Co‐transport of Pesticide Acetamiprid and Silica Nanoparticles in Biochar‐Amended Sand Porous Media

Abstract

The role of biochar as a soil amendment on the transport of acetamiprid, a widely used neonicotinoid pesticide, is little known. We conducted saturated column experiments to examine cotransport of acetamiprid and silica nanoparticles (NPs) in pure and biochar-amended sands. Retention of acetamiprid was minor in the pure sand, whereas application of biochar in the sand significantly increased retention. Retention was greater at lower ionic strengths and near neutral pH values and was attributed to biodegradation and sorption through π-π interaction and pore filling. The convection-diffusion equation with inclusion of first-order sorption, desorption, and degradation well described the transport of acetamiprid in the biochar-amended sand. The simulation results show that the sorption rate did not change with pH. This is because the acetamiprid is nonionic and cannot be bonded with the biochar by protonation or deprotonation. The desorption rate was independent of variation of solution chemistry, indicating that desorption was a physical process (i.e., pore diffusion). Application of biochar in the sand had little influence on the transport of silica NPs in NaCl but caused complete attachment in CaCl. Energy dispersive X-ray spectroscopy suggested that the enhanced attachment was due to cation bridging between silica NPs and functional groups in biochar by the Ca. The co-presence of acetamiprid and silica NPs in the solutions enhanced transport of acetamiprid and NPs in the biochar-amended sand by competing for the binding sites on the biochar surfaces.

Influence of Biochar on Deposition and Release of Clay Colloids in Saturated Porous Media

“…The zeta potentials were less negative in CaCl 2 than in NaCl at a given IS due to specific adsorption of calcium ions to the clay particle surfaces (Chen and Elimelech, 2006). It should be noted that the measured zeta potentials are mean values, which cannot reflect surface chemical heterogeneities (e.g., functional groups on the biochar surfaces) (Li et al, 2017; Wang et al, 2016b). In addition to the chemical heterogeneity, the sand and biochar surfaces bear physical heterogeneities (i.e., surface roughness) (see the scanning electron microscopy images in Supplemental Fig.…”

“…If the clay colloids were initially attached in CaCl 2 , only a small fraction of the colloids were released by introducing DI water. The irreversibility of clay colloid deposition on biochar surfaces is likely due to formation of cation bridging between the clay colloids and the functional groups on the biochar surfaces (Wang et al, 2016b). Shen et al (2013) showed that the cation bridging eliminates the short‐range repulsion between the colloids and the collector surfaces, and the colloids cannot be released by reducing IS even if they are attached atop of nanoscale protruding asperities.…”

“…Using the mass ( m ) of packed sand and assuming a sand density (ρ) of 2.65 g cm −3 , the porosity ( f ) of the sand packed column was determined to be 0.37 using the following expression: where V is the volume of the column. Taking the biochar density as 0.54 g cm −3 (Wang et al, 2016b), the porosities of the biochar‐amended sand columns were determined to be 0.37, 0.38, 0.40, 0.45 for the amendment at 5, 10, 20, and 50%, respectively.…”

“…The detailed procedure for solving Eq. [2] and [3] is described in Wang et al (2016b). Briefly, a zero initial concentration, pulse input, and a zero concentration–gradient boundary were used.…”

“…To measure zeta potentials of the sand, the sand was sonicated for 5 min in electrolyte solutions of interest (Tufenkji and Elimelech, 2004), and the supernatant was diluted for the measurement. The biochar ( JINHEFU Agriculture Development Co., Ltd.) used in this study has the same properties as that used in Wang et al (2016b). The biochar was produced by pyrolysis of cotton straw, peanut shell, and sawdust in a muffle furnace at 400°C for 2 h. The biochar (size range, 0.25-1 mm) was thoroughly cleaned with DI water, dried at 80°C, and stored in a dessicator for column experiments.…”