The Impact of Sodium Hexametaphosphate Use in Soil Fractionation Protocols on Downstream Mineral Analyses

Sodium hexametaphosphate (NaHMP) is a common chemical dispersant used in soil analytical methods such as the fractionation of mineral-associated organic matter (MAOM) from bulk soil; however, its effect on downstream mineral analyses is not well understood. This study examined how pretreatment with NaHMP affected subsequent metal speciation analyses in soil studies. Emulating MAOM fractionation protocols, a simulated physical fractionation was performed by exposing technical replicates of three Florida soils with variable clay and organic matter content to a 0.5% NaHMP solution. NaHMP-treated and control soils were then sequentially extracted into exchangeable, carbonate-bound, Fe–Mn oxide-bound, organic matter-bound, and residual metal fractions using a modified Tessier extraction protocol. To assess treatment effects, metal concentrations were compared using a mixed-design factorial ANOVA. Tests of simple main effects indicated no significant difference between treatments (p = 0.733) in a soil with moderate amounts of clay and organic matter but significant three-way (p < 0.001) and two-way (p ≤ 0.001) interactions in sand-rich and clay-rich soils. P concentrations also differed significantly (p < 0.003) between treatments in at least one fraction of all soils, demonstrating that NaHMP-induced changes in P concentration may persist throughout a sequential metal extraction. The results highlight the importance of considering inherent soil properties and desired future analyses when selecting NaHMP as a dispersant in experimental protocols.