Multifunctional carboxymethyl cellulose sodium encapsulated phosphorus-enriched biochar composites: Multistage adsorption of heavy metals and controllable release of soil fertilization

• CMC@MBC has a super high capacity for adsorbing various heavy metals • CMC@MBC performs well in dynamic sewage input with fluctuating pH conditions • CMC@MBC has a long-term performance due to a multistage adsorption mechanism • CMC-Na encapsulation enables the controllable and sustained release of phosphorus • CMC@MBC is a low-cost, high-efficient adsorbent and P slow-release fertilizer Water and soil heavy metal contamination induced by wastewater irrigation endanger human health. To improve the long-term heavy metal removal performance in the complicated aqueous/soil system, a novel carboxymethyl cellulose sodium (CMC-Na) encapsulated phosphorus (P)-enriched biochar (CMC@MBC) was synthesized. The maximum adsorption capacities of the unmodified biochar for Pb(II), Cd(II), and Ni(II) were increased by 822, 464, and 256 mg/g, respectively. The CMC-Na encapsulation introduced abundant carboxymethyl groups and enables CMC@MBC a strong anti-pH interference ability. The sewage treatment result revealed that CMC@MBC functioned admirably with prolonged sewage flushing. The long-term effectiveness of CMC@MBC against heavy metals might be explained by a multistage adsorption process: immediate adsorption by the CMC-Na coating, fast adsorption at the interface of CMC-Na and P-enriched biochar (MBC), and delayed adsorption by the P compound progressively released from MBC. Heavy metal speciation analysis indicates that the contributions of different adsorption mechanisms vary with heavy metals. The macromolecular spatial network structure of CMC-Na could be a good cover for preventing P leaching, making CMC@MBC a great slow-release fertilizer (SRF) with a cumulative P release ratio of 61% after 28 days of application. This work suggests that CMC@MBC is a low-cost and high-efficiency adsorbent and phosphorus SRF.