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Research article - Peer-reviewed, 2018

Magnetite-coated biochar as a soil phosphate filter: From laboratory to field lysimeter

Riddle, Matthew; Cederlund, Harald; Schmieder, Frank; Bergstrom, Lars

Abstract

Agriculture is one of the largest anthropogenic contributors of phosphorus (P) to surface waters and reducing these P loads is a target set by many countries. One potential mitigation strategy could be to utilize biochar, functionalized by co-precipitating metal ions onto its surface to adsorb phosphates. We evaluated such an approach in a series of laboratory experiments and a three-year field lysimeter study. Leached phosphates were trapped in a 3-cm layer of a commercially available 20% hardwood/80% Norway spruce biochar, coated with magnetite and placed under the soil root zone. Langmuir maximum adsorption potential (Qmax) was 3.38 mg P g(-1) biochar, as determined from batch adsorption studies performed at pH 6.5. Further laboratory experiments revealed that adsorption was strongly negatively correlated with pH (R-2 = 0.995). In a laboratory column study using high flow rates, no difference was found in phosphate adsorption between coated biochar and the control (no biochar), after application of ammonium phosphate at a rate corresponding to 22 kg P ha(-1). However, differences were observed at higher application rates (285 or 570 kg P ha(-1)). Calculation of a Fe:P molar ratio to evaluate the magnetite treatment method from laboratory (2:1) to lysimeter (248:1) suggested a relatively effective laboratory performance, but overall poor field efficiency. Nevertheless, biochar in the lysimeter study still reduced phosphate leaching from two organic soils, by 62% (P < 0.05) and 35% (NS). This suggests that, even though the method of magnetite-coating biochar is not necessarily resource-efficient, it is effective in removing P from soil leachate.

Keywords

Biochar; Leaching; Lysimeter; Magnetite; Phosphorus; Soil

Published in

Geoderma
2018, Volume: 327, pages: 45-54