What controls the chemical speciation of abundant heavy metals during wastewater treatment: insights from combined spectroscopic and modeling analyses

Abstract

Speciation of heavy metals (HMs) in sewage sludges (biosolids) is fundamental to understanding important physicochemical and biological processes in wastewater and sludge treatments, operation improvement, risk evaluation of biosolid land applications. Speciating even the most abundant HMs remains analytically challenging and the main processes and factors controlling the speciation remain poorly understood. In this work, we conducted meta-analysis on spectroscopic speciation data of sewage sludges with different generation and processing conditions, and revealed the consistency and variation in the chemical speciation of iron (Fe), zinc (Zn), and copper (Cu). Formation/prevalence conditions for the main chemical species identified spectroscopically were simulated using the PHREEQC code. Modeling results were consistent with experimental results and showed how specific redox conditions, elemental composition, and pH affect the prevalence of the main chemical species. Combined analysis of the spectroscopic and modeling results also revealed some discrepancies that are originated from characteristics of wastewater treatment processes and different kinetics of chemical species under redox or compositional fluctuations. Results from this work provide a comprehensive and predictive insight into the dynamic transformation of several abundant elements and their coupling during wastewater and sludge treatments.