Influence of algal blooms on the efficacy of La/Al-based phoslock in the control of phosphorus release from sediment in shallow lakes: a microcosm study

One of the most significant problems in eutrophic lakes is the presence of algal blooms, which may affect phosphorus (P) inactivation agents (PIAs) in the control of P release from sediment. Therefore, in this study, lanthanum/aluminium co-modified thermally treated calcium-rich attapulgite (LA@TCAP) is used to analyse the influence and mechanisms of algal blooms on P inactivation effect to provide technical support for the application of PIAs. In August 2020, lake water, sediments, and algae were collected from Zhushan Bay in Lake Taihu and LA@TCAP was prepared in the laboratory. These samples were used to establish three groups of microscopic simulation experiments. Through sequential extraction and 31P nuclear magnetic resonance, the mobile forms of P (Mobile-P) of sediment in each experimental group were measured and analysed, and the influence of algal blooms on the inactivation effect of the P immobilisation of LA@TCAP was determined. Physical and chemical properties of overlying water (DO, pH, algae organic matter (AOM), etc) and microbial community structure of capping layers were used to understand the influence mechanisms of algal blooms on the inactivation effect of PIAs. The concentration of dissolved oxygen (DO) concentration in the overlying water was reduced due to the decomposition of algal blooms, which increased the relative abundance of P-solubilising bacteria, and transformed more inert forms of P (Inert-P) into Mobile-P in the capping layer. Simultaneously, the algal blooms released OP, which passed through the capping layer and increased the OP content in the sediment. Under this dual effect, the Mobile-P content in the sediment increased, making LA@TCAP unable to inactivate the increased mobile-P, which was bound to affect the inactivation effect of LA@TCAP. Besides, the AOM released from the algal blooms combined with the metal ions in LA@TCAP, resulting in the reduction of adsorption sites of LA@TCAP for P. The algal blooms significantly decreased the DO concentration in the overlying water, thereby affecting the microbial community and transforming more Inert-P into Mobile-P. In addition, the adsorption performance of LA@TCAP for P also reduced owing to AOM competed with P for the metal ions in the LA@TCAP. Thus, algal blooms reduced the ability of LA@TCAP to control the P release from sediment.