Exploration of the distribution of heavy metals in extracellular polymeric substances of sludge

Due to the adsorption of heavy metals (HMs) onto extracellular polymeric substances (EPS) in dewatered sludge, the distribution of HMs among various EPS fractions remains unclear, thus impeding the advancement of efficient HM removal technologies. To investigate the persistence of EPS within sludge flocs following physical extraction and to determine the distribution of HMs in EPS, a systematic set of procedures and analytical tests were designed and implemented. The experimental results revealed a novel type of EPS, which is located closer to the cell wall and can be effectively extracted using chemical rather than physical methods. The phenomenon was elucidated through a combination of EPS extraction analysis, SEM analysis and soluble chemical oxygen demand (SCOD) analysis of the residual sludge following pretreatment with Ethylenediaminetetraacetic acid (EDTA) and citric acid (CA). A newly identified EPS fraction, termed chemical-extracted tightly bound EPS (C-TB-EPS), was observed, and 0.125 mol/L EDTA was determined to be the most appropriate extraction method for C-TB-EPS without causing significant disruption to the sludge flocs. Furthermore, the spatial distribution of HMs within EPS exhibited a distinct gradient, with their concentrations showing a marked increase in proximity to the cell wall. Quantitative analysis revealed that Zn, Ni, and Cu contributed 26.36%, 16.56%, and 0.51%, respectively, to the total HM inventory in the respective original sludge. The findings not only offer valuable insights into the spatial distribution and classification of EPS in dewatered sludge but also lay a foundation for further investigations into the development of HMs targeted removal strategies.