Abstract
Understanding the sources and controlling processes of various groundwater contaminants and their removal methods is extremely important, as groundwater contamination is intricately linked to human health. Chromium (Cr) is a common groundwater contaminant with both natural and anthropogenic origins. Dissolved Cr exists in hexavalent and trivalent forms – while the former is carcinogenic and more soluble – the latter is a micronutrient at low levels and is less soluble. Therefore, most chromium removal methods rely on reducing the hexavalent chromium to its trivalent state to decrease the Cr-toxicity. In recent years, several experimental methods have been attempted for hexavalent chromium removal from aqueous media/groundwater. This paper reviews the recent findings on Cr removal by important, effective, and widely used methods such as adsorption by nanoscale zero-valent Fe-based and conventional materials, electrocoagulation (EC), and bioremediation. The reaction pathways, mechanisms, and effectiveness of each method are also highlighted. The role of parameters such as solution pH and temperature, initial Cr(VI) concentration, contact time with the reducing agent, adsorbent dose, and the presence of competing ions on Cr removal was evaluated. Many of the methods exhibit high (>90%) Cr removal efficiency; the main challenge would be to apply these methods for large-scale water treatment.
HIGHLIGHTS
Adsorption, electrocoagulation, and biological methods for Cr removal are reviewed in this study.
Stabilizer-based nanoscale zero-valent iron has proven to be an emerging technique for chromium mitigation.
pH of the aqueous medium seems to be the most influential parameter in determining Cr removal.
Future research should focus more on scaling-up the existing methods.
Abbreviations
INTRODUCTION
Disproportionate industrial and population growth in most countries of the world has undoubtedly led to the pollution of surface water and groundwater resources through the accumulation of various contaminants in different chemical forms. As a significant population consumes untreated groundwater in developing and underdeveloped countries, groundwater contamination poses a serious and direct threat to its consumers. Chromium 
has a crustal abundance of 100 ppm (Barbalace 2007) and is a carcinogenic groundwater contaminant. It is known to have geogenic (e.g., from chemical weathering of Cr-bearing minerals) and anthropogenic (e.g., electroplating, dyes, leather industries) origins. Likewise, for other groundwater contaminants, the presence of dissolved Cr in groundwater (and, in general, aqueous media) beyond the safe limits, presents a challenge to stakeholders involved with groundwater resource management. Therefore, it motivates researchers to find novel, cost-effective, and benign methods of mitigation. Chromium removal methods thus require a comprehensive understanding of the aqueous chemistry of Cr.
. In contrast, Cr(III) occurs mainly as hydroxide complexes (Cr(OH)2+, Cr(OH)2+, Cr(OH)3, and Cr(OH)4−) and is stable in acidic and low Eh conditions (Figure 1). Because of its low solubility, Cr(III) has low mobility in water. Compared with Cr(VI) and Cr(III), divalent Cr exists in exceptionally low pH and very low Eh (−400 to −800 mV; Figure 1) conditions. Cr(VI) is thus the predominant species at Eh–pH values generally observed in groundwater. 
In terms of toxicity, Cr(VI) is carcinogenic, whereas Cr(III) is a micronutrient at low levels. Therefore, if the groundwater is contaminated with Cr, the predominant, stable, and toxic form (i.e., Cr(VI)) primarily enters into the human food chain through groundwater ingestion (Saha et al. 2011; Hausladen et al. 2018). Therefore, from the standpoint of human health, it is important to remove Cr(VI) from groundwater. Many of the Cr removal approaches (from aqueous media), therefore, rely on converting the Cr(VI) into Cr(III), as the latter has very low solubility and gets easily removed from the aqueous phase.
Cr(VI) in groundwater originates both by natural and anthropogenic routes. If groundwater is contaminated by natural pathways (e.g., from oxidative weathering of the aquifer Cr-containing minerals), then Cr has to be removed from the entirely contaminated groundwater. Anthropogenic contamination usually occurs by seepage of industrially discharged (contaminated) waters through the soil layers into the groundwater. Therefore, to reduce the extent and volume of groundwater contaminated by the anthropogenic pathway, it is required to treat industrial waters and effluents before it contributes to Cr in groundwater. Moreover, to comply with environmental regulations, effluents containing hexavalent chromium must be treated before being discharged into the environment (Hamdan & El-Naas 2014).
In the last decade, researchers have experimented with multiple technologies and methods to remove Cr(VI) from groundwater. Chemical removal methods mainly comprise inorganic and organic methods. While organic methods primarily deal with microbial remediation, inorganic methods employ various approaches. These inorganic methods include adsorption by novel and conventional materials, coagulation, ion exchange, membrane filtration, electrochemical precipitation, and catalytic reduction (Farooqi et al. 2021). Among the adsorptive removal methods, Rajapaksha et al. (2022) reviewed different adsorbents like activated carbon (AC), zeolite, polymeric substances, and modified magnetite. The overall sorption mechanisms of the above adsorbents are believed to proceed through pore diffusion, Cr(VI) reduction to Cr(III), precipitation, and ion exchange. Based on statistical analyses of the reviewed articles on the above adsorbents, Rajapaksha et al. (2022) concluded that nanomaterials and mineral-doped biochar were the most efficient remediation methods, and low pH (approximately 4) conditions were better suited for adsorption than at high pH (6–8) values. Contrary to the conventional expectations, they found that adsorbents with a lower surface area have the highest adsorption capacity, implying that the chemical properties of adsorbents influence the adsorption more than their specific surface area.
Over the last two decades, different review articles have been published on chromium removal from groundwater. Mulligan et al. (2001) summarized different remediation approaches such as isolation and containment, mechanical and pyrometallurgical separation, chemical treatment, permeable treatment walls, electrokinetics, biochemical processes, phytoremediation, soil flushing and washing, and treatments of sediments. Sharma et al. (2008) summarized Cr(VI) removal methods such as adsorption, coagulation, ion exchange, electrodialysis, and biological removal. Since then, in the last decade, many studies were carried out in every domain. Additionally, novel methods such as bioelectrochemical (BES) systems have been used for groundwater bioremediation. Cecconet et al. (2018) published a review paper on this particular approach. In BES methods, Cr(VI) is reduced to Cr(III) either at the abiotic cathode or the biocathode. Tolkou et al. (2020) reported applications of novel nanoparticles such as NiO, graphene oxide functionalized with magnetic cyclodextrin–chitosan, polypyrrole–graphene oxide nanocomposite (PPy–GO NC), and magnetic graphene/Fe3O4 composite. Among these, maximum sorption capacity was observed for the PPy–GO nanocomposite; however, the higher removal efficiency of these adsorbents between pH 2 and 5 restricts their use to acidic waters, rather than groundwater, wherein pH remains within 6.5–8.5. Almeida et al. (2019) reviewed nanomaterials-based removal methods published from 2007 to 2017. In the studies reported by Almeida et al. (2019), experiments were carried out under varying conditions of temperature, contact time, pH, amount of sorbent, and the initial chromium concentration. It was found that removal capacity increased with higher temperature but decreased with pH. An important suggestion by these authors was to conduct studies in the presence of other competing trace elements.
Compared with the traditional physical and chemical methods, bioremediation approaches provide suitable alternatives. Recently, these approaches were summarized for their feasibility toward large-scale applications (Fernández et al. 2018). A variety of algae (e.g., Spirulina), bacteria (e.g., Escherichia coli, Bacillus), yeast (e.g., Pichia), and fungi (e.g., Aspergillus) could be used for bioremediation of Cr(VI). Bioremediation occurs through the mechanisms of biosorption, bioaccumulation, and biotransformation. Among these, biosorption showed promising success toward large-scale Cr(VI) detoxification.
Review articles covering both organic and inorganic methods published during the last one to two decades are, at best, sparse. This paper presents a short review of the works primarily published in the last decade. With the vast amount of literature on each of the methods covered here, this review may not necessarily be exhaustive; however, it provides a brief account of the methods such as nanoscale zero-valent Fe-based adsorption, electrocoagulation (EC), adsorption by conventional materials, and bioremediation. The mechanisms and effectiveness of these methods are highlighted. Towards the end, a brief account of the attenuation of Cr(VI) that occurs naturally is also presented.
SOURCES OF CHROMIUM
Developing an appropriate mitigation strategy for groundwater contaminants requires knowledge of its primary sources. Pertinent to this review, aqueous Cr is naturally derived from the weathering of major Cr-containing minerals. Major chromium-bearing minerals are chromite (FeCr2O4), eskolaite (Cr2O3), chromatite (CaCrO4), crocoite (PbCrO4), magnesio-chromite (MgCr2O4), and uvarovite (Ca3Cr2(SiO4)3) (Liu et al. 2017). These minerals are generally found in metaquartzites, serpentinites, and ultramafic rocks. In nature, weathering processes occur wherein different physical agents such as wind, precipitation, river, and glacier break down fine particles from chromium-bearing rocks through physical and chemical weathering (Robles-Camacho & Armienta 2000; Fantoni et al. 2002; Ball & Izbicki 2004; Oze et al. 2007). After weathering, various agents carry and deposit the weathered materials on the soil. The contaminants can then infiltrate into the underground water by leaching.
One primary source of chromium in terms of anthropogenic origins is the chrome-mining industry (Das et al. 2021). During the mining process, overburdened materials are deposited alongside the mine. Through wastewater and rainfall, the water percolates via the overburden to the soil and finally into the groundwater. With this process, several heavy metals also come leaching down and mixing with the groundwater. For example, the Sukinda mine in Odisha (India) is the biggest chromite mine in the country, where large-scale chromium contamination has been reported (Mishra & Sahu 2013; Nayak & Kale 2020; Das et al. 2021).
Apart from mining, in the leather industry, chromium sulfate is used as a tanning material to make the leather sturdy and long-lasting (Mia et al. 2020). Wastewater dumping by these tanneries contaminates the surface water, soil, and groundwater (Kankaria et al. 2011). In India, the states of Uttar Pradesh, West Bengal, and Tamil Nadu have several tanneries where the groundwater environment is prone to chromium contamination (Mandal et al. 2011; Sharma et al. 2012; Nithya & Sudha 2017). In the electroplating industry, different heavy metals are used. For stripping purposes, nitric, hydrochloric, and sulfuric acids are used, while for the cleaning process, chromic acid is used. Due to these processes, acids, along with heavy metals, get released into wastewater (Kumar & Thatheyus 2013), thus making the soil and water contaminated. Similarly, the battery and dye industries use chromium compounds for different purposes, and the effluents from these industries may also pollute the groundwater (Mishra & Bharagava 2016; Bhattacharya et al. 2019; Karunanidhi et al. 2021). Environmental regulations are often flouted in developing and underdeveloped countries; therefore, ineffective treatment of industrial discharges is a pathway for groundwater contamination.
ADVERSE HEALTH EFFECTS OF CHROMIUM
Cr(VI) is carcinogenic, whereas Cr(III) is considered a micronutrient (Mishra & Bharagava 2016). According to the World Health Organization (WHO), the permissible limit for total chromium in drinking water is 50 μg/L. Cr(VI) could enter the human body through oral, dermal, or inhalation pathways and reach the lungs, liver, kidney, spleen, brain, aorta, and lymph nodes. Dermal contamination may result in skin inflammation, dermatitis, eczema, etc., whereas inhalation of Cr(VI) could cause diseases like pharyngitis, bronchitis, asthma, and pneumonitis. Cr(VI) binds with hemoglobin on entering the body and forms a very stable chromium-hemoglobin complex. Cr(VI) induces cytotoxicity and carcinogenicity through the oxidative deterioration of biologically important molecules (Alvarez et al. 2021). Thus, Cr(VI) could pose concerns for humans and animals by forming components that could set off different physical conditions, such as several types of cancer, anemia, nervous impairment, and blood circulation failure (Saha et al. 2011; Qu et al. 2017).
CHROMIUM REMOVAL METHODS
In the earlier sections, we discussed the aqueous geochemistry of Cr, the major sources, the processes by which Cr(VI) is accrued in groundwater, and the detrimental effects of consumption of such Cr-contaminated groundwater. There is a need to reduce Cr levels in groundwater before its use for drinking. These various removal methods are developed in the laboratories, some of which are further scaled up to treat large volumes of contaminated water. We discuss some of these methods and their important findings in the following sections.
Adsorption by nanoscale zero-valent iron
The nanoparticles are composed of Fe(0) core, while the shell has the Fe-oxide layers of Fe(II) and Fe(III). In aqueous solutions, ZVI nanoparticles react with water and oxygen to form an outer layer of iron hydroxide, which provides sites for the formation of chemical complexes'. The core acts as an electron donor and the outer layer of iron hydroxide acts as an effective adsorbent of heavy metals. Heavy metals (here chromium) are first reduced and then adsorbed onto the structure of nZVI (Tarekegn et al. 2021). However, because of nanoparticles' high specific surface area and ultra-high surface energy, and also due to the van der Waals force and magnetic characteristics, nZVI particles frequently coalesce, which limits their efficiency in decontamination (Xie & Cwiertny 2010; Noubactep et al. 2012; Li et al. 2019). Furthermore, oxidation occurs when nZVI is not in contact with the target pollutant, resulting in nZVI deactivation (Li et al. 2019). Researchers, therefore, have utilized several stabilizers to enhance the electrostatic friction between particles, decreasing agglomeration and increasing surface transport (Yu et al. 2020). Some of the stabilizers are discussed below:
- (i)Activated carbon filter supported nZVI (ACF-nZVI): According to Qu et al. (2017), using an ACF is a good way to stabilize the nZVI for the removal of Cr(VI). After the procurement of commercially available ACF, it was purified by soaking with 5% HCl for 24 h, and washed with de-ionized water until the supernatant became pH approximately 7, followed by drying at 378 K for 24 h. For this experiment, nZVI particles were produced from the reduction of hydrous ferric chloride (FeCl3·6H2O) using NaBH4 as a reductant. The reduction equation is as follows (Zhang et al. 2014; Qu et al. 2017):
It was observed that Cr(III) produced by reducing Cr(VI) by ACF-nZVI disappeared from the solution within 15 min of the reaction. It was proposed that a significant fraction of Cr(III) was captured by the ACF surface, while that chances of Cr(III) deposition on the ACF-nZVI were significantly reduced. The above allows microscopic galvanic couples to form between nZVI and ACF (Zhou et al. 2013), where electrons flow from the nZVI anode to the ACF cathode (Qu et al. 2017). It was confirmed that nZVI acted as the reductant, while ACF only facilitates e-transfer and provides adsorption support.
- (ii)
Sodium alginate dispersed nZVI (SA-nZVI): Heavy metals and hazardous chemicals are widely removed from groundwater using sodium alginate (SA) (Huang & Wang 2018; Li et al. 2019). SA's molecular chain has many carboxyl groups that can adsorb metal ions (Cho et al. 2018). Furthermore, SA is an excellent dispersant utilized to increase the encapsulation of biological enzymes in nanomaterials (Yu et al. 2014). Li et al. (2019) compared the effect of initial pH on Cr(VI) removal by critical micellar concentration (CMC)-nZVI and SA-nZVI. After 20 min of reaction, the removal by CMC-nZVI was 75.6% at pH 6.0 and 60.4% at pH 9.0, while the corresponding values were 96.4 and 85.7% for SA-Alginate. These results suggest a higher removal efficiency for SA-nZVI compared to CMC-nZVI. Under alkaline conditions, the removal efficiency decreases; however, it is still high (85.7%), making it an effective remediating agent for groundwater. With the increase in pH (e.g., 4.5–8.5), reduced efficiency was also reported for CMC-nZVI (97.5–45.9%; Li et al. 2019). It is believed that under alkaline conditions, the passivation of Fe0 hinders electron transfer from the highly active Fe(0) core, which results in a decreased removal rate (Mondal et al. 2004). In addition, Li et al. (2019) also investigated the effect of NO3− as a competing ion, as it is a major component in groundwater. At solution pH 8.5 and after 11 min of reaction time, they observed that Cr(VI) removal efficiency decreased to 81% (from 86%) in the presence of 5 mg/L of NO3−.
- (iii)
Sodium carboxymethyl cellulose nZVI: CMC stabilizes nZVI and increases the removal efficiency of Cr(VI) by reducing surface agglomeration while improving mobility (Yu et al. 2020). A series of batch and column experiments on Cr(VI) removal showed that CMC-nZVI had greater reducing power than nZVI. This study reported a maximum chromium removal of 88.5% at pH 6 and 200 mg/L of CMC-nZVI with 5 mM ionic strength. With increasing CMC concentration, the removal efficiency first increased from 0 to 200 mg/L but started decreasing as CMC increased further to 1,000 mg/L. The above was explained by the excess coating of nZVI by the CMC molecules, which hinders the interaction between Cr(VI) and nZVI. Cr(VI) removal decreased when the solution pH increased from 4 to 10. Under acidic conditions, Fe(0) corrosion is augmented, and thus, a higher surface activity is developed, which ultimately increases the Cr(VI) removal. Under the alkaline condition, precipitation of Fe(OH)2 leads to the passivation of CMC-nZVI surfaces, thus decreasing the removal efficiency.
- (iv)
Sepiolite-nZVI: Fu et al. (2015) used the sepiolite clay mineral because of its surface properties. Sepiolite-nZVI was used in batch experiments to reduce Cr(VI). With increasing pH from 4 to 9, the Cr(VI) removal efficiency was found to drop from 98.5 to 30.7%. This suggests that acidic conditions strongly preferred reduction relative to alkaline conditions. At higher pH, the sepiolite-nZVI surface charge becomes negative, which then causes electrostatic repulsion between the nZVI surface and Cr(VI), which are present as oxyanions. This further decreases the Cr(VI) adsorption rate (O'carroll et al. 2013). The Cr(VI) removal also depends on the dosage of the sepiolite-nZVI absorbent. When the sepiolite-nZVI concentration was increased from 0.05 to 3.2 g/L, Cr(VI) removal efficiency increased from 45.1 to 99.2%. The optimum concentration of sepiolite-nZVI for chromium removal was 1.6 g/L. The removal mechanism was proposed to proceed by a two-step process that included sorption of Cr(VI) onto the surface layers or inner layers of sepiolite-supported nZVI, followed by its reduction to Cr(III). It was also found by Fu et al. (2015) that Cr(VI) removal was largely unaffected by the presence of competing ions such as phosphate, silicate, calcium, and bicarbonate. Regarding kinetics, the removal followed a pseudo-first-order rate law with both Langmuir and Freundlich isotherm models (Fu et al. 2015).
- (v)nZVI-Green tea (GT): Zhu et al.(2018) synthesized composites of GT extract with iron (Fe-II) solution (GT-nZVI) and with mixed Fe(II) and Cu(II) solutions (GT-nZVI/Cu). GT-nZVI/Cu exhibited a higher removal efficiency (94.7%) than GT-nZVI (73.2%), which was attributed to the addition of Cu(II). The presence of Cu significantly promoted the formation of Cu–Fe galvanic couples, further enhancing the generation and transfer of electrons. Furthermore, it was also observed that acidic conditions favor reduction. In acidic conditions, the high H+ concentration shifts the balance between GT-nZVI/Cu and Cr (Ⅵ) toward the reduction of Cr(VI) following the reactions below:
The acidic environment hinders any accumulation on the surface of GT-nZVI/Cu, which forms more reactive sites and increases the Cr(VI) reduction process (Yang et al. 2014). Higher temperature also enhances the Cr(VI) removal efficiency, though marginally, from 93.4 to 98.8% for a temperature increase of 293 to 313 K. It is thought that adsorption–reduction ability increased with higher temperature as it escalated the molecular mobility. Higher temperature increases the diffusion and transfer of Cr(VI) onto the reactive sites (Xia et al. 2012). It was also found by Zhu et al. (2018) that the presence of humic acid inhibits the Cr(VI) reduction process. It was understood that humic acid competes with Cr(VI) for adsorption sites and further accelerates the agglomeration of GT-nZVI/Cu particles.
Electrocoagulation method
Anode and cathode reaction in the EC cell (reproduced after Hamdan & El-Naas (2014)).
In a different study, Maitlo et al. (2019) experimented with an iron air fuel cell EC (IAFCEC) system in a synthetically prepared wastewater containing mainly chromate, chloride, phosphate, and silicate. Two main findings of this study were as follows: (i) the removal efficiency decreased with the increase in initial chromium concentration, and (ii) the presence of phosphate greatly hindered the removal capacity of Cr(VI). This study aimed to optimize the processing cost, and they were able to remove 100% of Cr(VI) within 4 h with an estimated cost of 0.2 USD per m3 of water. This IAFCEC is one of the most cost-effective methods of reduction. Another study (Martín-Domínguez et al. 2018) dealt with EC and chemical coagulation, using iron as the coagulation agent. Reduced Fe(II) converts Cr(VI) to Cr(III), which gets absorbed into the iron oxide, and through agglomeration, becomes bigger and gets precipitated from water. EC showed a high efficiency of 99.7%, whereas chemical coagulation had a similar efficiency of 99.9%.
Removal by conventional adsorbents
Direct and indirect methods of chromium reduction through adsorbents (reproduced after Park et al. (2005) and Pakade et al. (2019)).
Direct and indirect methods of chromium reduction through adsorbents (reproduced after Park et al. (2005) and Pakade et al. (2019)).
Kan et al. (2017) worked on adsorption experiments in which the adsorbent was silica sand coated by dried-out wastewater residuals and compared its adsorption behavior with silica sand. It was observed that the metal uptake was greater for coated silica sand than uncoated silica sand; however, the Cr ion selectivity was low. It was proposed that different types of metals present in the wastewater residual weaken the Cr adsorption mechanism. In another adsorption experiment with a novel adsorbent, Song et al. (2020) experimented with Cr(VI) removal using nano-magnetite-modified biochar. The nano-Fe3O4 enhances the porosity of the biochar, which in turn exhibits excellent adsorption capacity. In acidic conditions, the magnetite-biochar functional groups had more affinity toward Cr(VI) reduction than in alkaline conditions. However, the Cr(VI) removal efficiency was limited to 44–60%.
Polymer-based adsorbents (e.g., polyaniline (PANI) and their composites) are also used for decontamination. Generally, functional groups would be bound to PANI to form composites such as Fe3O4/PANI, HCl/PANI, citric acid/PANI, PANI nanofibres, and magnetic mesoporous silica composites (Jiang et al. 2018). For example, Gu et al. (2012) reported the use of Fe3O4–PANI nanoparticle composites, which successfully removed 100% Cr(VI) within 5 min when the initial Cr(VI) concentration ranged from 1 to 3 mg/L. Characterization techniques (e.g., FT-IR, XPS) revealed that Cr(VI) is reduced by PANI, followed by Cr(III) adsorption to the polynanocomposites.
Biological treatments
Many physicochemical methods used for Cr(VI) removal suffer from limitations such as high operational and maintenance costs, the formation of secondary chemical waste, and waste disposal problems. In contrast, biological methods have low costs and are environmentally safe. The biological Cr(VI) reduction process can occur by aerobic, anaerobic, or as a combination of both processes. Ishibashi et al. (1990) experimented with Pseudomonas sp., whereas Shen & Wang (1993) worked with E. coli for Cr(VI) removal. Later, researchers worked with sulfate-reducing and denitrifying bacteria to remove Cr(VI) (Mabbett & Macaskie 2001; Vainshtein et al. 2003; Asatiani et al. 2004).
In recent work, Mamais et al. (2016) added milk to the groundwater samples for biological growth. K2Cr2O7 was used for Cr(VI) concentration of 200 μg/L, and for nutrient supply (N: 12 mg/L and P: 2 mg/L) NH4Cl and K2H(PO4)3, respectively, were used. Furthermore, aerobic, anaerobic, and anaerobic–aerobic conditions were maintained. Very high removal efficiencies were observed for anaerobic conditions (96.5%) and mixed anaerobic–aerobic conditions (95%). Whereas for aerobic conditions, the reduction gets hindered as dissolved oxygen behaves as a Cr(VI) competitor. In the same work, Mamais et al. (2016) also used sugar and cheese hay for biological growth. They found sugar to have more removal efficiency than milk and cheese hay; the latter of which was described as a complex fermentable substrate. In another recent study, Zhang et al. (2019) built a reactor using adsorption and a microbial fuel cell where Latanus acerifolia leaves were used. With an initial Cr(VI) concentration of 50 mg/L, this reactor could adsorb up to 98% of Cr(VI) within 16 h.
Biosorbents based removal is an important bioremediation process in which Cr(VI) adsorbed onto different biomaterials. Biosorbents can be a wide range of materials such as lignin, cellulose, hemicelluloses, algae, fungus, neem leaves, wheat bran, and mosambi peel. Saha et al. (2013) used devil tree sawdust as a biosorbent for Cr(VI) removal. The adsorption capacity was highest in acidic conditions due to the formation of more H+ ions. These hydrogen ions hinder the hydroxyl group adsorption, which assists in the diffusion of Cr(VI) ions. The removal percentage also increases with increasing temperature. Mukherjee et al. (2014) used an aqueous sugarcane bagasse extract as a biosorbent. The water-soluble part of this material consists of lignin, polysaccharide, and several other compounds which help reduce Cr(VI) to Cr(III). Here, the complete reduction took place after a contact period of 72 h. Certain neutral surfactants were used as a catalyst to speed up the process to 24 h. The reduction rate was also found to be highest at pH 2. In a different study, Moringa oleifera flower extract was used to remove Cr(VI) (Mukherjee et al. 2015). It comprises mainly hydroxyl and phenolic groups, which act as the electron donor groups for the biomaterials. The biosorbent (Moringa oleifera), coupled with surfactants as catalysts, was mixed with Cr(VI) solutions to check its adsorption capacity, which was found to be a slow process. Even with catalysts, it took 285 h to complete the 40 ppm Cr(VI) reduction to Cr(III). Mondal et al. (2017) experimented with soapnut extract to remove Cr(VI) with different solvent-extractants. The importance of this study is that this saponin extract acts as both biosorbent and surfactant. It produces the micellar structure above CMC, enhancing the rate. This process reports the successful completion of Cr(VI) reduction to Cr(III) within 24 h.
In another study, the role of natural iron-bearing minerals was assessed in bioremediation (Lu et al. 2020). Here, four minerals (mackinawite, magnetite, wustite, and pyrite) were used as the electron donors to reduce Cr(VI). In the batch experiments, the minerals were powdered and added to the bioreactors, which then proceeded in anaerobic settings. The removal efficiency varied from 87.5 to 98% after the completion of the 96 h experiment. Among the four minerals, mackinawite showed better prospects. The iron minerals generally showed better efficiency than the typical gaseous electron donor (Lu et al. 2020). Němeček et al. (2015) reported Cr reduction by combining the abiotic (nZVI) and biotic (microbes) approaches. They used nZVI and whey-containing sulfate-reducing bacteria in succession, which could completely remove Cr(VI). This combination of chemical and microbial reduction processes thus proves to be a cost-effective and sustainable solution for removing Cr(VI)-contaminated groundwater.
NATURAL ATTENUATION
In natural settings (e.g., groundwater environments), it is also important to monitor chromium reduction by natural processes (i.e., natural attenuation), as it does not involve any cost. Fe-minerals and organic matter are common subsurface natural reductants of Cr(VI). A few conditions need to be met for natural attenuation to become a viable option: (i) the relative rate of Cr(VI) reduction must be greater than its advective transport, (ii) relatively higher residence time of the contaminated water than the time it takes for reduction of Cr(VI), (iii) the total amount of contaminant within a given volume must be less than the total reduction capacity of the aquifer, and (iv) no net oxidation from Cr(III) to Cr(VI) should occur within the aquifer (Palmer 1994).
There have been a few experiments regarding the natural attenuation of Cr(VI). Truex et al. (2015) performed laboratory experiments with different soil columns (sediments pre-treated with biocide, added microbial substrate, and treated with 1% hydrogen peroxide) at a sediment-to-water ratio of 4.24 g/mL under anaerobic conditions. Although natural attenuation was observed, sometimes it was masked by a constant flow of chromium from the source. Attenuation became prominent only if source flux was hindered. Since laboratory results show higher rates than at actual field sites, experiments done at actual field sites are also important.
In a recent study, Jiang et al. (2021) experimented with the reduction process in a natural wetland situated at the downslope of chromite ore storage. Wetlands are used for water decontamination because they contain both biotic and abiotic reducing agents. The wetland was divided into three parts (contaminated area, transitional area, and uncontaminated area) concerning the contamination source. In the groundwater samples from the contaminated and transition areas, Cr(VI) was found. The presence of Fe2+ could cause a reduction in the transitional area but not in the contaminated area because of the latter's high concentration of Cr(VI). It was observed that in the highly contaminated area, the population and diversity of (Cr-reducing) microbes lessened with time as they became Cr-resistant. However, at the transition zone, both Fe2+ and microbial activity reduce the chromium and naturally attenuate the groundwater.
Natural attenuation of chromium-contaminated groundwater is possible; nonetheless, it requires the fulfillment of several conditions and depends on several factors. Therefore, we need a better understanding of the environmental parameters, reducing agents, and the reduction capacity of the aquifer for the source flux.
CONCLUSION AND FUTURE PERSPECTIVES
This review article presents the main findings on several methods of Cr(VI) removal published in the past decade. Different methods like EC, adsorbents, and biosorbents used in recent studies have been discussed in detail, along with their efficiency and novelty. The chromium removal process depends on several parameters, like pH, temperature, initial Cr(VI) concentration, contact time with the reducing agent, adsorbent concentration, and the presence of competing ions. Additionally, some findings from a few other studies are listed in Table 1. Two types of removal strategies are used in all these methods; some processes remove Cr(VI) directly, while some convert Cr(VI) to Cr(III).
Comparison of different chromium removal methods in addition to the reviewed literatures in this paper
| Removal method categorization | Removal approach | Remarks | References |
|---|---|---|---|
| Adsorption by nanoparticles | nZVI entrapped Chitosan beads | Reduction rate is directly proportional to nZVI concentration and inversely related to pH and initial Cr(VI) concentration | Liu et al. (2010) |
| Silica fume-supported nZVI | 88.0% within 120 min | Li et al. (2011) | |
| nZVI-incorporated mesoporous silica (SBA-15) | 99.7% removal in 10 min at pH 5.5; 92.8% in 120 min at pH 9 | Sun et al. (2014) | |
| PANI/Ferric oxide nanocomposites | 51.2% removal | Ramachandran et al. (2017) | |
| Fe-sulfide nanoparticles | 92.5% removal efficiency | Wang et al. (2019a) | |
| Permeable reactive barrier (nZVI, bimetallic nZVI, AC, Sand/Zeolite mixture-SZ) | Removal efficiency: 89.7% – nZVI, 84.1% – Fe0/Cu, 23.01% – AC, 14.0% – SZ | Maamoun et al. (2020) | |
| Fe/Al bimetallic nanoparticles | 70.7% removal efficiency after 105 h | Ou et al. (2020) | |
| Adsorption | Mg/Al layered double hydroxide (LDH Adsorption) | 66% efficiency with 0.024 mol Mg and 0.0008 mol Al | Chao et al. (2018) |
| Biological method | Ferrous sulfate | 99–100% removal at pH 5.8–7.8 with a high Fe(II) dosage | Guan et al. (2011) |
| Rhizofiltration by halophyte Juncus acutus | 100–140 mg/L initial concentration to below permissible limit in 5 days | Dimitroula et al. (2015) | |
| Bio-wastes and Vermiculite | High absorption capacity (approximately 1,400 mg/kg) for 50% coir pith + 50% vermiculite | Tripathi & Chaurasia (2020) | |
| Sulfur-based mixotrophic bioreduction process | 95.5% removal efficiency within 48 h | Zhang et al. (2020) | |
| Fe-biochar composite | 66.9% removal after 18 cycles of infiltration | Chen et al. (2021) |
| Removal method categorization | Removal approach | Remarks | References |
|---|---|---|---|
| Adsorption by nanoparticles | nZVI entrapped Chitosan beads | Reduction rate is directly proportional to nZVI concentration and inversely related to pH and initial Cr(VI) concentration | Liu et al. (2010) |
| Silica fume-supported nZVI | 88.0% within 120 min | Li et al. (2011) | |
| nZVI-incorporated mesoporous silica (SBA-15) | 99.7% removal in 10 min at pH 5.5; 92.8% in 120 min at pH 9 | Sun et al. (2014) | |
| PANI/Ferric oxide nanocomposites | 51.2% removal | Ramachandran et al. (2017) | |
| Fe-sulfide nanoparticles | 92.5% removal efficiency | Wang et al. (2019a) | |
| Permeable reactive barrier (nZVI, bimetallic nZVI, AC, Sand/Zeolite mixture-SZ) | Removal efficiency: 89.7% – nZVI, 84.1% – Fe0/Cu, 23.01% – AC, 14.0% – SZ | Maamoun et al. (2020) | |
| Fe/Al bimetallic nanoparticles | 70.7% removal efficiency after 105 h | Ou et al. (2020) | |
| Adsorption | Mg/Al layered double hydroxide (LDH Adsorption) | 66% efficiency with 0.024 mol Mg and 0.0008 mol Al | Chao et al. (2018) |
| Biological method | Ferrous sulfate | 99–100% removal at pH 5.8–7.8 with a high Fe(II) dosage | Guan et al. (2011) |
| Rhizofiltration by halophyte Juncus acutus | 100–140 mg/L initial concentration to below permissible limit in 5 days | Dimitroula et al. (2015) | |
| Bio-wastes and Vermiculite | High absorption capacity (approximately 1,400 mg/kg) for 50% coir pith + 50% vermiculite | Tripathi & Chaurasia (2020) | |
| Sulfur-based mixotrophic bioreduction process | 95.5% removal efficiency within 48 h | Zhang et al. (2020) | |
| Fe-biochar composite | 66.9% removal after 18 cycles of infiltration | Chen et al. (2021) |
Remediation based on nZVI and its composites are novel methods that rely on reduction and exhibit high removal efficiency, and these hold promise in future remediation processes. However, pH was found to be one of the most sensitive. In acidic conditions, the Cr(VI) removal process is consistently found to be highly effective because of the difference in the basic chemistry of these two oxidation states. Eh–pH diagrams suggest Cr(III) is more stable in acidic environments, while Cr(VI) is stable in alkaline conditions. In lower pH, the corrosion rate of nZVI increases significantly because of the abundance of H+ ions. However, metal hydroxides could be precipitated in higher pH conditions over the nZVI surface, hindering the electron transfer process or blocking the reactive areas, thus decreasing the reduction process (Zhou et al. 2015). Even though alkaline conditions documented lower removal capacity than acidic conditions, it is still noticeable even in higher pH conditions. In the pH range (7–9), the removal efficiency showed some promising results, like 80–95% at pH 9 (ACF-nZVI), 85.7% at pH 9 (SA-nZVI), 70% at pH 7 (sepiolite-nZVI), and 100% at pH 8 (EC). Additionally, EC and biological treatments are cost-effective, regenerative, easy to maintain and handle, and efficient Cr(VI) removal methods.
Some studies reported that coexisting ions, such as Mg2+ and Ca2+, might negatively affect Cr(VI) reduction (Qian et al. 2014). However, Dorosti et al. (2020) showed that the presence of CaCO3 in the cathode bar had no interference with the Cr(VI) reduction sites. He et al. (2020) reported an experiment showing that the presence of nitrate might enhance the Cr(VI) reduction to some extent. Sun et al. (2014) showed that humic acid did not significantly affect the final reduction capacity, but the Cr(VI) reduction decelerated with increasing humic acid concentration. In comparison, Wang et al. (2019b) reported that at pH 4.0, humic acid increased the Cr(VI) removal, but at pH 6.0, the effect slowed down. Therefore, it is evident that the coexisting components in the groundwater must be analyzed to properly investigate Cr(VI) reduction and removal.
Many of these methods in this short review reported high (>90%) Cr removal efficiency, even at moderately high pH. It is borne out of the present study that future research should focus on the following: (i) applying the efficient methods for large-scale water treatment with minimization of cost; (ii) using actual groundwater as the medium to study the Cr removal rates so that the role of the competing major ions and trace elements is clearly understood; (iii) evaluating the impact of other inorganic catalysts in enhancing the Cr removal rates; and (iv) in some stabilizer-based nZVI removal methods, the initial reduction created a barrier between the adsorbent surface and Cr(VI) hindering further reduction and decreasing the reduction rate; therefore, more studies could be done on stabilizers to help resolve this problem. Finally, it is vital to ensure that secondary contamination does not arise from these chemical removal methods.
FUNDING
S.A. would like to acknowledge the Science and Engineering Research Board, Department of Science & Technology (India) for funding the fellowship through project EMR/2016/000697.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.
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