Evaluation of anaerobic sludges as metal biosorbents and development of a biotechnological process for metal ion removal from selected wastewater

Abstract

As a result of rapid expansion of the industrial sector and increasing population, the environment has been under phenomenal stress. The volume of sewage and other effluents has increased tremendously in the last century. Globally, approximately .12 million tonnes of dry sludge biomass is produced and discarded of by landspreading, landfilling, incineration or dumping in lagoons and oceans. The discharge of industrial effluents into receiving waters has been documented to be the cause of severe environmental contamination. Heavy metals have been the cause of particular environmental concern. Their toxic and carcinogenic potentials at low concentrations, as well as the large quantities disposed to the environment, have prioritised them as leading contaminants. Current technologies of remediating heavy metal containing effluents are expensive and, in most cases, ineffective. Locally, most industries are merely diluting their effluents, thus resulting in the loss of valuable water resources. Waste sludges have shown the ability to adsorb heavy metals from their aqueous environment. Therefore, the current study attempted firstly, to compare biosorptive capacities of various waste sludges for a range of heavy metal ions, and secondly, to establish a relationship, if any, between biosorptive capacity and sludge surface charge. Finally, a laboratory scale biosorption process, encompassing desorption and recovery of metal ions from sludge surfaces, would have to be developed. Effluents used included pure, metal solutions of divalent zinc, cadmium, copper, nickel, trivalent and hexavalent chromium. In addition, synthetic effluents comprising a cocktail of the above-mentioned metal ions as well as an industrial effluent from a metal plating company were used. Five waste digested sludges were prepared and challenged against pure metal solutions to determine and compare their respective biosorptive capacities. Mechanisms of biosorption were elucidated using the Langmuir adsorption isotherm model. Sludge surface charge was determined using the millivolt quantification method. Upscaling of bioreactor trials to fully mixed laboratory scale was also investigated. These experiments encompassed the use of three sludges showing the greatest potential for biosorption and desorption using the selected mineral acid, H2S04, In addition, a simultaneous fully mixed biosorption and desorption process was designed and optimised. Subsequent trials involved comparing the latter process with a packed bed configuration whereby biomass was immobilised using poly sulfone resin. The overall comparative adsorptive capacities of the sludges (SI-SS) for metal ions in single solutions was S3 > S2 > S4 > SS > SI. Surface charge determination showed S3 to contain the most electronegative charge, with other sludges following in the same descending order as mentioned above. These findings supported the theory of a direct correlation between sludge surface charge and biosorptive potential. The affinity series of the sludges for metal ions followed the descending order of Cd2+ > Cu2+ > Ni2+ > Zn2+ > Cr6+ > Cr3+. Fully mixed studies, using mixed synthetic effluents, resulted in lower biosorptive capacities being recorded by the three selected sludges ie., S2, S3 and S4, as compared to single solution experiments. Biosorption studies with industrial effluent, containing Zn2+ as the most prevalent metal at 119.4 mg.F'. resulted in S3 biosorbing a maximum of 4.5 mg.g' of the cation. Sulphuric acid (H2S04) at O.2N, hydrochloric acid (HCI) at O.2N and acetic acid (CH3COOH) at O.4N were tested for their desorptive efficiencies. Sulphuric acid proved to be the most effective desorbing agent. Using S3 as biosorbent and O.2N H2S04 as desorbent, the manipulation and operation of a simultaneous process proved to be successful since both biosorption and desorption occurred concurrently, thus reducing time required for successful remediation considerably. Immobilised biomass, in a packed bed configuration, produced acceptable final effluent regarding standards as stipulated by the Durban Municipality for trade effluents. However, biosorption capacity of the sludge was compromised, with subsequent reductions in desorption being recorded, when the process was compared to fully mixed trials. Affinity series determined for the packed bed process wasC~+ >Cd2+>Zn2+>Cu2+>Cr6+ >Ni2+. Waste digested sludge has shown potential as metal biosorbent on an industrial scale. The present findings have succeeded in demonstrating a novel laboratory scale biotechnological process for the remediation of metal laden industrial effluents.