2D and 3D carbon-based adsorbents for an efficient removal of HgII ions: A review
Graphical abstract
Graphical representation of mercury removal using carbon-based adsorbents.
Introduction
Man’s direct intervention in natural processes has resulted in an imbalance in ecosystems. Reckless use of natural resources for urbanization is polluting the environment to a far greater extent than before. After the 1800s, the industrialization has changed the face of human lifestyle from manual to the machine at the expense of deterioration in living quality. Irrational growth for human comfort is continuously degrading water and air standards.
Groundwater and water in freshwater bodies make up roughly 0.7% of total water to be accessible for human consumption. All living beings require ions for a sound metabolism and water is the best source. Presence of Na, K is highly desirable, but metal ions like Hg, Pb, Co, As, Cd, etc. above their permissible limits make the water unsuitable for common use. In the last few decades, a rapid decrease in water quality has been recorded due to the direct disposal of industrial effluents in rivers and lakes [1]. Heavy metal contaminants exist in waste streams of many industries, such as tanneries, mining, metal plating, dye and paints and petro-based industries. Even the soil surrounding military bases and these industries are contaminated and pose a serious threat to groundwater and surface water [2]. The presence of toxic metals in drinking water even at a very low level can be lethal to human beings [3]. Since biological systems are not that much capable of eliminating heavy metals, these can accumulate in living bodies and can amplify concentration through food chain which can cause various ailments and disorders [4]. The presence of heavy metals in natural or industrial wastewater and their potential impact on living species has been a subject of research in environmental science for a long time.
Inorganic Mercury (Hg) in the form of salts are used in dentistry [5], cosmetics [6], disinfectants, photography, dyes and pigments, and electrical equipment [7]. Organic forms of mercury like methyl mercury [CH3.Hg]+ and ethyl mercury [(CH3)2Hg] are formed by the methylation of Hg2+ by microorganisms and bacteria in Hg-contaminated water bodies [8], [9]. Long-term exposure to large amounts of Hg has detrimental effects on the kidneys, gastrointestinal tract, central nervous system, and heart [10], [11]. A severe case of methylmercury poisoning was recorded in Minamata City, Japan where intake of mercury-contaminated fish and shellfish resulted in numerous deaths [12]. Presence of methylmercury in pregnant women can cause severe damage to the nervous system of unborn babies [11].
In recent years, alternatives to mercury usage including composites and ceramics in place of dental amalgam [13], diaphragm and membrane technologies in chlor-alkali plants [14], etc. have been adopted but are still in their initial stages of development. According to the Environmental Protection Agency guidelines, the maximum contaminant level of mercury in drinking water has been set to 2 µg L−1 [15]. For maintaining the quality of water, it is highly required to explore efficient and affordable methods for the treatment of mercury-contaminated water.
Many different techniques like chemical precipitation, adsorption, coagulation, and filtration have been tested for the removal of mercury from wastewater [16]. Out of these techniques, adsorption is the most widely accepted technique as it is simple and economic [17]. A wide variety of solid-phase adsorbents including carbon-based adsorbents, iron oxide [18], composites [19], modified silica gels [20], polymers [21], biosorbents [22], [23], zeolites [24], clays [25], etc. have been studied for HgII adsorption. Though pristine form of a carbon-based adsorbent has a good mercury removal capacity but the performance can be enhanced by modifying the adsorbent by acid/base treatment [83], sulphur doping [54], [113], forming composite [26], [27], [39], [40], or incorporating binding ligands like PAMAM [47], MBT [28], EDA [29], 3-aminopyrazole [32], [63], EDTA [38], xanthate [50], cysteine [28], phytic acid [103], polyethenimine [114], etc. on the adsorbent’s surface. Furthermore, the incorporation of magnetic nanoparticles of Fe3O4 [50], [57], [60], Fe2CuO4 [27], and CoFe2O4 [56] with carbon-based adsorbents as composites makes the recovery of Hg loaded adsorbents much easier.
In this review, efforts have been made to compile the results that have been published dealing with the mercury removal using carbon-based adsorbents. This review presents valuable information about schemes employed for modification of pristine carbon-based adsorbents. The thermodynamics, kinetics, and isotherm of HgII adsorption onto carbon-based adsorbents along with adsorption mechanisms have been discussed in detail. Efforts have been made to validate the fact that carbon-based adsorbents are novel adsorbents for the removal of HgII from industrial wastewater.
Section snippets
Physical properties of carbon-based adsorbents
Adsorption is a surface phenomenon which is strongly governed by the availability of active sites on the surface. An adsorbent with a very large SSA favours the adsorption process as it provides a comparatively large contact area for an adsorbate to interact with the adsorbent [85]. The SSA of an adsorbent can be measured using N2 adsorption-desorption isotherm at 77.3 K with Brunauer–Emmett–Teller equation [86]. It is evident that the SSA for graphene-based adsorbents has shown a broad range
Synthesis and modification of carbon-based adsorbents
Carbon-based adsorbents, i.e., graphene-based and CNT-based adsorbents both in pristine and modified forms have been used for the adsorption and removal of HgII from aqueous solution. The grafting of functional groups containing N, S, and O as donor atoms onto the surface of carbon-based adsorbents is known to increase the HgII adsorption capacity of adsorbents by many folds.
Graphene-based adsorbents
In the published work reviewed here, raw and modified forms of GO and rGO have shown superior adsorption performance than any other carbon-based adsorbent which may be due to the strong involvement of both physical and chemical forces governing the adsorption process. Various graphene-based adsorbents: Au/Fe3O4/MoS2CAs (1527.0 mg g−1) [109], and GN-DE-MPTMS (881.0 mg g−1) [43]; rGO-based adsorbents: PANI/RGO (1000.0 mg g−1) [26], Fe2CuO4/rGO (1250.0 mg g−1) [27], IT-PRGO (624.0 mg g−1) [33],
Effect of pH
The pH of a solution plays a significant role in the adsorption process as it strongly governs the speciation of metal ions in the solution and the surface polarity of an adsorbent. The characteristic acid-base property of functionalities on the surface of an adsorbent is entirely dependent on the pH and thus governs the affinity of these functional groups for metal ions. The pH-dependent study provides valuable information on the adsorption-desorption profile of an adsorbent. In many of the
Adsorption isotherms
At a particular temperature (T), for any sorption system at an equilibrium, the distribution of metal ions in the bulk and at the solid-liquid interface is well defined. The adsorption isotherms relate the metal uptake per unit mass of adsorbent (qe) to the adsorbate concentration at equilibrium (Ce) in the bulk liquid phase i.e., . Due to the structural and energetic heterogeneity of the adsorbent surfaces, the analytical forms of adsorption isotherms are complex. But after taking some
Adsorption kinetics
Sorption kinetics is controlled by various mechanisms of chemical reactions, mass transfer, and particle diffusion. For the interpretation of adsorption kinetics, several mathematical models have been proposed which can be broadly categorized under adsorption reaction models and adsorption diffusion models. The adsorption reaction models describe the chemical reaction kinetics without considering diffusion pathway. On the contrary, the development of adsorption diffusion models is based on the
Thermodynamic studies
For HgII ions to get sorb onto the surface of a carbon-based adsorbent, energy is required to strip the water molecules present in the hydration sphere of HgII ions (fully or partially). Also, when these fully or partially dehydrated HgII ion gets sorbed onto the surface of a carbon-based adsorbent, a sufficient amount of energy is released. To understand these inherent energetic changes involved in the sorption process, it is necessary to evaluate the thermodynamic parameters i.e., ΔG°, ΔH°,
Regeneration and reusability
An efficient regeneration of an adsorbent is highly desirable for its application on an industrial scale. The process of removing adsorbed Hg from the loaded adsorbent can be achieved simply by treating it with stripping agents in the aqueous phase. Most often pH-dependent adsorption profile of HgII onto carbon-based adsorbents have shown a negligible adsorption capacity at lower pH which was found to rise with the increasing solution pH [30], [70]. These pH-dependent profiles indicate that the
Adsorption mechanisms
According to the Pearson’s hard and soft acids and bases (HSAB) theory, soft acids react faster and form stronger bonds with soft bases, whereas hard acids react faster and form stronger bonds with hard bases. Being a soft acid, Hg2+ should interact strongly with soft bases like R2S, RS−, C6H6, I−, ArNH2, etc. and would result in higher removal efficacies. In most of the studies reviewed here, ligands containing N and S atoms formed strong complexes with HgII which was found in accordance with
FT-IR analysis
The functional groups of an adsorbent can be identified by the Fourier-transform infrared spectroscopy (FT-IR). A functional group is marked by a distinct IR frequency range (fingerprint frequency range) which can get altered due to metal binding [102]. A shift in vmax (of a functional group), broadness in bands, and a decrease in spectral intensities is expected during the adsorption of a metal ion onto an adsorbent. For MBT-GO, large shifts in CN (1569 cm−1 → 1575 cm−1), CS (1217 cm−1
Conclusion
This review highlighted the removal of HgII from aqueous solution using carbon-based adsorbents. Carbon-based adsorbents, i.e., CNTs, and graphene in their pristine, acid modified and ligand functionalized forms have been employed for the removal of mercury. The mutated forms of carbon-based adsorbents showed extraordinary adsorption capacity for HgII compared to that of pristine forms making adsorbents more economic. More often adsorption processes fitted well to the Langmuir isotherm and the
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Conflicts of interest: The authors declare no conflicts of interest.