Mechanism of synchronized adsorption of As-I in groundwater by Zr/Cu modified biochar

The natural coexistence of arsenic (As) and iodine (I) in the Kuytun region of Xinjiang has attracted increasing attention. Simultaneous removal of As and I remains a challenge due to competition for adsorption sites. In this study, wheat straw biochar (XM) was used as a carrier, and zirconium (Zr) and copper (Cu) were loaded onto it to optimize the preparation of a biochar-based composite material (Zr/Cu—XM). The effects of different Zr/Cu ratios and pH on the adsorption of As(Ⅴ) and I⁻ were investigated. The material was characterized using techniques such as SEM-EDS, FTIR, and XPS, and the simultaneous adsorption mechanism of As and I was explored. The results demonstrated that by fine-tuning the mass ratio of the modified metals Zr and Cu to 3∶1, the resultant Zr/Cu(3∶1)-XM material exhibit mesoporosity and possess an exceptional surface structure. This unique composition facilitat the simultaneous and rapid adsorption of both As(Ⅴ) and I⁻ ions from aqueous solutions within a mere 2 hours, achieving a remarkable removal efficiency of up to 98%. Compared to the original biochar, the adsorption capacity for As(Ⅴ) and I⁻ increased by nearly 40 times, with maximum adsorption capacities of 22.77 mg·g⁻¹ and 23.40 mg·g⁻¹, respectively. The isotherm adsorption model fit the Langmuir isotherm model, and the adsorption of As(Ⅴ) and I⁻ is more consistent with the pseudo-second-order kinetic equation. The adsorption process is not only controlled by intraparticle diffusion but also depend on mechanisms such as surface adsorption. Competitive adsorption of As(Ⅴ) and I⁻ occurr during the adsorption process. When pH < 6.77, electrostatic adsorption and chemical adsorption play crucial roles in the adsorption of As(Ⅴ) and I⁻. Zr-OH has a stronger affinity for adsorbing As, forming a more stable Zr—O—As complex, while Cu-OH provided additional sites. I⁻ precipitated in the form of CuI and could form strong chemical bonds. When the pH＞6.77, electrostatic adsorption vanishes. As (V) then occupies the active adsorption sites and establishes a more stable Zr—O—As linkage. I⁻ predominantly swaps with the surface metal-OH of the adsorbent to form a surface metal complex. At this stage, Cu-OH becomes pivotal in the adsorption of iodide. Concurrently present with As (V), iodide will also vie for the surface metal hydroxyl groups.