Chitosans as soil amendments for the remediation of metal contaminated soil

摘要

Research was conducted to evaluate the potential of chitosan, a fishery waste-based material, and its derivative cross-linked chitosans, as soil amendments for the remediation of metal contaminated land. This research comprised modification of chitosan followed by a characterisation study, a batch sorption study, two pot experiments and a biodegradation study. Chitosan was modified with three cross-linking reagents, namely glutaraldehyde (GLA), epichlorohydrin (ECH) and ethylene glycol diglycidyl ether (EGDE). The characterisation study used X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and Fourier transform infrared spectroscopy (FTIR) methods to investigate the effect of cross-linking treatment on the surface and physical properties of chitosan, the effect of metal interaction on the surface properties of chitosan and cross-linked chitosans, and the binding mechanism(s) of metal ions onto the chitosans. Cross-linking treatments on chitosan enhanced its chemical stability in acidic media and increased its BET surface area. Metal interaction reduced the crystallinity and changed the surface morphology of the chitosans. FTIR analysis revealed that the complexation of metal ions was through dative covalent interaction with the amino and hydroxyl groups of the chitosans. The batch sorption study evaluated the ability of chitosan and cross-linked chitosans to bind heavy metals. The effects of contact time, initial metal concentration and background electrolyte on metal binding were assessed. The binding behaviour was described by several kinetic and isotherm models. The maximum binding capacity (Q) values, estimated using the Langmuir isotherm model for the chitosans were comparable with other low-cost sorbents reported in the literature. The sorption-desorption study showed that the chitosans were able to retain metal ions on their surfaces, even at dilution factor of x11. The pot experiments evaluated the effectiveness of chitosan and chitosan-GLA in immobilising heavy metals in the contaminated soil. Their effects on plant growth and metal accumulation in plant tissue were determined using Lolium perenne (perennial ryegrass) and Brassica napus (rapeseed). For perennial ryegrass, the results were dependent on the rate of addition of the chitosans. Low application rates (up to 1% w/w) resulted in an increase in metal uptake, whereas 10% (w/w) addition decreased metal uptake. For rapeseed, metal uptake was decreased at all rates of application of chitosans. The ammonium acetate extractable metals in soil decreased following application of chitosan and plant growth. The biodegradation study measured microbial breakdown of the chitosans in both non-contaminated and contaminated soils. It was estimated that a longer period is required to complete the breakdown of the cross-linked chitosans (up to approximately 100 years) than unmodified chitosan (up to approximately 10 years). The influence of biodegradation on the bioavailable fraction of heavy metals in soil was studied concurrent with the biodegradation trial. It was found that the binding behaviour of chitosan for heavy metals in soils was not affected by the biodegradation process.