Tuning the synthesis of aluminosilicates to enhance metal cation uptake for heavy metal immobilization

Abstract

Many aluminosilicates are characterized by a pronounced ability to bind heavy metals from aqueous solutions. This work demonstrates the great potential and adaptability of clay materials for use in the emerging field of in-situ groundwater remediation of heavy metals. The synthesis of aluminosilicates was adapted to achieve the highest possible uptake affinity and capacity for Co²⁺, Cu²⁺ and Ni²⁺. This optimization enabled maximum loadings of up to 1.1 mol Me²⁺ kg⁻¹ and K_D values well above 3000 L kg⁻¹. Elevated temperatures during synthesis or drying lead to reduced uptake affinity for metals, e.g., by the factor of 3 for Ni²⁺ when changing drying temperature from 100 °C to room temperature. On the one hand, metal uptake takes place via interaction with variable surface charges, which is strongly pH-dependent and more pronounced for Cu²⁺. On the other hand, there is also a certain degree of interaction with permanent surface charges through isomorphous substitution leading to more stable bonds. Individual experiments showed comparable uptake of the various metals with 0.6 mol kg⁻¹ at pH 3 up to 1.1 mol kg⁻¹ at pH 5. However, Ni²⁺ was preferably taken up in direct competition with the other metals. Thanks to the simple low-temperature synthesis, the high metal loadings and robust metal uptake achieved even at low pH, amorphous aluminosilicates represent a valuable alternative to other products for heavy metal removal from aqueous solution. Future research should focus on their interaction with different metals simultaneously and metal uptake in flow-through systems.