Molecular-scale characterization of groundwater treatment sludge from around the world: Implications for potential arsenic recovery

Iron (Fe)-based treatment methods are widely applied to remove carcinogenic arsenic (As) from drinking water, but generate toxic As-laden Fe (oxyhydr)oxide waste that has traditionally been ignored for resource recovery by the water sector. However, the European Commission recently classified As as a Critical Raw Material (CRM), thus providing new incentives to re-think As-laden groundwater treatment sludge. Before As recovery techniques can be developed for groundwater treatment waste, detailed information on its structure and composition is essential. To this end, we comprehensively characterized sludge generated from a variety of As-rich groundwater treatment plants in different geographic regions by combining a suite of macroscopic measurements, such as total digestions, leaching tests and BET surface area with molecular-scale solid-phase analysis by Fe and As K-edge X-ray absorption spectroscopy (XAS). We found that the As mass fraction of all samples ranged from ∼200-1200 mg As/kg (dry weight) and the phosphorous (P) content reached ∼0.5-2 mass%. Notably, our results indicated that the influent As level was a poor predictor of the As sludge content, with the highest As mass fractions (940-1200 mg As/kg) measured in sludge generated from treating low groundwater As levels (1.1-22 µg/L). The Fe K-edge XAS data revealed that all samples consisted of nanoscale Fe(III) precipitates with less structural order than ferrihydrite, which is consistent with their high BET surface area (up to >250 m2/g) and large As and P mass fractions. The As K-edge XAS data indicated As was present in all samples predominantly as As(V) bound to Fe(III) precipitates in the binuclear-corner sharing (2C) geometry. Overall, the similar structure and composition of all samples implies that As recovery methods optimized for one type of Fe-based treatment sludge can be applied to many groundwater treatment sludges. Our work provides a critical foundation for further research to develop resource recovery methods for As-rich waste.