Studies on the removal characteristics of heavy metals in aqueous solution by apple residues

Abstract

Apple residues(AR) from apple juice plant were used to remove heavy metals in aqueous solution. These residues contain acidic groups such as carboxylic and phenolic functional groups which can be enchanced by means of chemical modification. The removal characteristics of copper, lead and cadmium by AR were investigated in batch and column experiments. The effects of solution pH, ionic strength, co-ion, ligands, particle sizes of AR were studied in batch experiments. Blank tests were also performed without AR to investigate the metals removal which might occur via precipitation. In order to improve the structural stability on prolonged use and metal removal capacity, AR were chemically treated with phosphorus(V) oxychloride according to the procedure of Peska et al. The effect of chemical treatment of AR was studied. Adsorption isotherms and kinetic studies for metal removal by AR were also carried out to investigate the adsorption equilibrium and removal capacity. Modification of carboxyl groups of AR with acidic methanol was performed by the procedure of Wilcox to investigate the contribution of carboxyl groups to the metal binding. The extent of esterification was monitored by analyzing the amount of methanol released in the sample hydrolysates by gas chromatography. The effect of esterification on metal binding was investigated in batch experiments by unmodified and modified AR. The optimal pH ranges for copper, lead and cadmium removal are shown to be from pH 5.5 to 7, pH 6.5 to 8.0, and pH 8.0 to 9.5, repectively. The maximum removal was 91.2% for copper, 95.3% for lead, and 91% for cadmium. Monovalent cations, $Me(OH)^{+}$ shown to be dominant ion species at optimal pH regions. Based on the results of blank tests, precipitation does not contribute to removal mechanism of copper, lead, and cadmium in batch experimental limit. Based on experimental results and speciation of metal ions, metals removal by AR may be occurred by complexation between the negat...