Arsenic is removed from aqueous phase through precipitation as arsenosulphides and/or co-precipitation and adsorption on iron sulphides. Studies were carried out to ascertain the stability of reduced biogenic arsenic and iron sulphide precipitates formed in an attached growth reactor (AGR) through a series of experiments based on Toxicity Characteristic Leaching Procedure (TCLP), aging and long term leaching tests. About half of the AGR was initially added with waste activated carbon (WAC) to support the growth of mixed microbial consortia and used for treatment of arsenic and iron contaminated simulated groundwater. The X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy results indicated that the biosolids were mainly composed of arsenosulphides and iron sulphides. While TCLP and aging tests were conducted in anoxic as well as oxic conditions with the aim to evaluate stability of biomass containing biogenic sulphides, long term leaching test was conducted through supply of aerated distilled water to evaluate the stability of spent WAC as well. Results generated from the research indicate that the concentration of leached arsenic never exceeded 123 mu g/L under all conditions tested, thus biosolids not imposing an environmental hazard.