A Comparative Study on Chemical Speciation of Trace Metals as a Function of Solution pH: A Multivariate Analysis and Simulation Approach

Biogeochemical activity of trace elements in the environment significantly relies on their chemical speciation. Solution pH influences the chemical speciation and biogeochemical activity of trace elements in the soil-plant continuum. The current study was conducted to simulate the influence of solution pH on chemical fractions of 11 trace elements (arsenic, lead, zinc, copper, chromium, cadmium, mercury, nickel, strontium, antimony, and selenium) using Visual Minteq, a chemical equilibrium model used for the calculation of metal speciation, sorption etc. Results delineated that pH-mediated variations in the chemical speciation of trace elements differed with their type. The results of the current study revealed a dominance of chemical trace element species at acidic (1–6), neutral (7.0), and alkaline (8–14) pH. Divalent trace elements (zinc, cadmium, copper, chromium, nickel, and lead) were the dominant species at acidic pH. However, mercury dominance has a narrow pH range (1–3), while strontium dominance has a wider pH range (1–13). Nitrate, sulfate, and phosphate fractions of trace elements also occurred at pH <8. However, at pH >8, trace elements existed in hydroxide (OH) fractions. In the case of arsenic (arsenite As³⁺ and arsenate As⁵⁺), selenium (selenite Se⁴⁺ and selenate Se⁶⁺), and antimony (antimonite Sb³⁺ and antimonate Sb⁵⁺), their dominant species get oxidized with increasing pH. Multivariate analysis was conducted for various metals for their dominance as a function of pH. Moreover, Principal Component Analysis verified a linear relation of trace element fractions with pH. Results revealed that solution pH affects the chemical fractions and the phytoavailable fraction of trace elements in growth culture and associated health hazards through soil-plant transfer.