors have to be considered like large upscale adsorbent production costs, adsorbent commercial viability, effectiveness and selectivity of adsorbent for Hg removal from contaminated water, rapid adsorption kinetics, high adsorption capacity and finally reliability with regards to non-leaching of toxic material and complete separation of adsorbent from aqueous solutions. Since some thiols are costly use of low-cost support materials can lower adsorbent production cost and make sulphur based adsorbents a viable option for treatment of Hg(II) contaminated wastewater. Declaration of Competing Interest We hereby declare that we have no conflict of interest. Acknowledgment The authors would like to sincerely thank the Faculty of Science at the University of Johannesburg for the financial support. References [1] A.R. Kadam, G.B. Nair, S.J. Dhoble, Insights into the extraction of mercury from fluorescent lamps: a review, J. Environ. Chem. Eng. (2019), https://doi.org/10. 1016/j.jece.2019.103279. [2] C.-C. Lin, N. Yee, T. Barkay, Microbial trnasformations in the mercury cycle, Environ. Chem. Toxicol. Mercur. (2012) 155–192, https://doi.org/10.1002/ 9781118146644.ch5. [3] R. Awual, Novel nanocomposite materials for efficient and selective mercury ions capturing from wastewater, Chem. Eng. J. 307 (2017) 456–465, https://doi.org/

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