Surface Interactions between Gold Nanoparticles and Biochar

Abstract

Engineered nanomaterials are directly applied to the agricultural soils as a part of pesticide/fertilize formulations or sludge/manure amendments. No prior reports are available to understand the surface interactions between gold nanoparticles (nAu) and soil components, including the charcoal black carbon (biochar). Retention of citrate-capped nAu on 300-700 °C pecan shell biochars occurred rapidly and irreversibly even at neutral pH where retention was less favorable. Uniform organic (primarily citrate ligands) layer on nAu was observable by TEM, and was preserved after the retention by biochar, which resulted in the aggregation or alignment along the edges of multisheets composing biochar. Retention of nAu was (i) greater on biochars than a sandy loam soil, (ii) greater at higher ionic strength and lower pH, and (iii) pyrolysis temperature-dependent: 500 < 700 ≪ 300 °C at pH 3. Collectively, carboxyl-enriched 300 °C biochar likely formed strong hydrogen bonds with the citrate layer of nAu. The charge transfer between the conduction band of nAu and π* continuum of polyaromatic sheets is likely to dominate on 700 °C biochar. Surface area-normalized retention of nAu on biochars was several orders of magnitude higher than negatively charged hydroxyl-bearing environmental surfaces, indicating the importance of black carbon in the environmental fate of engineered nanomaterials.

Figure 1

Phase contrast TEM images of individual nAu particles supported on a carbon lacey grid. Images were obtained from two different regions of TEM grid (a) and (c), each at two different magnifications (a and b) and (c and d). The region around the edge of the particles pointed by the white arrows is composed of citrate.

Figure 2

(a) Influence of pH on 22 mg L⁻¹ nAu (citric acid, pH 3) retention by 300–700 °C biochars and Norfolk soil (1 g L⁻¹, 3 d equilibration). (b–e) Influence of added (10–100 mM) MgCl₂ on q_e (squares for left y-axes) and TOC (crosses for right y-axes) for biochars (1 g L⁻¹) and soils (20 g L⁻¹). Horizontal lines in b–e represent TOC of nAu stock solution (solid line) and biochar/soil (without nAu, dashed line).

Figure 3

Retention (a) and release (b–c) isotherms of nAu on 300–700 °C biochars and a model sandy loam. Sorption (a): 22.9 mg L⁻¹ nAu, 0.5–5 g L⁻¹ sorbent. Release 1 (b): replaced 18 mL supernatant by DDW immediately following (a). Release 2 (c): replaced 18 mL supernatant by DDW and adjusted pH to 7, after 5 months aging. Each step employed 3 d equilibration and 20 mL total volume.

Figure 4

Phase contrast TEM images of nAu + PS700 at different magnifications showing (a) nAu aggregates on the biochar multi-sheets and (b) alignments along the edges of sheets composing PS700. (c) and (d) Higher magnification images show the uniform citrate layers preserved on the surface of nAu particles retained by biochar.