A lab study of an iron-coated biochar as a phosphate mitigation tool

Sammanfattning: Nutrients in leachate water resulting from manmade activities are recognized as a major problem leading to eutrophication worldwide. In Sweden and other Baltic countries, agricultural activities are a big contributor to eutrophication of the Baltic Sea. In order to eliminate these problems, different techniques have been trialled over the years with mixed results. One method using magnetite biochar has been explored due to its adsorption properties allowing it to attract negatively charged chemical ions such as phosphate in leachate water. Biochar itself has a limited capacity to attract phosphate via surface-surface molecular interaction, but chemical treatment of plain biochar can change its chemical behaviour. The main objective of this present experiment was to determine the adsorption properties of magnetite biochar as a phosphate mitigation tool in a laboratory study. Biochar, “Skogens kol”, was treated with an iron(II) /iron(III)-mixture resulting in formation of an iron oxide, magnetite, on the biochar surface. Adsorption experiment results revealed the magnetite biochar adsorbed twenty times as much phosphate in comparison to the non-treated biochar, where adsorption was little to none. A follow up column leaching experiment showed that the highest phosphate application rates had accumulated an additional ± 5.6 mg PO4 and ± 9.3 mg PO4 respectively for columns containing magnetite biochar in comparison to columns containing only sand, indicating adsorption of phosphate at the magnetite biochar. The contact time between the biochar and the phosphate, < 10 hours, was much lower in the column experiment in contrast to the adsorption experiment and is therefore suggested as a main contributor to the differing results. Both the adsorption and column leaching experiments showed distinct adsorption abilities when using the Fe-coated biochar in comparison to the control. The results suggest that application of Fe-coated biochar as a phosphate mitigation tool has potential for commercial application. However, further research on the importance of contact time with phosphate, pH conditions and stability of the biochar’s adsorption properties over time is suggested.