Industrial Wastewater Treatment Past and Future Perspectives in Technological Advances for Mitigation of Cr(VI) Pollutant

Abstract

It is well known that among heavy metals, chromium in its hexavalent form appears to be one of the major water contaminants globally in this century. It has toxicity, persistency and bio-accumulation tendency in nature. It is carcinogenic, genotoxic and mutagenic to living organisms. Hexavalent chromium (Cr(VI)) can interfere with photosynthesis, seed germination and nutrient intake, as well as the overall plant growth and functionality. Because of these issues, this proposed chapter entitled Industrial Wastewater Treatment Past and Future Perspectives in Technological Advances for Mitigation of Cr(VI) Pollutant is of importance. This chapter mainly focuses on chromium toxicology in humans and the environment and conventional industrial wastewater treatment methods and technical advances including adsorption using membrane technology for chromium removal from wastewater.

1. Introduction

Water is a quintessential resource for the sustenance of life utilized for industrial, aquatic, domestic and agricultural purposes [1]. About 70% of the earth’s surface is comprised of water while <1% is freshwater thus freshwater supply must be adequate and safe to drink [1]. However, the pollution of water resources due to the discharge of heavy metals has been an increasing worldwide concern for the last few decades leading to freshwater scarcity [2]. Industries discharge effluents containing heavy metal byproducts directly or indirectly into the aquatic environment without any adequate treatment thus deteriorating our water quality [3]. These heavy metals are at least five times heavier than water and with a density greater than or equal to 6.0 g.cm⁻³ [4]. The prevalent metals include mercury (13.53 g.cm⁻³), cadmium (8.65 g.cm⁻³), copper (8.95 g.cm⁻³), chromium (7.19 g.cm⁻³), nickel (8.91 g.cm⁻³), cobalt (8.90 g.cm⁻³), zinc (7.14 g.cm⁻³) and lead (11.34 g.cm⁻³) [5]. Most of these heavy metals are highly inimical even at very low concentrations of 0.1–0.3 mg.L⁻¹ and affect the plant biological factors and enter the food chain on consumption of these plant materials thus posing significant health risks to human and animal health due to their non-degradability [4, 6]. Among these metals, chromium in its hexavalent form appears to be one of the major water contaminant globally in this century [5].

2. Chromium

Cr(VI) is ascribed to be the top 16th hazardous substance by the Agency for Toxic Substances and Disease Registry (ATSDR) [7]. Chromium is a silvery-white transition metal with an atomic number of 24, a relative atomic mass of 52.996 g.mol⁻¹, a melting point of 1900°C and a boiling point of 2642°C discovered in 1797 by a French chemist Louis Nicolas Vauquelin [4, 8]. Chromium is the seventh most abundant element on earth and it exists in three predominant forms in the environment the metallic Cr (0) is present in ores, the trivalent Cr [Cr(III)] occurs naturally and Cr(VI) is generated from anthropogenic activities [4, 9]. Cr(VI) is usually released into the water bodies via leakage, poor storage, and unsafe disposal of industrial practices [10]. These industries include electroplating, leather tanning, cement, dyeing, metal processing, wood preservatives, paint and pigments, textile and steel fabrication industries which release chromium and its compounds i.e. chromates, dichromates, chromic acid, chromic sulfate and chromic oxides into the environment [5, 11]. The leather manufacturing units are considered the main cause of chromium pollution because they use a large quantity of chromium salts thus generating large quantities of Cr waste [12]. Each ton of processed leather results in more than 0.12 kg of Cr pollution [12]. These industries produce large quantities of toxic metal wastewater effluents, in concentrations above discharge levels [5]. However, the permissible limit of Cr for effluent discharge from electroplating is 2500 mg L⁻¹ Cr (VI), which must be controlled to acceptable levels before discharge to the environment [13]. Although Cr(III) has poor cellular permeability and is an essential nutrient for living organisms in maintaining cholesterol, fat and glucose metabolism, the oxidized state of Cr(VI) is more toxic due to being highly soluble and thus mobile across the membranes in humans and the environment [14]. Herein, this literature review presents the Cr(VI) source, toxicity on living organisms and the environment, conventional industrial wastewater treatment methods and adsorption using membrane technology in removal of Cr(VI).

2.1 Chromium toxicology

2.1.1 Chromium toxicology on humans

Cr (III) can be oxidized to Cr (VI) following the reaction: $C{r}_2{\text{O}}_3+3{\text{O}}_2\to 4Cr{O}_3$ under suitable conditions to increase its toxic exposure impact [8]. Human exposure to chromium is mainly through inhalation, skin contact, or ingestion depending on the absorbed quantity, route and exposure period [9, 11]. Cr (VI) exposure can be classified into acute (14 days), intermediate (75–364 days) and chronic (365 days) [11]. If Cr(VI) can enter humans through the respiratory tract it can trigger acute respiratory effects including pulmonary irritation, airway irritation and obstruction among others [8]. Chromium skin effects include irritation or dermatitis due to allergies [8]. The dermatitis effects include dryness, papules, erythema, fissured skin, small vesicles and swelling [8]. Cr(VI) can easily penetrate the cell membrane to react with the cell nucleic acids and proteins while being deoxygenated to Cr³⁺ and the reaction with the genetic matter is responsible for the carcinogenic characteristics of Cr(VI) [15]. Cr(VI) is classified to be a group one human carcinogen by the International Agency for the Research (IARC) on Cancer because it frequently occurs in the respiratory system, predominately as nasal, lung and sinus cancers [8, 9]. Other health effects of chromium include, cardiac, liver, gastrointestinal, kidney, hematologic or reproductive disorders, growth problems dizziness, eye irritation, weakness, teeth erosion and stain, accompanied by appreciable Cr concentration in the tongue papillae [8]. Cr(VI) can even become lethal when its concentration reaches the body weight of 0.1 mg.g⁻¹ [16].

2.1.2 Chromium toxicology in the environment

Chromium can exist in the air, soil and water [17]. Fossil fuel combustion, iron and steel manufacturing processes give off Cr into the atmosphere in the form of minute separate particles and most of the Cr eventually settles and ends up in the soils and water [17]. Excess oxygen in the environment oxidizes Cr(III) to the toxic Cr (VI) [9]. Cr(VI) easily leaches from the soil to water and persists as sediments, with a high potential of accumulating in aquatic life [17]. Chromium exists in the two most stable states in an aquatic environment either as Cr(III) or Cr(VI) [17]. Cr (VI) is an oxyanion that exists as chromate (CrO₄²⁻) ions predominately at 6.5–14 pH, hydrogen chromate (HCrO₄⁻), chromate (CrO₄²⁻), and dichromate (Cr₂O₇²⁻) ions at 0.7–6.5 pH, and chromic acid (H₂CrO₄) at <0.7 pH and the dichromate is the most toxic [18]. In water or soil, chromium goes through oxidation, reduction, sorption, desorption, precipitation and dissolution [15]. The aquatic toxicology of Cr, for example, on fish is affected by biotic and abiotic aspects [17]. The biotic factors include the species type, developmental stage and age [17]. The abiotic factors include pH, temperature, salinity, Cr oxidation state and concentration, water alkalinity and hardness [17]. Besides, metal speciation, lethal and sub-lethal concentrations also determine the organism sensitivity [17]. Cr collects predominantly on plant roots and in small levels is moved to the shoots, independently of the Cr species [15]. Cr uptake is an active mechanism carried out by sulfate carriers [15]. Cr is injurious to plants at concentrations as low as 5 mg.kg⁻¹ in soils and 0.5 mg.L⁻¹ in solution resulting in delayed seed germination, photosynthetic impairment, damaged roots, reduced root growth, reduced plant height and biomass, damaged membrane, leaf chlorosis, necrosis, low grain production and even plant death [11]. Therefore, it is obligatory to treat chromium-containing water before disposal into the aquatic environment to preserve human health and for environmental sustainability. Consequently, the World Health Organization (WHO) reported the discharge standard of Cr(VI) to be 0.05 mg. L⁻¹ in drinkable water and 0.1 mg. L⁻¹ effluent discharge into surface and groundwater [3].

2.2 Water treatment methods

In adherence to discharge regulations, industries have implemented conventional methods such as reduction [19], solvent extraction [20], precipitation [21], membrane filtration [22], ion exchange [23] and adsorption just to name a few for Cr(VI) removal [24]. A selection of a suitable technique depends on the plant reliability and flexibility, initial Cr(VI) concentration, operational costs and environmental conditions [14]. Photoelectrocatalytic reduction sequesters Cr (VI) by simultaneous organic pollutant oxidation and Cr reduction to Cr (III) in an electrochemical reaction. Zhao et al. [25] provided a review on the photocatalytic reduction of Cr(VI) for its removal and the technique was found to be promising for high-efficiency Cr (VI) reduction. Zhoua et al. [19] reported 91.5% Cr (VI) removal efficiency from wastewater using a nanogenerator-driven self-powered electrochemical system. Martinez et al. [26] employed an electrochemical reduction technique with ring iron rotary electrodes to remove Cr(VI) from synthetic plating wastewater and the Cr(VI) concentration was reduced from about 500 mg.L⁻¹ to values lower than 0.5 mg.L⁻¹. However, photocatalytic, chemical and electrochemical reduction methods suffer from high operational costs and long processes [3]. The solvent extraction method extracts Cr (VI) from wastewater by agitating a metal-laden solution with an organic solvent. The extraction takes place due to the relative solubility of the contaminant in two immiscible liquids [20]. Agrawal et al. [20] presented a review on the efficient and steady reagent cyanex 923 to remove hexavalent Cr from industrial effluents. However, the solvent extraction-wide application is limited due to the generation of large of organic solvent quantities [27]. The precipitation method sequesters Cr from industrial effluents following the addition of counter-ions to reduce their solubility in an aqueous media [28]. The dissolved metals are turned into insoluble components by chemical precipitators like sodium hydroxide, calcium hydroxide, calcium magnesium carbonate and magnesium oxide under optimum pH conditions [11, 28]. Ghejua et al. [21] reported high Cr (VI) removal from wastewater by reduction with scrap iron and subsequent chemical precipitation of cations with NaOH. The chemical precipitation technique is economically feasible and can handle bulk wastewater with ease [28]. However, it generates large sludge volumes, has low Cr(VI) selectivity removal efficiency and is unable to maintain optimal pH for precipitation [28]. The ion exchange technique can remove Cr ions from water by exchange with non-polluting ions [1]. The heavy metals physically adsorb the resins to form a complex [1]. Energy efficiency and low maintenance make ion exchange a suitable technique for Cr(VI) removal [14]. Shi et al. [23] employed D301, D314 and D354 anion-exchange resins for efficient removal of Cr (VI) from an aqueous solution with maximum sorption capacities of 152.52, 120.48 and 156.25 mg.g⁻¹, respectively. Rengaraj et al. [29] reported that ion exchange resins have good potential to remove chromium from aqueous solutions. However, low sensitivity and cost implications limit the diverse application of this technique [30]. Compared to the conventional methods, adsorption stands out to be a promising technique because of its fast removal process and simplicity and thus high potential in Cr(VI) abatement from wastewater [31]. Adsorption is a process whereby the adsorbate binds to the adsorbent by physical attractive forces, ion exchange, or chemical bonds [2]. The driving force for adsorption is the concentration ratio to the analyte solubility and is affected by the adsorbent dosage, initial adsorbent concentration and agitation time [5, 14]. Cr(VI) abatement from an aquatic system occurs through the reduction of chromium from Cr(VI) to Cr(III) and Cr(VI) adsorption, resulting in the adsorption of Cr(VI) species mainly [16]. A variety of adsorbents including activated carbons (ACs) [32] chitosan [33] and carbon nanotubes [34] just to name a few have been employed for Cr(VI) adsorption [3]. Among these materials activated carbon is an attractive material for chromium abatement from wastewater streams due to its large surface area and well-developed microporous internal structure of a wide variety of surface functional groups among them carboxylic groups [14]. Wang et al. [32] provided evidence of the high porosity and reducing ability of AC materials critical for Cr(VI) removal from wastewater. However, commercial ACs are cost-intensive and adsorb a few milligrams of metal ions per gram [14]. Therefore, the circular economy practice was considered to derive AC from inexpensive, available, renewable energy waste materials into valuable AC products for waste management and to alleviate environmental pollution and waste disposal costs [35]. However, it is difficult to recover AC from wastewater streams post-wastewater treatment thus limiting its application as an adsorbent [36].

2.2.1 Adsorptive membranes

Nowadays, membranes using adsorption technology have gained tremendous attention as effective adsorptive membrane adsorbents due to their specific adsorptive groups and exclusive membrane morphological properties for the removal of heavy metal ions from wastewater streams [37]. In recent years, membranes have been utilized to reject Cr(VI) from the permeate based on the membrane pore size exclusion and operating pressure in a short period due to its easy operation and high removal efficiency (Figure 1) [2, 31].

Figure 1.

Membrane separation mechanism.

In addition, membranes consist of various separation processes including ultrafiltration (UF) which removes e.g. macro molecules, nanofiltration (NF) removes large molecules and reverse osmosis (RO) removes monovalent ions from waste streams in the range of 0.1–0.01, 0.01–0.001 and 0.001–0.0001 μm respectively (Figure 2) [2]. The removal of chromium by the membranes is affected by numerous factors including pressure, pH, feed water flow rate, initial concentration, etc. [33]. Darwish et al. [38] reported Cr(VI) removal from an aqueous solution by a polymer enhanced UF membrane. The polymer was enhanced with starch and polyethylene glycol (PEG) and the removal efficiency of 100% with starch and 91% PEG was achieved at pH 10 [38]. Hafiane et al. [39] reviewed NF membrane to be a very promising method for the treatment of Cr(VI) bearing wastewater. Cimen et al. [40] employed RO membranes to effectively reject chromium from wastewater at a rate of 91–96%.RO membranes possess excellent structural stability and can effectively remove chromium from wastewater at reduced operating pressure [40].

Figure 2.

Membrane filtration processes for remediation of Cr(VI) ions.

Interestingly, Vatsha et al. [41] demonstrated that fillers as pore-forming agents to tune the membrane morphology for the intended application. Various fillers including titanium oxide [42], multi-wall carbon nanotubes [43], polyvinylpyrrolidone (PVP) [41], and polymer coordination polymers (PCPs) have been widely utilized to fabricate mixed-matrix membranes [44]. Jacob et al. [45] reported the flux recovery ratio of psf/montmorillonite clay modified with methyl dihydroxy ethyl hydrogenated tallow ammonium membrane (mMMT) with 3% mMMT increase to 83%, which is higher than the pristine polysulfone membrane and the total fouling reduction to 39%. Zr-based PCPs are highly porous crystalline materials constructed by metal centers connected to organic linkers with the potential to remove inorganic pollutants [46]. Polymeric membranes have been used at the industrial level for Cr(VI) removal owing to their inexpensive, higher flexibility, smaller space for installation and easy pore-forming mechanism in comparison to inorganic membranes [47]. The prevalent polymers employed include polysulfone (PSf), polyethersulfone (PES) and polyvinylidene fluoride (PVDF), to name a few [48]. However, these polymers hydrophobic nature makes them prone to fouling resulting in a decline in the membrane performance and high operational costs [47]. However, the direct incorporation of PCPs into the membrane polymer matrix cast into a flat sheet following the phase inversion is incompatibility with the polymer matrix because PCPs readily agglomerate leading to void defects in modified membranes thus affect the membrane separation performance [44]. Therefore, polymers incorporated with functional groups of the connecting materials like polydopamine (PDA) and ethylenediaminetetraacetic acid (EDTA) can enhance PCP-polymer interaction [44, 49]. Dopamine (DA) is a small molecule that imitates mussel adhesive protein and binds to inorganic and organic surfaces to induce the polydopamine (PDA) layer [44]. PDA possess abundant functional groups including hydroxyl, amino, etc. and the organic PDA backbone can form intermolecular hydrogen or electrostatic and covalent bonds with various polymer membrane matrix materials [44]. EDTA is a chelating agent for heavy metal ions owing to strong binding sites including two softer tertiary amines and relatively four hard carboxyl groups [49]. However, the resulting metal-ligand complex is always water-soluble thus hindering its application. Therefore, binding EDTA on porous materials could lead to easy separation and recovery for reuse while maintaining adsorption sites for heavy metal ion capture [49].

3. Conclusions

The presence of Cr(VI) above safe concentration limits in an aqueous media it’s an environmental issue due to its toxicity, persistency and bio-accumulation tendency in nature.it is carcinogenic, genotoxic, and mutagenic to living organisms. Cr(VI) can interfere with photosynthesis, seed germination, and nutrient intake, as well as the overall growth and functionality of the plant. To comply with legal requirements and to preserve the water quality various conventional techniques for Cr(VI) removal have been widely employed however most of them suffer from high operational costs, incomplete Cr(VI) removal, and excessive generation of toxic sludge which hinders their practical and effective use. Nowadays, membranes using adsorption technology have gained tremendous attention as effective adsorptive membrane adsorbents due to the synergistic effects between the specific adsorption groups and exclusive membrane morphological properties characterized by better Cr(VI) removal not met by the pristine materials.

Conflict of interest

The author declares no conflict of interest.