Insights into the response of Anammox sludge to the individual and combined stress of neodymium and salinity and the role of hydroxylamine regulation

Abstract

Neodymium (Nd(III)) and sulfate (Na2SO4) are commonly present in neodymium mining wastewater. However, the effects of their coexistence on microorganisms responsible for removing nitrogen from wastewater have been poorly reported. In this study, the combined inhibition of Nd(III) and Na2SO4 on anaerobic ammonium oxidation (Anammox) and its recovery were investigated in terms of performance, kinetics, Nd(III) distribution characteristics and sludge morphology. The results shown that ≥ 150 mg·L−1 Nd(III) or ≥ 3 g·L−1 Na2SO4 decreased specific Anammox activity (SAA), and intracellular Nd(III) was identified as the primary factor for individual Nd(III) inhibition. Furthermore, the combined inhibition of Nd(III) and Na2SO4 on Anammox was stronger than individual inhibition, but low doses of hydroxylamine (NH2OH) could restore the Anammox. Notably, the combined effect of low doses of Na2SO4 (1 g·L−1) and Nd III (10 mg·L−1) on the long-term performance of Anammox reactor was not significant. However, when the dose of Nd(III) was kept constant and the dose of Na2SO4 was increased to 7 g·L−1, the total nitrogen removal efficiency (TNRE) decreased by 56.88%. Following the elimination of inhibitors and the addition of 5 mg·L−1 NH2OH, the TNRE of the Anammox reactor was increased from 20.83% to 29.78%. The Hill, modified Michaelis-Menten, and modified Boltzmann models were able to properly describe the inhibition effects of Nd(III) and Na2SO4 and recovery process of Anammox. In addition, scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) analysis revealed that the content of extracellular polymeric substances (EPS) and neodymium increased with increasing Na2SO4 concentration.