Mitigation of phosphorus and arsenic pollution in soils: from the laboratory to the field

Phosphorus (P) is an essential element for biomass growth and is a major component of fertilizers applied to crops. Excessive application of P to agricultural soils may lead to P-leaching and increasing concentrations of this element in aqueous system. This may cause the degradation of water quality through eutrophication processes. Arsenic on the other hand is a very toxic element that may be present at high concentrations in soil and aqueous environments due to weathering processes or to anthropogenic sources as mining and agricultural activities.

To design adequate remediation techniques for the mitigation of phosphorus (P) and arsenic (As) pollution it is key to understand the geochemical processes and the environmental drivers controlling their availability. In soil systems, the mobility of P and As is controlled by their interaction with soil minerals. Among these minerals, iron and aluminum oxyhydroxides are known to form especially strong surface complexes with both components, limiting its availability to the environment. On the other hand, the presence of soil organic matter may compete with these pollutants for the mineral surface sites, increasing the contamination risk in the soil system.

In the present work, we have studied at laboratory scale the As and P adsorption process on iron mineral surfaces under variable conditions. This includes changes of pH, redox conditions, presence of major ions (i.e. calcium, sulfate) or other trace elements (i.e. chromate, copper), and the presence of natural organic matter (i.e. humic substances or simple organic acids). Also, surface complexation models were applied in order to simulate and predict the behavior of such systems. The obtained results reveal that the presence of organic matter or other anionic species are important factors capable of increasing the mobility of P and As contaminants, whereas changing the redox potential or the carbon content in the organo-mineral aggregates has little effect.

The information obtained, allowed to design remediation solutions that could be applied at field scale. The soil characteristics and the biogeochemical processes can be improved by the application of amendments based on circular economy aspects, i.e. compost, biochar, Technosols. We have studied the effectiveness of the superficial application of these materials to immobilize both P and As, varying the main environmental drivers affecting to the sorption or desorption processes.