Assessment of trace heavy metals dynamics during the interaction of aqueous solutions with the artificial OECD soil: Evaluation of the effect of soil organic matter content and colloidal mobilization

- The accumulation and mobility in soil of trace Ni, Cd and Cu was evaluated.
- SOM enhanced linearly HMs mobility.
- The mechanism implies the interaction with colloidal phases present in the soil.
- Metals accumulate mainly in the top layer of soil (≤1 cm).
- In absence of colloidal mobilization, very low mobilization of HMs was observed.

A micro-contamination phenomenon was reproduced and studied at lab-scale, mimicking the irrigation of a standard artificial soil with a water solution containing three Heavy Metals (HMs) at trace concentration level. To assess the dynamics of micro-pollutants accumulation and migration trough the soil, the organic matter in the soil was varied, together with sodicity of the irrigation water. Accumulation of the investigated contaminants was observed mainly in the top layer (≤1 cm) of the irrigated soil. This was attributed to the high interaction capacity of the soil compared to the low HM concentrations in the water phase. HMs transport pattern was described assuming a multi-component mechanism including: i) the interaction of HMs with the colloidal phase of the soil; ii) the slow and constant release of small molecular weight ligands detaching from the soil immobile matrix; iii) the transportation of HMs through the soil by these low molecular weight chaperon molecules. The mobility was directly related to the soil organic matter (SOM), since higher amount of SOM correspond to a higher number of chaperon molecules. In the first centimetre of the soil the metals were mostly bound to the acid labile fraction. Very low mobilization was observed with increasing sodicity in the leaching water, since such conditions were unfavourable to the colloidal mobilization of SOM. This indicated that water/soil transfer of pollutant is not only related to the contaminant concentration in the irrigation water but also to the characteristics of the aqueous solution and to the physical-chemical properties of the soil.

Proposed mechanism for trace Cu2+ fate after irrigation of OECD standard soil.178