The removal of uranium (VI) from aqueous solution by the anaerobically digested sewage sludge with hydrothermal pretreatment

Anaerobic digestion (AD) with hydrothermal (HT) pretreatment (sequential HT-AD treatment) is a novel technology for sludge management. HT-AD sludge is rich in functional groups and its applications as pollutant sorbents might be a win-win strategy. This study investigated the removal of uranium (VI) from water using HTAD sludge as affected by solution pH, temperature, and ion strength. The reusability and heavy metal risk of HTAD sludge were also assessed. The batch sorption experiments demonstrated that even at an acidic initial pH of 3.2, the maximum adsorption of HT-AD sludge for uranium (VI) reached 117.13 mg/g, higher than that of most carbon-based materials. The inner-sphere and out-sphere complexation between uranium (VI) and the HT-AD sludge dominated the adsorption when pH was in the range of 2-6 and 6-11, respectively. The FTIR and XPS analysis indicated that the primary mechanisms of uranium (VI) adsorption by the HT-AD sludge were the surface complexation and the electric attraction between uranium (VI) and the functional groups (e.g. -COO-) on HT-AD sludge. The removal rate of uranium (VI) by HT-AD sludge only decreased by -7% after 3 consecutive adsorption cycles. Leaching experiment showed that less than 5% of the total heavy metal were released from HT-AD sludge. Our research proved that HT-AD sludge can be used as an efficient uranium (VI) adsorbent with good reusability and environmental safety.