Eutrophication, driven by excessive nitrogen (N) and phosphorus (P) in aquatic environments, remains a significant global water pollution challenge. While constructed wetlands (CWs) are widely recognized for their efficiency in wastewater treatment, the potential of enhancing natural sediment wetlands (SWs) with adsorbent materials has remained less explored. A recent study published in Scientific Reports by Wei Zhou, Hao Chen, Xinjuan Cui, Dongxu Cui, and Qingqing Cao investigates the impact of biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More and activated carbonActivated carbon is a form of carbon that has been processed to create a vast network of tiny pores, increasing its surface area significantly. This extensive surface area makes activated carbon exceptionally effective at trapping and holding impurities, like a molecular sponge. It is commonly More on the purification efficiency of both CWs and SWs under varying pollution loads.

The researchers established experimental CWs and SWs using biochar and activated carbon as substrates and subjected them to both high and low N and P pollution loads. The study revealed a significant interaction between the pollution load and wetland type, indicating that the purification capacities of biochar and activated carbon are indeed influenced by the pollution load.

Under low pollution loads, all wetland units demonstrated over 80% total phosphorus (TP) removal, with the highest efficiency observed in biochar-amended constructed wetlands (BCW). However, surprisingly, the addition of activated carbon and biochar in SWs actually limited TP removal under low loads. For ammonium (NH4+​), CWs showed significantly higher purification efficiency (84.90%) than SWs (77.88%) under low loads, while the type of substrate addition had an inconspicuous effect. Nitrate (NO3−​) removal was also notably superior in CWs (66.79%) compared to SWs (33.13%), and interestingly, biochar or activated carbon additions were not effective in CWs for nitrate removal. In SWs, however, the addition of biochar and activated carbon did significantly enhance NO3−​ removal.

The picture changed significantly under high pollution loads. CWs maintained a superior TP removal efficiency compared to SWs. Specifically, the BCW demonstrated a high TP removal of 94.57%, while the biochar-amended natural sediment wetland (BSW) achieved an impressive 93.13% TP removal. This indicates that the addition of biochar to SWs significantly promotes TP removal under high pollution loads. For NH4+​, CWs showed an average purification efficiency of 91.28% under high loads, significantly higher than SWs (67.68%). Biochar addition benefited NH4+​ purification in SWs, but activated carbon limited microbial metabolism. In terms of NO3−​, BCW (77.12%) and activated carbon-amended constructed wetlands (ACW) (75.94%) exhibited the highest and most stable removal rates, significantly outperforming unamended CWs. However, nitrate removals remained very low in SWs, even with biochar or activated carbon.

A key takeaway is that the self-purification ability of SWs is limited under increasing pollution loads, with TP removal efficiency decreasing from 86.56% to 75.97% as the load increased. However, the addition of biochar significantly enhanced this removal efficiency to 94.57% in SWs, demonstrating its crucial role in complementing the wetland’s natural treatment capacity under stressed conditions. This suggests that biochar can promote the sustainability of SW purification. In contrast, activated carbon was found to limit TP removal in SWs, potentially due to its unsuitable surface charge properties and pore structure for P adsorption, and its negative impact on microbial abundance and pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More stability. Microbial analysis further supported this, showing that activated carbon reduced bacterial richness and diversity in CWs.

This study confirms the hypothesis that the addition of biochar can promote the sustainability of SW purification and that both biochar and activated carbon exhibit varying adsorption characteristics depending on the wetland type and pollution load. These findings offer valuable practical insights for sustainable wastewater treatment and model development, particularly highlighting the improved purification potential in biochar-added SWs.

Source: Zhou, W., Chen, H., Cui, X., Cui, D., & Cao, Q. (2025). The impact of biochar and activated carbon on the purification efficiency of two wetland systems under varying pollution loads. Scientific Reports, 15(19927).