Zr-modified cattle manure biochar composite for efficient phosphate removal: Mechanisms and waste valorization

Addressing the dual global challenges of livestock waste accumulation and phosphate-induced aquatic eutrophication, this study develops a facile and sustainable strategy to convert cattle manure (CM) into monodisperse granular zirconium-modified biochar composites (Zr-CMB) via sequential pyrolysis, ZrOCl 2 ·8H 2 O functionalization and sodium alginate (SA) encapsulation. Unlike most reported single-modification powdered biochar adsorbents, this work simultaneously optimizes the granular structure for engineering applicability and Zr-based active sites for phosphate selectivity, overcoming the bottlenecks of poor solid-liquid separation and insufficient selectivity of powdered materials. Batch adsorption experiments show that the optimized Zr-CMB achieves exceptional phosphate removal efficiency (>99% at pH 3–6). The adsorption process is dominated by chemisorption (pseudo-second-order kinetics, R 2 = 0.999) and monolayer adsorption (Langmuir isotherm model). Material characterization verifies Zr-OH groups as the core active sites, and the synergistic removal mechanism involves ligand exchange at Zr 4+ active sites, electrostatic attraction and porous mass transfer enhanced by hierarchical granular structure. Under optimized conditions, the uniform 2–3 mm Zr-CMB exhibits a maximum phosphate adsorption capacity of 8.47 mg/g, maintaining stable effluent phosphate concentration below 0.2 mg/L at a dosage of 7 g/L. Importantly, Zr-CMB retains 76.92% of its initial adsorption capacity after 5 adsorption-desorption cycles with negligible heavy metal leaching risk. This work clarifies the synergistic phosphate removal mechanism of Zr-OH ligand exchange, electrostatic attraction and hierarchical pore mass transfer, and establishes a circular economy paradigm that converts livestock manure into a high-performance regenerable phosphate-selective granular adsorbent, providing a feasible and cost-effective solution for eutrophication control and agricultural waste valorization.