Predicting chemical speciation of metals in soil using Visual Minteq

Abstract

● Soil processes affect metal chemical speciation and their biogeochemical activity.

● The current study predicted chemical speciation of eight metals in two soil layers.

● Divalent forms of metals predominated in both soil layers (79.9%).

● Chromium showed a chemical speciation that varied from that of the other metals (95.8% as CrOH⁺).

● Mean percentage ages of all metal ions were similar for all 15 field locations investigated.

From soil contamination and risk assessment perspectives, it is imperative to understand the ecological processes occurring in soils. Certain soil processes greatly affect chemical speciation of potentially toxic metals (PTMs), and thus also influence their biogeochemical activity. The current study analyzed chemical speciation of eight PTMs (Cd, Cr, Fe, Cu, Mn, Ni, Zn, and Pb) in upper and lower soil layers for 15 agronomic fields of Vehari-Pakistan using Visual Minteq software. The divalent forms of most PTMs (PTM²⁺) generally predominated in both soil layers (79.9% overall occurrence). However, chromium revealed a different pattern of chemical speciation (95.8% as CrOH⁺) compared to other PTMs. The mean percentage of all the PTMs²⁺ was slightly higher for the lower soil layer (81.3%) than in the upper layer (78.4%), the trend being same for all the PTMs, except Cr. This higher PTMs²⁺ percentage in lower soil layers than upper layers was due to lower content of organic matter and other anions such as Cl⁻ and HCO₃⁻. The mean percentage ages of all the PTMs²⁺ was similar among all the 15 agronomic fields, which was confirmed by strong Pearson correlation values (R² > 0.95). The PCA graph grouped all the agronomic fields and PTM²⁺ closely, except Cr²⁺ and Cu²⁺. This grouping confirmed the similar chemical speciation of PTMs, except Cu and Cr in studied fields.

Graphical abstract

Keywords

heavy metals / free ionic forms / agricultural soils / multivariate analysis / PCA

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