Mn-C micro-electrolysis filler for nutrient removal in constructed wetlands: Performance evaluation and biochemistry processes

Abstract

Insufficient provision of electron donors for denitrification often hinders nitrogen removal performance. While using a combination of iron and carbon can mitigate this problem, the surface passivation and limited electrical potential difference between Fe and C electrodes restricted the electron transfer efficiency. In this study, a manganese‑carbon carrier (Mn-C) was prepared and used for micro-electrolysis (Mn-C-ME) in conjunction with biological denitrification. Four constructed wetlands (CWs), including three Mn-amended CWs filled with low/middle/high content of Mn-C and one control, were established to investigate nutrient removal performance and underlying mechanisms under low C/N conditions. The results showed that Mn-amended CWs significantly enhanced total nitrogen removal from 31.01% to an optimal 91.06%. Nitrate was directly reduced by Mn(0), while Mn-C-ME generated inorganic electron donors ([H]/H2 and Mn2+) in situ for denitrification. The use of Mn ores oxidized ammonium and immobilized residual Mn2+ produced by Mn-C-ME. High-efficiency total phosphorus removal confirmed the formation of Mn-P precipitation. Moreover, the addition of Mn-C and Mn ores resulted in a noticeable difference in microbial community structures. Autotrophic denitrifying bacteria were abundant and positively correlated with Mn-oxidizing bacteria (MnOB) suggesting Mn(II)-driven autotrophic denitrification occurred. The high proportions of proteins, along with the detection of Mn2+ in extracellular polymeric substances, indicated biofilms might regulate the release of Mn2+ and supply a medium for Mn cycling, thus improving electron utilization. Our findings provide a new strategy to enhance nitrogen removal by coupling with multiple Mn-containing substrates.