Immobilization of heavy metals in tannery sludge by subcritical water treatment

Abstract

Waste sludge discharged from tannery industry is a potential source of environmental pollution due to the hazardous heavy metals (HMs) such as chromium, copper and led. Surface water and soil pollutions by these HMs as a result of improper treatment and disposal of tannery sludge have become a great health-hazard concern to the society.There are various immobilization techniques for HMs that have been proposed and/or are being practiced. The hydro-Thermal treatment utilizing subcritical water (SCW) is one of the new technologies, which is regarded as environmentally friendly and sustainable. The SCW technology can stabilize and reduce the volume of organic sludge as well as immobilize HMs to reduce direct toxicity and the risk. The study investigates immobilization of HMs, which are contained in tannery sludge in high concentrations, under the SCW conditions by forming a particular crystalline mineral that captures HMs. Based upon the preliminary SCW treatment experiments, which were conducted at moderate temperature of up 280°C and under the saturated pressure with amended silica and calcium, and leaching tests, it was observed that heavy metal concentrations in the leachate from tannery sludge decreased after the SCW treatment. This implies that the SCW treatment has a potential as an immobilization technology for HMs contained in tannery sludge. Experiments are currently being undertaken in order to optimize the SCW conditions and silica/calcium amendment ratio, to identify the mechanism of immobilization and to evaluate stability of the immobilized HMs.