Effect of vitamin-C surface modification on Pb2+ and Zn2+ adsorption by nanosilica under saline conditions: influence of surface properties and adsorption performance

The application of nanostructured adsorbents for heavy metal removal from saline industrial wastewater remains challenging because elevated ionic strength can alter surface interactions and reduce adsorption efficiency. This study investigated the effect of vitamin C surface modification on the physicochemical characteristics and adsorption performance of nanosilica (NSi) for the removal of Pb 2 ⁺ and Zn 2 ⁺ under saline conditions. Pristine nanosilica and vitamin C-modified nanosilica (VC–NSi) were evaluated at different salinity levels (1, 10, and 20 dS m⁻ 1 ) and initial metal concentrations (50, 100, and 200 mg L⁻ 1 ). Surface characterization revealed substantial changes in the physicochemical properties of the silica structure after vitamin C modification, with a decrease in specific surface area from 195.47 to 106.90 m 2 g⁻ 1 , while the average pore diameter increased from 12.597 to 22.935 nm and the total pore volume remained nearly unchanged. These changes suggested partial occupation of accessible pores and reduced availability of adsorption sites after vitamin C modification . Although both adsorbents effectively removed Pb 2 ⁺ and Zn 2 ⁺, pristine NSi consistently exhibited superior adsorption performance, particularly under highly saline conditions. The removal efficiency of NSi remained above 90% for most Pb 2 ⁺ and Zn 2 ⁺ treatments, whereas VC–NSi showed a considerable decline in metal uptake across salinity levels. Adsorption kinetics followed the pseudo-second-order model (R 2 = 0.997 for Pb 2 ⁺ and 0.996 for Zn 2 ⁺), indicating that surface-mediated interactions controlled the adsorption process, while equilibrium data were better described by the Langmuir isotherm. The results demonstrate that surface modification does not necessarily enhance adsorption capacity and that modification-induced changes in pore accessibility and surface area can outweigh the contribution of newly introduced functional groups. This study provides new insights into the structure performance relationship of silica-based adsorbents for heavy metal remediation in saline wastewater environments.