Responses in sediment phosphorus and lanthanum concentrations and composition across 10 lakes following applications of lanthanum modified bentonite

A combined field and laboratory scale study of 10 European lakes treated between 2006 and 2013 with a lanthanum (La) modified bentonite (LMB) to control sediment phosphorus (P) release was conducted. The study followed the responses in sediment characteristics including La and P fractions and binding forms, P adsorption capacity of discrete sediment layers, and pore water P concentrations. Lanthanum phosphate mineral phases were confirmed by solid state 31P MAS NMR and LIII EXAFS spectroscopy. Rhabdophane (LaPO4 · nH2O) was the major phase although indications of monazite (LaPO4) formation were also reported, in the earliest treated lake. Molar ratios between La and P in the sediments were generally above 1, demonstrating excess La relative to P. Lanthanum was vertically mixed in the sediment down to a depth of 10 cm for eight of the ten lakes, and recovery of La in excess of 100% of the theoretical aerial load indicated translocation of the LMB towards the deepest areas of the lakes. Lanthanum was generally recovered from bed sediment samples following sequential chemical extraction from the HCl fraction. Soluble reactive P (SRP) release experiments on intact sediment cores indicated conditions of P retention (with the exception of two lakes) by sediments, indicating effective control of sediment P release, i.e. between two and nine years after treatment

A meta-analysis of water quality and aquatic macrophyte responses in 18 lakes treated with lanthanum modified bentonite (PHOSLOCK®)

Lanthanum (La) modified bentonite is being increasingly used as a geo-engineering tool for the control of phosphorus (P) release from lake bed sediments to overlying waters. However, little is known about its effectiveness in controlling P across a wide range of lake conditions or of its potential to promote rapid ecological recovery. We combined data from 18 treated lakes to examine the lake population responses in the 24 months following La-bentonite application (range of La-bentonite loads: 1.4 to 6.7 tonnes ha-1) in concentrations of surface water total phosphorus (TP; data available from 15 lakes), soluble reactive phosphorus (SRP; 14 lakes), and chlorophyll a (15 lakes), and in Secchi disk depths (15 lakes), aquatic macrophyte species numbers (6 lakes) and aquatic macrophyte maximum colonisation depths (4 lakes) across the treated lakes. Data availability varied across the lakes and variables, and in general monitoring was more frequent closer to the application dates. Median annual TP concentrations decreased significantly across the lakes, following the La-bentonite applications (from 0.08 mg L-1 in the 24 months pre-application to 0.03 mg L-1 in the 24 months post-application), particularly in autumn (0.08 mg L-1 to 0.03 mg L-1) and winter (0.08 mg L-1 to 0.02 mg L-1). Significant decreases in SRP concentrations over annual (0.019 mg L-1 to 0.005 mg L-1), summer (0.018 mg L-1 to 0.004 mg L-1), autumn (0.019 mg L-1 to 0.005 mg L-1) and winter (0.033 mg L-1 to 0.005 mg L-1) periods were also reported. P concentrations following La-bentonite application varied across the lakes and were correlated positively with dissolved organic carbon concentrations. Relatively weak, but significant responses were reported for summer chlorophyll a concentrations and Secchi disk depths following La-bentonite applications, the 75th percentile values decreasing from 119 μg L-1 to 74 μg L-1 and increasing from 398 cm to 506 cm, respectively. Aquatic macrophyte species numbers and maximum colonisation depths increased following La-bentonite application from a median of 5.5 species to 7.0 species and a median of 1.8 m to 2.5 m, respectively. The aquatic macrophyte responses varied significantly between lakes. La-bentonite application resulted in a general improvement in water quality leading to an improvement in the aquatic macrophyte community within 24 months. However, because, the responses were highly site-specific, we stress the need for comprehensive pre- and post-application assessments of processes driving ecological structure and function in candidate lakes to inform future use of this and similar products

Eutrophication management in surface waters using lanthanum modified bentonite: a review

This paper reviews the scientific knowledge on the use of a lanthanum modified bentonite (LMB) to manage eutrophication in surface water. The LMB has been applied in around 200 environments worldwide and it has undergone extensive testing at laboratory, mesocosm, and whole lake scales. The available data underline a high efficiency for phosphorus binding. This efficiency can be limited by the presence of humic substances and competing oxyanions. Lanthanum concentrations detected during a LMB application are generally below acute toxicological threshold of different organisms, except in low alkalinity waters. To date there are no indications for long-term negative effects on LMB treated ecosystems, but issues related to La accumulation, increase of suspended solids and drastic resources depletion still need to be explored, in particular for sediment dwelling organisms. Application of LMB in saline waters need a careful risk evaluation due to potential lanthanum release

Lanthanum in Water, Sediment, Macrophytes and chironomid larvae following application of Lanthanum modified bentonite to lake Rauwbraken (The Netherlands)

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.</p

Geo-Engineering in Lakes: A Crisis of Confidence?

The effective management of lakes suffering from eutrophication is confounded by a mosaic of interactions and feedbacks that are difficult to manipulate. For example, in lake processes can delay the relinquishment of legacy phosphorus (P) manifested within bed sediments for decades, even after effective catchment management. This recovery time is often deemed unacceptable and researchers have explored many in-lake management measures designed to “speed-up” recovery. The manipulation of biogeochemical processes (commonly targeting P) using materials to achieve a desired chemical and/or ecological response has been termed geo-engineering in lakes, and is becoming a commonly considered eutrophication management tool (Figure 1). Although this approach has been employed for many years it remains contentious largely due to variable results reported in the literature. This uncertainty risks ineffective management based on poorly designed or inappropriate applications. To address this, it is important that current levels of confidence in the approach be effectively communicated and that methods of increasing confidence are clearly demonstrated. We draw here on experiences of researchers and water managers at a global scale to demonstrate recent advances and consensus on recommendations (numbered below) for best practice. This information, although vital to underpinning successful management, has not been available in the peer reviewed literature