Abstract

Lanthanum Modified Bentonite (LMB; Phoslock®) is used to mitigate eutrophication by binding phosphate released from sediments. This study investigated the fate of lanthanum (La) from LMB in water, sediment, macrophytes, and chironomid larvae in Lake Rauwbraken (The Netherlands). Before the LMB application, water column filterable La (FLa) was 0.02 µg L−1, total La (TLa) was 0.22 µg L−1. In sediment the total La ranged 0.03–1.86 g m−2. The day after the application the maximum FLa concentration in the water column was 44 µg L−1, TLa was 528 µg L−1, exceeding the Dutch Maximum Permissible Concentrations (MPC) of 10.1 µg L−1 by three to fourfold. TLa declined below the MPC after 15 days, FLa after 75 days. After ten years, FLa was 0.4 µg L−1 and TLa was 0.7 µg L−1. Over the post-application years, FLa and TLa showed statistically significant downward trends. While the LMB settled homogeneously on sediment, after 3 years it redistributed to 0.2–5.4 g La m−2 within shallow zones, and 30.7 g m−2 to 40.0 g La m−2 in deeper zones. In the upper 20 cm of sediment, La concentrations were 7–6702 mg kg −1 dry weight (DW) compared to 0.5–7.0 mg kg−1 before application. Pre-application anaerobic sediment release of FLa was 0.006 mg m−2 day−1. Three months after the application it was 1.02 mg m−2 day−1. Three years later it was 0.063 mg m−2 day−1. Before application La in plants was 0.8–5.1 mg La kg−1 DW, post-application values were up to 2925 mg La kg−1 DW. In chironomid larvae, La increased from 1.7 µg g−1 DW before application to 1421 µg g−1 DW after one month, 3 years later it was 277 µg g−1 DW. Filtration experiments indicate FLa is not truly dissolved free La3+ cations.

Highlights

  • Eutrophication of freshwater lakes is considered the most important water quality problem worldwide (Smith and Schindler, 2009)

  • This study reports on the fate of La via the analysis of water column and sediment samples, in addition to that within macrophytes and chironomid larvae following the LMB application in Lake Rauwbraken

  • Including the two extreme concentrations, the post-application TLa ranged from 0.20 mg LÀ1 to 200 mg LÀ1 with a mean 5.5 mg LÀ1, (SD = 9.8 mg LÀ1, n = 603), excluding the extremes, Filterable La (FLa) concentrations ranged from 0.20 mg LÀ1 to 39.1 mg LÀ1 with a mean 5.1 mg LÀ1, (SD = 5.5 mg LÀ1, n = 601)

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Summary

Introduction

Eutrophication of freshwater lakes is considered the most important water quality problem worldwide (Smith and Schindler, 2009). A concomitant reduction of external- and internal P loads is considered essential to effectively mitigate cyanobacterial blooms (Cooke et al, 2005; Hilt et al, 2006). Internal P loading from lake sediments often hampers the lake recovery for years to decades following external nutrient load reduction (Gulati and Van Donk, 2002; Schindler and Hecky, 2009; Søndergaard et al, 2001). Internal Ploads can be reduced by sediment capping techniques that provide a physico-chemical barrier between the sediment and the overlaying water column. To this end, aluminium-, calcium-, and ironsalts have long been applied as the remediation technique of choice Studies have included testing the effect of La on zooplankton (Lürling and Tolman, 2010), effect of humic substance in La complexation (Lürling et al, 2014), incorporation of La in animals both in a laboratory study (Oosterhout et al, 2014), and after field application (Waajen et al, 2017a), the vertical distribution of La over the sediment following application (Meis et al, 2012, 2013), La:P ratios in the sediment of a LMBtreated lake (Yasseri and Epe, 2016) and the confirmation of the presence of rhabdophane (LaPO4ÁH2O) formation (Slade and Gates, 1999) in sediments of 10 treated lakes (Dithmer et al, 2016b)

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