Effect of particle size of nanoscale zero-valent copper on inorganic phosphorus adsorption-desorption in a volcanic ash soil

Abstract

Zero-valent copper engineered nanoparticles (Cu-ENPs) released through unintentional or intentional actions into the agricultural soils can alter the availability of inorganic phosphorus (IP) to plants. In this study, we used adsorption-desorption experiments to evaluate the effect of particle size of 1% Cu-ENPs (25 nm and 40-60 nm) on IP availability in Santa Barbara (SB) volcanic ash soil. X-Ray Diffraction results showed that Cu-ENPs were formed by a mixture of Cu metallic and Cu oxides (Cu₂O or/and CuO) species, while specific surface area values showed that Cu-ENPs/25 nm could form larger aggregate particles compared to Cu-ENPs/40-60 nm. The kinetic IP adsorption of SB soil without and with 1% Cu-ENPs (25 nm and 40-60 nm) followed the mechanism described by the pseudo-second-order (k₂ = 0.45-1.13 x 10^-3 kg mmol^-1 min^-1; r² ≥ 0.999, and RSS ≤ 0.091) and Elovich (α = 14621.10-3136.20 mmol kg^-1 min^-1; r² ≥ 0.984, and RSS ≤ 69) models. Thus, the rate-limiting step for IP adsorption in the studied systems was chemisorption on a heterogeneous surface. Adsorption equilibrium isotherms without Cu-ENPs were fitted well to the Freundlich model, while with 1% Cu-ENPs (25 nm and 40-60 nm), isotherms were described best by the Freundlich and/or Langmuir model. The IP relative adsorption capacity (K_F) was higher with 1% Cu-ENPs/40-60 nm (K_F = 110.41) than for 1% Cu-ENPs/25 nm (K_F = 74.40) and for SB soil (K_F = 48.17). This study showed that plausible IP retention mechanisms in the presence of 1% Cu-ENPs in SB soil were: i) ligand exchange, ii) electrostatic attraction, and iii) co-precipitate formation. The desorption study demonstrated that 1% Cu-ENPs/40-60 nm increased the affinity of IP in SB soil with a greater effect than 1% Cu-ENPs/25 nm. Thus, both the studied size ranges of Cu-ENPs could favor an accumulation of IP in volcanic ash soils.

Keywords: Adsorption models; Agricultural soil; Emerging pollutants; Engineered nanoparticles; Inorganic elements.