The influence of pH on iron speciation in podzol extracts: iron complexes with natural organic matter, and iron mineral nanoparticles.

The quantities of natural organic matter (NOM) and associated iron (Fe) in soil extracts are known to increase with increasing extractant pH. However, it was unclear how the extraction pH affects Fe speciation for particles below 30 nm. We used flow field-flow fractionation (FlowFFF) and transmission electron microscopy (TEM) to investigate the association of Fe and trace elements with NOM and nanoparticulate iron (oxy)hydroxides in podzol extracts. For extracts prepared at the native soil pH (~4), and within a 1-30 nm size range, Fe was associated with NOM. In extracts with a pH≥7 from the E and B soil horizons, Fe was associated with NOM as well as with iron (oxy)hydroxide nanoparticles with a size of approximately 10 nm. The iron (oxy)hydroxide nanoparticles may have either formed within the soil extracts in response to the increase in pH, or they were mobilized from the soil. Additionally, pH shift experiments showed that iron (oxy)hydroxides formed when the native soil pH (~4) was increased to 9 following the extraction. The iron (oxy)hydroxide nanoparticles aggregated if the pH was decreased from 9 to 4. The speciation of Fe also influenced trace element speciation: lead was partly associated with the iron (oxy)hydroxides (when present), while copper binding to NOM remained unaffected by the presence of iron (oxy)hydroxide nanoparticles. The results of this study are important for interpreting the representativeness of soil extracts prepared at a pH other than the native soil pH, and for understanding the changes in Fe speciation that occur along a pH gradient.