Modelling rhizosphere transport in the presence of goethite, including competitive uptake of phosphate and arsenate

A modelling approach was used to extend the knowledge about the processes that affect the availability of the nutrient P and the toxic agent As-V in the rhizosphere in the presence of a strong sorbent. Based on compartment system experiments in which Zea mays was grown the following hypothesis were assumed: a) measured P concentration gradients can be explained by the mobilisation of P by the root exudate citrate, and b) measured As-V concentration gradients can be explained by the simultaneous effect of the competitive sorption of As-V and P and the competitive uptake of As-V and P. First, the feasibility of the applied description of soil chemical processes was justified. Then competitive uptake was implemented in the computer code using two different mathematical approaches. Our model calculation provided support for hypothesis a) and suggested that hypothesis b) has to be extended. The results show that the competitive uptake of As-V and P has an influence on As-V concentrations in the rhizosphere, but including competitive uptake was not sufficient to predict observed As-V concentration profiles. Recent results on plant As-metabolism like As-III efflux and Si As-III interaction probably have to be included in addition for simulation of measured As-V concentration profiles.