Utilization of fruit waste materials for Cr(III) biosorption and fungal biomass for As(V) biosorption

Abstract

Heavy metal contamination in the aquatic environment has adverse effects on human beings, aquatic life and the environment.Conventional wastewater treatment methods for heavy metal removal have their inherent constraints. Biosorption has been considered a potent and inexpensive alternative for treatment of wastewater laden with heavy metals.In the present study,biosorption of Cr(III) by fruit waste materials and removal of As(V) by pristine and immobilized filamentous fungal biomass were performed to evaluate their biosorption performances and mechanisms. Low-cost fruit waste materials have been investigated as biosorbents because of their abundant availability and effective biosorption capacity. Two local fruit waste materials, pomelo peel and watermelon rind,were utilized in this study to evaluate their potential in Cr(III) removal. The surfaces of both types of biomass were characterized with the determination of zeta potential, while potentiometric titration and spectroscopic analyses including scanning electron microscopy (SEM) and energy dispersive X-ray (EDX),Fourier transform infrared (FTIR) spectroscopic and X-ray photoelectron spectroscopic (XPS) analyses were also performed.The determination of zeta potential illustrated that the surface of both materials was negatively-charged at a pH value beyond 3. The presence of K and Ca on pomelo peel and K,Ca and Mg on watermelon rind was revealed by EDX and XPS analyses. The findings from potentiometric titration, FTIR and XPS analyses demonstrated the presence of carboxyl and hydroxyl groups on both fruit waste materials. Batch Cr(III) biosorption experiments were carried out to evaluate the influence of different parameters on Cr(III) removal by the two fruit waste materials. Influential factors included pH, presence of cation and anion, biomass concentration, initial metal concentration, contact time and temperature. The effects of these parameters on Cr(III) biosorption by pomelo peel were similar to those on Cr(III) biosorption by watermelon rind.The optimum pH was found to be 5.5. The removal of Cr(III) was not significantly affected by the presence of K+, Ca2+, Mg2+, Cl-, NO3- or SO42- but it was reduced in the presence of C6H5O73-(citrate anion) and (H2EDTA)2-. The Redlich-Peterson isotherm model provided the best simulation of the equilibrium data and the fractional power model could best describe the Cr(III) kinetics.The biosorption capacity increased at elevated temperature and the thermodynamics parameters,including changes in Gibbs free energy (△Go),enthalpy (△Ho) and entropy (△So), illustrated that the Cr(III) biosorption processes were feasible,spontaneous and endothermic in nature for both types of biomass. The determination of Ca2+, Mg2+, K+ and Na+ released in the Cr(III) biosorption process indicated the displacement of light ions by Cr(III) ions during the biosorption process for both fruit waste materials. Esterification of carboxyl groups significantly reduced the Cr(III) biosorption capacity for both types of biomass. The results obtained from SEM-EDX, FTIR and XPS analyses of pristine and Cr(III)-loaded fruit waste materials were compared. For both fruit waste materials, the integrated results from ion displacement, EDX and XPS analyses demonstrated the occurrence of ion exchange while the findings from FTIR and XPS analyses indicated the involvement of carboxyl and hydroxyl groups in the Cr(III) biosorption process. Surface complexation is therefore proposed in addition to ion exchange as a major mechanism responsible for Cr(III) biosorption by pomelo peel and watermelon rind. Desorption studies on both Cr(III)-loaded fruit waste materials revealed that citric acid was the best desorbing agent among the nine desorbing agents tested. Beside the cationic heavy metal contamination in wastewater, anthropogenic release of anionic As(V) into water sources also poses a threat to both humans and the environment because of its high toxicity and potential health hazards. Mucor rouxii exhibited the highest As(V) removal ability among the various fruit waste materials including pomelo peel and watermelon rind and selected filamentous fungal strains tested. Batch As(V) biosorption experiments were carried out to evaluate the effects of pH, biomass concentration, initial metal concentration and contact time on As(V) removal by Mucor rouxii. As(V) biosorption was found to be optimum at pH 4. The Sips isotherm model provided the best description of the equilibrium data and the Elovich model could best simulate the kinetic data of As(V) removal.The SEM images illustrated the filamentous morphology of Mucor rouxii and smoothened surface of the fungal biomass after As(V) biosorption.FTIR analysis of Mucor rouxii before and after As(V) biosorption suggested that amine/amide groups could be involved in the As(V) removal. Immobilization of Mucor rouxii based on the cross-linking between the carboxymethyl cellulose (CMC) and Fe(III) demonstrated significant enhancement in uptake of As(V). The CMC-immobilized Mucor rouxii (CMC-M) bead was spherical and its surface was covered with small and prominent Mucor rouxii biomass as revealed by the SEM images. Batch As(V) biosorption experiments were carried out to evaluate the effects of pH, biosorbent dosage,initial metal concentration and contact time on As(V) removal by CMC-M beads. The As(V) removal by CMC-M beads decreased generally with the increase of pH. The Sips isotherm model could best describe the equilibrium data and the non-linearized pseudo-second model provided the best simulation of the kinetic data of As(V) removal by CMC-M beads.Simulation of kinetic data with the intraparticle diffusion model revealed that intraparticle diffusion should be the predominant rate-limiting step in the As(V) biosorption process. The CMC-M beads before and after As(V) biosorption were examined by SEM-EDX, FTIR and XPS analyses. The findings from FTIR and XPS analyses indicated the involvement of protonated amino groups in the As(V) removal process while the integrated results from EDX and XPS analyses revealed possible ion/ligand exchange between As(V) and other ions coordinated with Fe(III) on CMC-M beads. This research demonstrated the effective Cr(III) uptake by two low-cost fruit waste materials and significant As(V) removal by CMC-immobilized fungal biomass. Results showed that fruit wastes have great potential to be developed into effective and economic biosorbents for removal of heavy metals including Cr(III) and As(V).