Hydrochars and phosphate enhancing the transport of nanoparticle silica in saturated sands

- nSiO2 aggregates can be restablized by the adsorption of P at low pH.
- Hydrochars facilitate the transport of attached nSiO2 at pH 6.0.
- The retention of nSiO2 is more significant with hydrochars or P at high pH (9.0).
- When hydrochars and P coexist, the enhancement of nSiO2 deposition occurs.

Due to the potential negative impact of nano silica (nSiO2) on human's health and living environments, it is important to investigate their transport in soil environments. Hydrochars has been widely used in agricultural soil and phosphate (P) is an important nutrient, thus the aggregation and transport of nSiO2 in saturated sand columns were investigated in single and binary systems of hydrochars and P. The experimental results showed that the nSiO2 aggregates can be restablized by the adsorption of P or the attachment of hydrochars at high IS (>100 mM) and low pH (<7.0). Accordingly, the transport of nSiO2 in sand columns is enhanced due to the smaller particle size. However, the nSiO2 presents the distinct surface characteristics at pH > 7.0 from that at pH < 7.0. Thus, nSiO2 has a better dispersivity in 300 mM NaCl solution at high pH (9.0). Nevertheless, their deposition to sands becomes pronounced in the presence of hydrochars and/or P. In particular, the formation of nSiO2-hydrochar-Phosphate clusters associated with the larger size mainly contributes to the enhancement of nSiO2 retention in sand columns during the wide pH range, when hydrochars and P coexist in suspensions. The two-site dynamic model fitting results showed that the reversible retention is related to k2 (First-order straining coefficient on site 2). The results in this study will provide the theoretical basis for assessing the retention of nSiO2 in soil environment with the presence of hydrochars and phosphate.

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