Highly efficient and selective capture of heavy metals by poly(acrylic acid) grafted chitosan and biochar composite for wastewater treatment

A novel poly(acrylic acid) grafted chitosan and biochar composite (PAA/CTS/BC) for highly efficient and selective capture of heavy metals (HMs) was prepared. PAA/CTS/BC showed efficient and rapid adsorption of Cu2+, Zn2+, Ni2+, Pb2+, Cd2+, Mn2+, Co2+, and Cr3+, well fitted by Langmuir and pseudo-second-order kinetic models, respectively, and it could be applied in a wide pH range (2–7) with high adsorption capacity. Attractively, PAA/CTS/BC showed strongly selective adsorption in the order of Cr3+ > Pb2+ > Cu2+ ≫ Cd2+ > Ni2+ > Zn2+ > Co2+ > Mn2+. The distribution coefficients of Cu2+, Pb2+, and Cr3+ (~104, ~106, and ~107 mL/g, respectively) nearly matched or exceeded that of reported adsorbents in literatures. Surface complexation via inner-sphere complexes with carboxyl, hydroxyl, or amine groups is proposed as the major removal mechanism of HMs by PAA/CTS/BC; however, the covalent bond strength between HMs and oxygen/nitrogen atoms significantly differed among HMs based on the principle of hard and soft acids and bases, essentially determining the adsorption affinity and selectivity of HMs on PAA/CTS/BC. Surface precipitation was also an important mechanism for adsorption of Cr3+ by PAA/CTS/BC. Additionally, Cr3+ adsorbed on PAA/CTS/BC was almost completely resistant to desorption by 0.01 mol/L Na2EDTA solution, while other HMs could be mostly recovered (>80%) by the same method, indicating its promising potential of recycling pure Cr3+ and excellent reusability. Finally, the case study on the treatment of metal finishing wastewater confirmed that PAA/CTS/BC could efficiently achieve both wastewater purification and metal recycling for specific industrial wastewater.