In-situ growth of ZIF-8 nanocrystals on biochar for boron adsorption

Removing boron (B) from aqueous solution is economically and environmentally important in the treatment of many industrial and agricultural wastewater streams. Here, we produced canola straw-derived biochar (BC), a low-cost and green adsorbent, as a template to prepare a BC/ZIF-8 (zeolite-like imidazolate framework) composite material. We then performed a series of batch experiments and material characterization to evaluate the adsorption processes and practical applications of the material. The results showed that the maximum adsorption capacity of BC/ZIF-8 for B from a low total dissolved solids (TDS) aqueous solution was 44 mg g-1 (adsorbent dosage is 0.5 g L−1, pH=6, C0 =300 mg L-1). Compared to ZIF-8 alone, the adsorption capacity of B to BC/ZIF-8 increased by 76%. In the adsorption tests conducted in the low TDS solution, the adsorption process was well-represented using a pseudo-first-order kinetics model and the Langmuir adsorption isotherm. Through six regeneration cycles, we demonstrated that BC/ZIF-8 remains a robust sorbent for B. Hydraulic fracturing (HF), a key technology for oil and gas stimulation, produces a large amount of flowback and produced water (FPW). BC/ZIF-8 was used to remove boron from FPW, achieving a maximum adsorption capacity of 15.45 mg g-1 (C0 =42 mg L-1), 5.99% lower than that of pure B solution with similar conditions. Physicochemical characterization of BC/ZIF-8 showed that it has a stable structure and suggested that B adsorption to BC/ZIF-8 was primarily due to hydrogen bonding and electrostatic attraction between B(OH)4- and the positively charged BC/ZIF-8 at pH < PZC. Our study shows that BC-based metal-organic frameworks (MOFs) have the potential for broader industrial applications and water treatment.