INVESTIGATION & SYNTHESIS OF HIGH CAPACITY ADSORBENTS FOR TRACE REMOVAL OF Cr(VI) FROM WATER

Abstract

The occurrence of hexavalent chromium even in trace concentrations is capable of jeopardizing the biological and ecological structure since it is genotoxic and carcinogenic in nature. While a lot of research has been done on Cr(VI) removal from industrial wastewater, treatment of contaminated groundwater with trace Cr concentrations is not well-documented since satisfactory removal of Cr(VI) beyond a certain low concentration becomes extremely difficult using conventional methods. The reasons being the non-specific interactions of Cr(VI) anions with the adsorbents and competition from other commonly occurring anions present in concentrations at least two orders of magnitude higher than Cr(VI). It was earlier observed that a weak base anion exchange resin, Duolite A7, showed an unusually high capacity for trace Cr(VI) removal, which would not be possible by the ion exchange process alone. Previous research has also demonstrated that some organic adsorbents derived from plants could remove Cr(VI), but none of the existing adsorption models could appropriately describe the adsorption process. Thus, in this research work, an attempt was made to investigate and establish the reaction mechanism responsible for Cr(VI) removal by Duolite A7 and organic materials with an aim that elucidation of the mechanism would help open up a new paradigm for the design of a special class of Cr-selective sorbents for effective remediation of Cr(VI) contaminated water. Firstly, coconut husk, a lignocellulosic material with no or low cost, was chosen as the representative organic sorbent for studying the reaction mechanism. The investigation was carried out in two significant stages. First, the adsorption phenomenon was studied using batch and column experiments to determine the isotherms and kinetic models and effects of process parameters like solution pH and contact time. Then, the virgin and exhausted materials were characterized using several techniques to understand the changes occurring in the sorbent before and after Cr(VI) removal. From the adsorption study, it was found that there was a poor fit of the observed data with the known adsorption models, leading to the inference that the removal mechanism involved processes other than simple adsorption. From the characterization results, it was observed that Cr(VI) was first adsorbed at the functional groups followed by oxidation of major components like hemicelluloses, cellulose, and lignin, present in the sorbent. Meanwhile, Cr(VI) itself got reduced to Cr(III) and precipitated as Cr(OH)3 inside the pores of the adsorbent. Thus, all the results conclusively pointed that Cr(VI) removal by organic matters such as coconut husk is essentially a sorption coupled redox process. i Next, for establishing the mechanism responsible for significant trace Cr(VI) removal capacity shown by Duolite A7, several ion exchange resins were selected based on different combinations of functional groups and matrix. The performance of the resins was evaluated through fixed-bed column study using an influent solution at pH 5 containing trace Cr(VI) and a background of competing ions at three orders of magnitude higher concentration. It was found that at acidic pH, Duolite A7 ran for about 30x more bed volumes than other anion exchangers. Further investigations indicated that redox reactions were taking place within the Duolite A7 resin in addition to ion exchange, where Cr(VI) was reduced to Cr(III) and precipitated inside the resin as Cr(OH)3. It was postulated that the phenol-formaldehyde matrix adjacent to the functional groups, due to its weakness against oxidative stress, might have been oxidized due to the redox reaction. However, further studies were required to know precisely the role of the resin matrix in demonstrating such high Cr(VI) removal capacity. Detailed reactions within the Duolite A7 resin were then studied through spectroscopic techniques such as ATR-FTIR (Attenuated total reflection – Fourier transform infrared) and XPS (X-ray photoelectron spectroscopy). It was established that the mechanism for Cr(VI) removal involved ion exchange followed by redox reactions occurring between Cr(VI) and the matrix and the functional groups within the resin. Ion exchange/adsorption is a fast process through which Cr(VI) anions were first attached to the surface of ion-exchanger at respective functional groups having an affinity towards Cr(VI) anions. Cr(VI) anions have strong oxidation potential. When sitting close to the matrix of the ion-exchanger, they initiated redox reactions with the matrix of the resin through which Cr(VI) got converted to Cr(III), which precipitated as Cr(OH)3 at neutral to alkaline pH. Conversion of Cr(VI) to Cr(III) acted almost like perpetual auto-regeneration of active adsorption/ion exchange sites, making it free to adsorb more Cr(VI) anions. Unfortunately, the phenol-formaldehyde matrix of Duolite A7 produced toxic reaction byproduct such as formaldehyde into the water, thus limiting its application for drinking water treatment. Nevertheless, motivated by the phenomenon of redox-active ion exchange, we synthesized a sorbent called PS-7 by immobilizing polyethyleneimine (PEI) polymer on the surface of a hydrophobic substrate which demonstrated highly selective Cr(VI) removal capacity - even better than Duolite A7 at neutral pH. An insightful investigation revealed that the use of hydrophobic matrix for PEI immobilization played a crucial role in Cr(VI) removal at neutral pH, opposed to the other reported PEI-based adsorbents with hydrophilic matrix which were capable of removing ii chromate anions at acidic pH only. The occurrence of redox reactions within the sorbent were further confirmed by ATR-FTIR and XPS techniques. The knowledge acquired from this study will be crucial for developing effective materials for remediation of Cr(VI) contaminated water and the redox-active ion exchange phenomenon studied here can motivate various essential applications in water and wastewater treatment. Keywords: Cr(VI) removal, Ion exchange, Adsorption, Redox reaction, Duolite A7, Phenol formaldehyde matrix, Low-cost adsorbents, Coconut husk, Biosorption, Redox-active sorbents, Functional polymers, Polyethylenimine, Hydrophobic interactions