Magnesium-Doped Phosphoric Acid-Activated Tea Branch Biochar for Efficient Removal of Cd ²⁺ and Pb ²⁺ from Water: Adsorption Behavior and Insight into the Quantitative Mechanism

Cd²⁺ and Pb²⁺ contamination in water systems causes a severe challenge to environmental sustainability and threatens human health, making efficient removal strategies urgent needed. Biochar is an attractive, green, and low-cost adsorbent for the removal of heavy metals from aqueous solutions, but its adsorption performance is often insufficient. Its modification significantly improves the adsorption performance of heavy metals. We synthesized a new tea branch biochar comodified with phosphoric acid and magnesium chloride (PMTB) for the efficient removal of Cd²⁺ and Pb²⁺ from aqueous environments. The physicochemical properties, adsorption performance, and contributions of different mechanisms of PMTB were investigated through batch adsorption tests and characterization analyses. PMTB was identified as a disordered mesoporous carbon matrix decorated with numerous oxygen-containing functional groups. PMTB possessed superior affinity for Cd²⁺ and Pb²⁺, with maximum adsorption values of 140.43 mg g^-1 for Cd²⁺ and 237.70 mg g^-1 for Pb²⁺ at pH 6.0 and an adsorbent dosage of 2 g L^-1. PMTB also demonstrated exceptional selectivity, maintaining a high removal efficiency even in the presence of common competing cations (K⁺, Ca²⁺, Na⁺, and Mg²⁺). In a binary metal system, PMTB exhibited a stronger affinity for Pb. Additionally, PMTB remained quite reusable. Kinetic and isotherm studies confirmed that the predominance of adsorption occurred via a monolayer chemical process. Through quantitative contribution analysis, complexation and precipitation were identified as the dominant mechanisms for Cd²⁺ (68.81%) and Pb²⁺ (73.75%) adsorption. Cation exchange and coordination of π electrons were also enhanced after comodification. Therefore, PMTB emerged as an efficient adsorbent with significant potential for treating wastewater containing Cd²⁺ or Pb²⁺.