Biological Weathering in Metal-Contaminated Soils: Influence of Biologically Derived Dissolved Organic Compounds on Mineral Dissolution.

Persistent trace metal pollution negatively affects ecosystem services and functioning in many areas globally, including the USA. In contaminated soils, trace metals are common impurities in Fe oxide minerals and can be liberated by organic compounds in the soil. Plants and soil microorganisms contribute organic compounds to soils, often in the form of low molecular weight organic acids (LMWOAs) and amino acids.

We synthesized a common soil mineral, goethite (α-FeOOH) as a series of coprecipitates with Cd and subjected them to dissolution using common biological exudates - oxalic acid, citric acid, cysteine and histidine. Substantial quantities of amorphous- (10–30 % of total) and surface-associated Cd (5–15 % of total) were present in all coprecipitates, with greater Cd solubilization from coprecipitates containing greater Cd. In addition, increasing Cd in the coprecipitates inhibited Fe release from the minerals. Finally, the presence of Cd altered mineral reactivity toward the organic ligands, with enhanced Fe solubilization occurring in the presence of cysteine rather than oxalic acid. This is important because many plants and microorganisms release specific organic (phyto)siderophore compounds to acquire mineral-bound nutrients from surrounding soils. In contaminated ecosystems, use of these (phyto)siderophores to liberate mineral-bound nutrients may be less effective because of the altered Fe solubility in trace metal-containing minerals.

Mixed amorphous–crystalline mineral aggregations are common in natural soils, and must be further studied to understand their role in nutrient and contaminant availability. In addition, Fe oxides, often considered sinks for toxic metals, may be less effective at reducing contaminant mobility and bioavailability in biologically active soils.