Stability of As-rich residues from pilot biofilters for passive treatment of neutral mine drainage: Laboratory vs field testing

Passive biofilters efficiently treat As-rich contaminated neutral drainage (As-CND), while the management of contaminated residues remains uncertain. Environmental stability of As-CND post-treatment residues from 3 pilot-scale biofilters lab vs field was assessed. Twenty solid samples were collected from four layers of each biofilter (I-1, A, B and I-2), while As removal mechanisms were assessed via physiochemical and mineralogical characterization. Environmental stability was evaluated using static leaching tests (TCLP, SPLP, and FLT). Results showed highly contaminated residues, with concentrations of As up to 3.6 g/kg, and of Fe up to 86.3 g/kg. Metal oxyhydroxides and silicates were the main mineral phases in the three biofilters. The retention of As was controlled by sorption onto Fe (III)-oxyhydroxides in the laboratory biofilter, whereas in field-pilot biofilters As removal involved sorption in the upper layers but precipitation in the form of FeAsS and co-precipitation with metal (Fe and Al)-oxyhydroxides was found in the deeper layers. Static tests results showed compliance (USEPA) classifying the residues of as non-hazardous and suitable for potential co-disposal with municipal waste. The TCLP results also met Quebec's regulation on mine effluents classifying them as low risk. These findings indicate passive biofilters as promising for the control of As-CND.