SULFATE REMOVAL BY ELECTROCOAGULATION BY PRECIPITATION OF ETTRINGITE

Abstract

The aim of this research work was to investigate sulfate removal from the industrial wastewater by electrocoagulation by means of precipitating ettringite (Ca6Al2(SO4)3(OH)12.26H2O) and thus, reducing the TDS of effluent. Aluminium anode was used as sacrificial electrode and stainless steel, and graphite as cathode. Calcium hydroxide (Ca(OH)2) was used as source of Ca2+ ion. Initial sulfate concentration was around 18,000 ppm which was brought below the drinking water regulation level (250 ppm – USEPA or 200 ppm – IS 10500 : 2012) in two stages of the experiment. First stage was of six hours’ duration and the second stage was of 8-10 hours’ duration. Experiments were done in batch mode in which sulfate containing wastewater sample was taken in a vessel and lime slurry was added continuously with the help of peristaltic pump. To optimize the EC process and make it less costly, electrode configuration and spacing selection, current density selection, and optimization of lime flow rate and concentration for both stages separately is discussed. Characterization of the precipitate was also done with DTA/TGA and powdered X-ray diffraction (P-XRD) which showed ettringite precipitation and presence of calcite as impurity. This dissertation also discusses the theoretical aspects related to different electrochemical processes of water/wastewater treatment and specifically EC process, its mechanism, its applicability for different contaminants, and advantages and limitations.