Degradation and transfer of polyacrylamide based flocculent in sludge and industrial and natural waters
Abstract
Flocculants are widely used in numerous industrial domains (food-processing industry, mineral industries, chemical industries, water purification, wastewater treatment) to im-prove the solid/liquid separation in water-containing fine par-ticles. Thanks to the addition of flocculants, particles gather together and form flocs that improve the materials aggregation and wastewater clarification. At the molecular level, floccu-lants can be mineral polymers or natural organic polymers, but synthetic organic polymers constitute the main flocculants. Generally, they are anionic polyacrylamides produced by polymerisation of acrylamide and the anionic co-monomer, the sodium acrylate. Monomers are only partially eliminated during polyacrylamide preparation and remain at the state of tracks in the end product. According to Smith et al. (1996), the residual content in monomers varies between 0.05 and 5 %. Sodium acrylate presents no harmfulness like the polyacryl-amide (Stephens, 1991). However, as the acrylamide is con-sidered as a carcinogenic molecule, mutagen and reprotoxic (Molak, 1991), all polyacrylamides (PAMs) used within the European Union are required to contain less than 0.1 % (w/w) of residual acrylamide (AMD) (European Parliament 1999) unless they are classified and labelled as a category 2 carcin-ogen (European Parliament 2006). More stringent thresholds are also set depending on additional regulations covering specific uses like, for example, when PAM-based flocculants are used for the treatment of drinking water (no more than 0.05 % of residual AMD). The potential risks on environment and health are thus linked to the spreading of acrylamide and polyacrylamide degradation products in the natural environment. Two levels of spreading have to be considered: – The water: about 95 % of the polymers adsorb on the