Purification of Acid Sulfate Soil Runoff Water Using Biochar: a Meso-Scale Laboratory Experiment

Acid sulfate soils worldwide pose a risk to water bodies due to acidic, metal-rich runoff. Efficient water protection methods to reduce this diffuse load in forestry sites do not exist currently. Biochar is a promising adsorbent due to its high porosity and ion exchange capacity but has not been studied for water protection for forestry in acid sulfate soils. Our objective was to study the metal adsorption capacity of biochar for acid sulfate soil runoff water, where several metals are competing for the adsorption. We also assessed whether the use of wood ash in biochar reactors can improve adsorption. Furthermore, we studied if desorption occurs when the metal concentrations in the water decrease. In a meso-scale laboratory experiment, hundreds of liters of runoff water from acid sulfate soils were circulated through biochar and biochar-ash filled reactors. We extracted water samples from the inlet and outlet of the reactors and determined the metal concentrations (Al, Fe, Co, Ni, Cu, Zn, and Cd). These were used to construct adsorption kinetics models. We studied desorption by diluting the water and measuring the concentration changes. Biochar increased the solution pH and adsorbed multiple metals simultaneously. The adsorption capacity and rate were higher in biochar-ash reactors than in the biochar reactors. Biochar-ash reactors adsorbed 70–99% of six of the seven metals. However, Al was released from ash. We observed no significant desorption after the dilutions. Therefore, biochar could be considered as a potential water protection tool in forests located on acid sulfate soils.