Removal and recovery of toxic metal contaminants from process effluents using integrated biosorption and electroplating

Abstract

The research study reported herein aimed to apply Micrococcus luteus, an activated sludge bacterial isolate, to the removal and recovery of copper ions from wastewater. The work included optimization of calcium-alginate/polyacrylamide beads for the immobilization of M. luteus; development of a fixed-bed biosorption column of the cell-immobilized beads for the treatment of copper-contaminated wastewater; regeneration of the copper-laden beads (copper desorption); characterization of the fixed-bed biosorption and desorption; and, recycling of the desorbed copper in electroplating. In the optimization of the cell-immobilized beads, a rehydration treatment (40 ℃ oven-drying followed by re-immersion in distilled-deionized water) stabilized the beads in a constant volume throughout the sequential batch copper biosorption/desorption cycles. No significant differences were found between the biosorption characteristics of the rehydrated form of the immobilized M. luteus (RIM) and those of the untreated beads, including their biosorption capacities, kinetics, equilibrium isotherms and diffusion mechanisms. Fixed-bed copper biosorption studies showed that the breakthrough (Ce = 4 mg-Cu/L) of the RIM column was deferred by increasing the bed depth and decreasing the influent concentration and the flow rate. The optimal conditions were a 50 cm bed depth, a 50 mg-Cu/L influent concentration, and a 1 mL/min inlet upward flow rate. The RIM beads in the column revealed better biosorption performances and less shrinkage than the untreated beads. A greater copper removal and a larger treatment volume were demonstrated by the RIM beads in the column reactor, compared with those in the continuous-stirred-tank reactor at the breakthrough and the batch reactor at the equilibrium. Binary-metal biosorption studies demonstrated the metal-binding preference of the RIM column as Pb(II) > Cu(II) >> Ni(II) > Zn(II). Converging with this order, the overshot biosorption profiles (Ce/Co > 1) of nickel and zinc were observed in the presence of Cu(II) ions, whereas Pb(II) ions promoted the copper overshoot. The Clark model, the Adams-Bohart model, the Yoon and Nelson model, and the Thomas model were applied to predict the breakthrough profiles of the fixed-bed copper(II) biosorption. The linearized form of the Thomas model provided the better simulations, with large r² values, small average percentages of errors, and low variances in the predictions of the process performances. Of 11 common desorbing materials, CaCl₂ displayed a moderate copper desorption ability and a better reusability of the RIM beads in the batch studies. Similar desorption efficiencies were obtained in the fixed-bed studies under different CaCl₂ concentrations and inlet flow rates. The optimal fixed-bed desorption conditions were a 1 M CaCl₂ concentration and a 1 mL/min inlet upward flow rate. The reusability of the RIM column was examined by introducing both synthetic and industrial electroplating wastewater in 10 successive biosorption/desorption cycles. Both the results showed that the copper removal decreased at the initial cycle(s) and remained steady afterward. A lower efficiency was found in the treatment of the industrial wastewater. The surface chemical composition of the RIM beads in the sequential fixed-bed biosorption and desorption was characterized by X-ray energy-dispersive analysis. The results coupled with the material balance calculation showed that the ion exchange between calcium and copper ions was one of the major mechanisms of the biosorption and desorption in the RIM column. After a two-step purification process using Na₂SO₄ and Na₂HPO₄, the recovered copper was isolated from the desorption effluent and directly re-used in an acid-sulfate-copper-electroplating process. The study showed that the addition of the recycled copper did not significantly change the properties of the deposited workpieces, including their overall appearance, surface smoothness, deposit distribution, coverage, copper purity on the deposits, and electrical resistance. In this research studies, the rehydrated form of the Ca-alginate/Polyacrylamide immobilized Micrococcus luteus was successfully applied in the fixed-bed column reactor with reasonable copper removal performance, desorption efficiency and biosorbent reusability. The desorbed copper after the purification could be reused in the electroplating process without any significant influence on the plating performances and the deposit characteristics. Thus, this integrated biosorption system could be developed as a promising technology for copper removal and recovery from industrial wastewater, and should be applied further in the practical treatment system.