Microbial action and mechanisms for Cr(VI) removal performance by layered double hydroxide modified zeolite and quartz sand in constructed wetlands

This study was conducted to evaluate the performance underlying the removal of hexavalent chromium Cr(VI) associated with Zn-layered double hydroxides (Zn-LDHs)-modified substrates utilized in simulated constructed wetlands (CWs) from a microbial perspective. To accomplish this, Zn-LDHs-modified substrates (zeolite and quartz sand (QS)) were synthesized at various Zn2+/Al3+ and Fe3+ molar ratios by co-precipitation under alkaline conditions. The experimental group was then compared with a control group to determine the microbial action responsible for Cr(VI) removal during the Cr(VI) removal experiments. The removal experiment revealed that the average Cr(VI) removal rates of the Zn-LDHs-modified substrates were superior to those of natural substrates. Subsequent evaluation of the microbial structure by Illumina high-throughput sequencing revealed that the relative abundance of Novosphingobium, Brevundimonas, Methylophilus, and Acidovorax related to Cr(VI) removal was relatively high in Zn-LDHs-modified substrates. Moreover, the extracellular polymeric substance (EPS) content was significantly influenced by the Zn-LDHs coating according to the relative microbial experiments. Similar trends were observed in enzyme activity. Taken together, these findings illustrated that the Zn-LDHs coating had a significant impact on microbial action, and the Cr(VI) removal efficiency of the Zn-LDHs-modified QS (zeolite) substrate was better than that of the natural substrate because of intracellular and extracellular removal mechanisms. Briefly, the microbial action of Zn-LDHs-modified QS played an important role in Cr(VI) removal, since the EPS content possessed the appropriate concentrations. Moreover, the microbial activity of ZnAI-LDHs-modified QS (zeolite) may have been higher than that of ZnFe-LDHs-modified QS (zeolite) because Al had a stronger promoting effect on Cr(VI) bio-removal than Fe. Therefore, the microbial Cr(VI) removal supported by ZnAl-LDHs-modified QS is a better choice for CWs.