Adsorption of Synthetic Organic Substances from the Aqueous Phase on Walnut Shell Biochar

Abstract

The effect of the maximum pyrolysis temperature on the adsorption capacity of biochars obtained from walnut shells (WNS) (Kyiv region, Ukraine) for the removal of synthetic organic compounds (SOCs) from the aqueous phase was studied. The thermolysis of WS samples was performed in an atmosphere with a limited oxygen content at an average furnace heating rate of 15°C and a maximum processing temperature from 340 to 800°C. Initially, the efficiency of the thermal process was estimated by the biochar specific adsorption activity with respect to iodine and Methylene Blue and the product yield and bulk density. The structural and sorption characteristics of the best samples were evaluated by the t-method using the isotherms of adsorption from the aqueous phase of a standardized organic adsorbent and the isotherms of nitrogen adsorption−desorption from the gas phase. The adsorption properties of biochars were determined on three samples carbonized at 410, 650, and 800°C. Phenol, p-chloroaniline (p-CA), and surfactant Triton X-100 (TX-100) were used as adsorbents, for which the isotherms of adsorption from the aqueous phase were measured in a static isothermal process. It was established that the adsorption activity with respect to I₂ for the materials carbonized with WS in the maximum temperature range of 420–800°C increases almost by a parabolic law by 4 times (from 85 to 350 mg/g). The yield of the carbonized product decreases (from 34 to 24%) with an increase in the maximum processing temperature. The best samples of biochar (W800 and W650) showed the adsorption of monoaromatic compounds and nonionic surfactants from dilute aqueous solutions is characterized by rather high energy and value as compared to the nonporous adsorbent. It was established that biochars have a microporous structure with undeveloped mesoporosity. According to the values of structural parameters (surface area, 300–400 m²/g; adsorption volume, less than 0.2 mL/g), new products can be attributed to biochars with initial nanoporosity. To obtain an efficient adsorption material from walnut shells under the studied thermal conditions, it is necessary to perform chemical activation procedures for the precursor.