Investigation of sodium silicate-derived gels as encapsulants for hazardous materials – The case of scorodite

Abstract

High content arsenic waste generated in the metallurgical industry can be converted into a synthetic mineral, scorodite, FeAsO4·2H2O, and deposited into a landfill site. Scorodite is most stable in weakly acidic to neutral pH range under oxic conditions. A novel way to enhance the range of stability for scorodite is to encapsulate it with an inert material. In this work, silicate gel is developed and investigated as a possible encapsulating material for scorodite. The initial method of gel formation in this study produced a silicate gel with high alkalinity (pH 10) that was incompatible with scorodite. A reverse titration method was developed producing a gel with optimum pH profile (5–6.5). This technique proved to have only marginal effect on scorodite stabilization prompting an investigation of different ageing techniques (drying; 22, 44°C and hydrothermal treatment; 110, 160°C) as a means of producing silica-like coatings with better stabilization potential. Interestingly most of these measures proved counterproductive as aged scorogels showed a higher release of As than scorodite alone. Through surface-sensitive depth profile analysis (XPS), and molecular-sensitive analysis (Raman and FTIR mapping), it was discovered that the silicate engaged into an “ion-exchange” type reaction on the surface of scorodite by bonding to iron, hence the observed release of arsenic. Development of a hydrothermally-induced iron silicate layer may lead to an effective encapsulant.