Synergetic effect of hydrochar on the transport of anatase titanium dioxide nanoparticles in the presence of phosphate in saturated quartz sand

Abstract

The rapid development of nanomaterials has led to the unavoidable leakage and release of nanoparticles (NPs) into soil and the underlying groundwater. It is possible for chars and phosphate introduced into soil to improve crop soil properties by improving contact with NPs. In this study, the influences of hydrochar and/or phosphate on the anatase nTiO₂ transport behaviors were investigated under different conditions. The breakthrough curves (BTCs) and retention profiles were obtained by the saturated sand column experiments. The additional analysis of zeta potentials, sedimentation kinetics, Raman mapping, and the two-site kinetic attachment model (TSKAM) was conducted to explore the possible underlying mechanisms. The simultaneous presence of phosphate and hydrochar acted in a synergetic fashion to enhance the transport of nTiO₂ in a sand medium compared to the facilitated effect of single phosphate or hydrochar. The higher levels of hydrochar induce the more nTiO₂ in the high IC solution passing through the saturated sand columns in the co-presence of phosphate. It was attributed to the competitive adsorption of hydrochar with nTiO₂ to the sand site and the phosphate adsorption on nTiO₂ occurred simultaneously through the sand columns. The fitting results of BTCs using TSKAM showed that the value of k₂ for nTiO₂ (the irreversible attachment coefficient at site 2) was smaller than that of k_1d/k₁ (the first-order reversible detachment and attachment coefficient at site 1, respectively), suggesting irreversible retention of anatase nTiO₂ at site 1. The value of k_1d/k₁ could be better used to explain the retention of nTiO₂ with combined phosphate and hydrochar. This study provides insight into the implications of phosphate and/or hydrochar for nTiO₂ transport in crop soil environments.

Graphical abstract

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