Reduction of chromium (VI) to chromium (III) using continuous emulsion liquid membrane

Abstract

Recently, the application of emulsion liquid membrane (ELM) process as an alternative technology for solute separation is highlighted due to the simple operation of simultaneous extraction and stripping process. The most important aspects for a successful ELM process are liquid membrane formulation and emulsion stability. This study was carried out to investigate the liquid membrane formulation for the reduction of chromium (VI) to chromium (III) from electroplating wastewater using continuous ELM process (CELM). Liquid membrane system comprises of three liquid phases which are external (electroplating wastewater), organic liquid membrane and internal phase. Liquid membrane and internal phase were emulsified and dispersed into the external phase to be treated. The experimental work consisted of four major parts which were ELM component formulation, stability study of ELM in batch process, screening of parameters and optimization of chromium removal efficiency by response surface methodology (RSM) in continuous operation process and recovery of the chromium at optimum process conditions. The results show that the favourable conditions for liquid membrane formulation are 0.04 M TOMAC as a carrier, palm oil as a diluent and 0.1 M thiourea in 0.1 M sulfuric acid as a stripping agent. The best condition of stable water-in-oil (W/O) emulsion was obtained at 7000 rpm of homogenizer speed, 5% (w/v) Span 80 as surfactant and 1 minute of emulsifying time. Meanwhile, the most stable water-in-oil-in-water (W/O/W) emulsion obtained during the continuous process operation was at 350 rpm agitation speed, pH<5 of external phase and 1 to 5 of treat ratio. The optimization results by RSM show that 99% of chromium was extracted at 2.83 minutes of retention time, 342 rpm rotational speed and 1 to 5 of treat ratio. As a conclusion, about 81% of less-toxic chromium (III) has been recovered into the internal phase using 2.0 M thiourea in 2.0 M sulfuric acid as the stripping agent. The favourable process condition of the formulated membrane study was satisfactory and is suitable to treat wastewater as low as 20 ppm up to 200 ppm of chromium concentrations. This study reveals that CELM is a simple process and practical technology to remove chromium (VI) from industrial wastewater while solving the environmental problem simultaneously.