Carbonaceous adsorbent as green silica by-product applied to the treatment of contaminated effluent by pesticide commonly sprayed on rice cultivation

Grain cultivation and its impacts on the environment have been the focus of many studies, especially due to generated solid waste and the wide use of agrochemicals aiming for greater productivity. In this context, the present study proposes a new and consistent step in constructing self-sustainability in rice farming. The proposed stage includes reusing green silica waste as an adsorbent to treat effluents contaminated by pesticides directly applied to rice cultivation. After nano silica production through the rice husks burning, followed by basic leaching and acid precipitation, a carbonaceous material remains. This material, naturally impregnated by Na₂SiO₃, was washed and dried, characterized, and used to remove the pesticide 2,4-dichlorophenoxyacetic acid (2,4-D). The adsorption essays were performed at 2,4-D at low concentrations (between 1 and 10 mg L^-1) at different temperatures. The washed and dried porous carbon (WDPC) surface is irregular and presents slit-shaped channels. The FT-IR analysis identified the siloxane, carbonyl, carboxylate, and methylene functional groups available to interact with the pesticide molecules. The washing/drying process eliminated impurities, improving the surface area from 539.67 to 619.67 cm² g^-1 and pore volume from 0.29 to 0.44 cm³ g^-1. Concerning the adsorption of 2,4-D on WDPC, the best pH was 6.0, where around 75% of the pesticide was removed from the water. The equilibrium isotherms presented an S-shaped form indicating a multilayer and cooperative adsorption, with maximum adsorption capacities of 7.504 and 7.736 mg g^-1. The estimated ∆G_ads, ΔH_ads, and ΔS_ads values suggested that pesticide adsorption was spontaneous, exothermic, and favorable. Finally, WDPC demonstrated a good potential to uptake 2,4-D from water, contributing to self-sustainability in rice farming.