Tailored mesoporous biochar sorbents from pinecone biomass for the adsorption of natural organic matter from lake water

Natural organic matter (NOM) raises major issues for drinking water treatment including undesirable taste and color, formation of carcinogenic disinfection by-products, and promotion of microbial regrowth in the water distribution system. As such, mesoporous biochars have been tailored from pine-forestry by-products for treating NOM and color causing compounds from drinking water sources, such as lakes. Herein, several tailored biochars are fabricated via two procedures: pre-pyrolysis/activation/post-pyrolysis and activation/post-pyrolysis processes, using NaOH and ZnCl₂ activators to improve the surface chemistry and porous structure for higher NOM adsorption. The mesoporous biochars, pristine biochars, and pinecone biomass are characterized via several analysis methods including Brunauer, Emmett and Teller surface area measurement (BET), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). Batch experiments are conducted to study the adsorption isotherm, kinetics and mechanism along with desorption. Characterization revealed effective characteristics of tailored biochars for NOM adsorption including mesoporous structure, remarkable surface area (up to 1470 m²/g), high thermal stability, and elevated carbon content. All the tailored biochars showed improved removal capacities for NOM and color compounds from real lake water samples compared with those of the pinecone biomass and pristine biochars. The most promising tailored biochar (herein named as TB-N-I) was developed by NaOH modification via pre- and post-pyrolysis processes. With the lowest optimized dosage (0.25 g/L), TB-N-I removed >80% of both NOM and color from the lake water (chemical oxygen demand (COD): 13.4 and color: 53.65 mg/L), superior to the removal capacity of commercial powdered activated carbon (PAC). Acidic conditions significantly favored the adsorption, e.g. NOM removal by TB-N-I from the lake water reached 97% at pH 2. Nonlinear regression provided a good fit for Freundlich and Sips (r² = 0.988 and Δq = 0.08) isotherms as well as pseudo-second-order kinetic models. This suggests the heterogeneous distribution of the adsorptive sites at the biochar surface and the multilayer nature of NOM adsorption. Our desorption study revealed that more alkaline solutions resulted in higher NOM desorption (30 mM NaOH > 3 mM NaOH > distilled water), yielding regenerated adsorbents with high re-adsorption capacity. Liquid chromatograph–organic carbon detection (LC-OCD) was used to study the removal of different size fractions of NOM, e.g. complex humic substances and low molar mass acids. Of significant interest, low molar mass (LMM) fraction, which are more hydrophilic and resistant to conventional treatments, were effectively removed. LC-OCD indicated that TB-N-I removed 20% more of the problematic LMM compared with that of PAC. Effective pore size distribution of tailored biochar (TB-N-I) was indicated by BET analysis and was confirmed by LC-OCD results for the adsorption of NOM fractions.