Efficient removal of phosphorus from constructed wetlands using solidified lanthanum/aluminum amended attapulgite/biochar composite as a novel phosphorus filter

Developing a suitable substrate with high phosphorus (P) sorption capacity, low solubility, and high hydraulic loading for constructed wetlands (CWs) is crucial for their functions. In this study, we used attapulgite and biochar as base materials to prepare a lanthanum/aluminum (La/Al) amended attapulgite/biochar composite as a novel P filter using a one-step drying process and subsequent high-temperature thermal treatments. Results indicated that the solidified poly aluminum chloride (PAC) amended attapulgite/biochar (SAl@AB) has a higher solubility than the solidified La-modified attapulgite/biochar (SLa@AB) and the solidified PAC and La co-modified attapulgite/biochar (SAlLa@AB). Therefore, SAl@AB is not suitable to be used as a substrate for constructed wetlands (CWs). Batch studies indicated that SLa@AB and SAlLa@AB have maximum P sorption capacities of 12.8 mg/g and 21.3 mg/g, respectively. The P sorption rates are higher than those found in most substrates used in constructed wetlands. Additionally, pH and coexisting ions exert minor effects on the P removal performance of SAlLa@AB. Column experiments indicated that longer hydraulic retention time (HRT) favors the removal of influent P. A 120-day column experiment indicated that an average of 95% of the P influent (10 mg P/L) could be removed by the SAlLa@AB with an HRT of 8 h. The P forms analyzed by sequential extraction indicated that P removed by SAlLa@AB occurs through the formation of calcium-bound and Al-bound P fractions, which can account for 68.7% and 18.8% of the total phosphorus, respectively. The formation of lanthanum/aluminum phosphate precipitation was the main P removal mechanism of SAlLa@AB. This was further confirmed by an XPS analysis, showing a formation of La-O-P and Al-O-P inner-sphere complexes after P sorption by SAlLa@AB. The results of this study indicate that SAlLa@AB was a promising substrate for future CWs.