Recovery of heavy metal ions through decontamination of synthetic solutions and industrial effluents using modified barks

Abstract

Raw and chemically treated tree barks were used for the decontamination of synthetic solutions containing 10, 100 and 1000 ppm of arsenic, cadmium, copper, chromium, iron, lead, mercury, nickel and zinc. Industrial effluents were decontaminated on the laboratory and pilot scales. Metal removal from solutions depends on the pH, the initial concentration, the bark species and, to some extent, on the anion. It varies from 20 to 99% of the initial heavy metal ions `HMI' content in the solution. The average retention capacity of the treated bark varies from about 0.25 to 4.36 mmol/g of dry modified bark. Full extraction of HMI from the saturated modified bark was made possible with dilute acid allowing the regeneration of the bark for reuse as an ion exchanger. Bark loaded with heavy metals was analyzed by scanning electron microscopy `SEM' and infrared `IR' spectroscopy. Metal distribution was uniform in the bulk of the bark. No metal segregation was observed. It seems that HMI were chelated to the acidic (phenolic and/or carboxylic) sites of the bark. Anions were not detected into the loaded bark with either the SEM–EPMA or the IR spectroscopy. Incineration of the bark loaded with copper or chromium resulted in ashes containing up to 70% of metal oxides. Pyrolysis of the bark loaded with copper led to solids containing 10% of metallic copper and about 85% carboniferous matter. Such materials can be recycled for the production of secondary metal using the current industrial units. The suggested process can be used for the decontamination of industrial effluents or polishing of treated effluents by conventional technology. It was possible to decrease the effluents' HMI content to ppb level. This research can be considered as a demonstration of the possibility of using a waste, of low economic value, to resolve an environmental problem such as effluents' decontamination. Moreover, the approach is to try to eliminate the pollution due to the HMI by their recycling instead of their disposal as waste.