Adsorption and Cosorption of Tetracycline and Copper(II) on Montmorillonite as Affected by Solution pH

TLDR

Land application of animal‑feed waste can lead to accumulation of tetracyclines and metals in soils, and their adsorption onto soil minerals strongly influences mobility. The study aimed to evaluate how tetracycline and Cu(II) interact during adsorption and cosorption on montmorillonite across different solution pH levels. Adsorption experiments were performed on montmorillonite at varying pH to assess TC and Cu(II) uptake and cosorption behavior. At pH below 6.5, TC enhances Cu(II) adsorption on montmorillonite through complex formation, and conversely Cu(II) boosts TC uptake across a broad pH range; the weighted‑sum model fits the data, and the TC–Cu(II) complexes exhibit higher sorption coefficients than individual species, indicating that their coexistence reduces mobility.

Abstract

Land application of wastes generated from concentrated animal feeding operations may result in accumulation of tetracyclines (TCs) and metals in agricultural soils. Adsorption of TCs and metals on soil minerals strongly affects their mobility. This study was conducted to evaluate the interaction between tetracycline (TC) and Cu(II) with regard to their adsorption and cosorption on montmorillonite as affected by solution pH. When solution pH was below 6.5, the presence of TC increased Cu(II) adsorption on montmorillonite, which could be due to increasing Cu(II) adsorption via the TC bridge, or due to the stronger affinity of TC−Cu(II) complex to the mineral than Cu2+ ion itself. Zeta potential of the montmorillonite significantly decreased after the adsorption of TC, suggesting a strong interaction between TC and montmorillonite. Addition of Cu(II) ions increased TC adsorption on the mineral in a wide range of pH. The experimental data were well fit with the weighted sum model. The complexes of TC and Cu(II) (CuH2L2+, CuHL+, and CuL) had higher sorption coefficients (Kd) than that of the corresponding TC species (H3L+, H2L, and HL−). Increasing adsorption of TC and Cu(II) on montmorillonite as they coexist in the normal pH environment may thus reduce their mobility.