Restoration of the shallow lake Kralingse Plas, the Netherlands, with emphasis on lanthanum-modified bentonite

Lake Kralingse Plas (Rotterdam, the Netherlands), with a surface area of 114 ha and an average depth of 2.3 m, regularly suffered from cyanobacterial blooms. To improve water quality and meet the requirements of the European Union Water Framework Directive (WFD), the responsible authorities, following a system analysis, applied 1064 tonnes of lanthanum-modified bentonite (LMB, commercial name Phoslock®) to the entire lake in November 2021. Water quality was monitored by the water authorities on a monthly basis. Just before the LMB application, at 3, 15, and 36 months post-LMB application, sediment phosphorus (P) release, sediment P fractionation and Lanthanum (La) fate in the sediment were estimated. Following the application, the sediment La content was significantly higher in the top 4 cm of the sediment layers compared to pre-application conditions. The "HCl-P" pool was increased after LMB application, reflecting LaPO₄ formation. It was the highest P fraction, increasing from 53.7% at pre-application to 59.5% after 3 months, further increasing to 62.6% of total P after 15 months, and 66.7% after 36 months. This suggested that LMB reduced sediment P release by increasing the non-bioavailable P in the sediment. The average filterable P (FP) fluxes in the sediment cores under anoxic conditions were 6.27 (±4.4), 0.29 (±0.21), -4.65 (±3.1) and 0.27 (±0.42) mg P m^-2 day^-1 in cores taken before the application, 3, 15, and 36 months after the application, respectively. In addition, to address the LMB efficacy on P release at different pH levels (pH 7 and pH 10), a 256-day sediment core incubation was conducted. The release rate of P following LMB application did not differ between pH 7 and pH 10, indicating the stability of La-bound P under alkaline conditions. This finding is particularly relevant for shallow eutrophic lakes, where high pH and internal P loading may occur.