Optimizing Heavy Metals Removal of Excess Sludge by Biodegradable Chelants GLDA Using a Response Surface Methodological Approach

摘要

High heavy metals content is the major drawback of sludge resource, especially for sludge derived from an industrial wastewater treatment plant. A novel and more environmentally friendly kind of chelating agent, GLDA, was employed to treat sludge contaminated by heavy metals in the present study. The effects of the GLDA concentration, pH, and reaction time on sludge heavy metals availability and removal were investigated, and the response surface methodology (RSM) based on the Box-Behnken design was employed to optimize these parameters. The results showed that the higher GLDA concentration and the lower pH led to the higher removal rate of heavy metals, but the effect of longer reaction time is not obvious. The optimum conditions of extraction reaction at a GLDA concentration of 0.05 mol.L-1, pH of 3.56, and reaction time of 2.05 h, under the optimum conditions that the comprehensive heavy metals removal rate was 76.40%, and the extraction efficiencies of Cd, Cu, Pb, and Ni can reach 81.04%, 77.35%, 67.75%, and 75.78%, respectively. Analyzing the response surface plots and contour plots indicated that the effect of pH was more significant than GLDA concentration and reaction time. Scan electron microscope (SEM) observation demonstrated that the flocculent structure of sludge was replaced obviously by mass structure and the layered structure after treatment. Meanwhile, adsorption ability and volume were reduced. The research results also showed that the GLDA treatment sludge can effectively reduce sludge heavy metals content and improve chemical stability, which is advantageous to the sludge dewatering and its further processing and utilization.