Performance of a floating Ca/Fe modified pumice in phosphorus removal from agricultural drainage water

Phosphorus (P) removal from low concentration agricultural drainage runoff is particularly challenging. Conventional powdered adsorbents are often difficult to recover from water bodies, while buoyant natural pumice, despite being easy to retrieve, suffers from limited adsorption capacity. Here we synthesized a buoyant Ca/Fe modified pumice (CFMP) via alkaline in-situ co-precipitation, integrating floatability with its P-binding capability to address multiple P fractions in agricultural runoff. Batch experiments showed CFMP maintained stable removal efficiencies above 60% across a wide pH range (2∼9), peaking at 77% at pH = 7 for a 5 mg-P L^-1 solution and achieving a Langmuir maximum adsorption capacity of 0.389 mg g^-1, exceeding that of unmodified pumice (0.294 mg g^-1). Response Surface Methodology (RSM) was employed to optimize operational parameters for real-world application in a CFMP-contained device. Mechanistic investigations indicated dominant removal pathways including Ca²⁺/Fe³⁺ mediated ligand exchange, precipitation for truly dissolved P (truly DP < 3 kDa), and electrostatic attraction for particulate (PP, >450 nm) and colloidal fractions (CP, 3 kDa-450 nm). Field tests with agricultural drainage containing 0.3 mg-P L^-1 demonstrated removal efficiencies-81.3% for truly DP, 75.0%∼85.8% for PP, 72.8%∼83.8% for fine colloidal P (FCP) and 73.3%∼83.8% for medium colloidal P (MCP) sustained over five reuse cycles. It was noted that the removal efficiency increased with higher influent P concentrations and with increasing water level from 5 cm to 10 cm, but declined at elevated flow rates (0.02∼0.04 m³ d^-1) and at water levels exceeding 10 cm. These findings highlighted CFMP's distinctive combination of buoyancy, recoverability, adsorption capacity, and fraction-specific removal performance, offering a practical solution for mitigating agricultural non-point source P pollution.