Inner Sphere Adsorption Mechanisms for Arsenate and Lead on Goethite: Theoretical Implications in Contaminated Soil Recovery

Studies that do not separate the adsorption of toxic elements and compounds by outer and inner spheres can lose precision regarding the potential pollution of the water table. We sought to establish details of inner sphere adsorption (ISA) mechanisms of arsenate (H2AsO4-) and lead (Pb2+) on synthetic goethite under different experimental conditions: pH 5.0 and 9.0; purified and unpurified samples; different contact times (24 and 240 hours). Goethite (Gt) was saturated separately with Pb and arsenate (noncompetitive adsorption) and the outer sphere forms was previously removed. The ISA of Pb (maximum of 67,682 mg kg-1) was more intense than that of As (maximum of 4,492 mg kg-1). ISA of arsenate on goethite was dependent on the contact time and was more intense in acidic soils due to the greater possibility of the occurrence of biprotonated ferrol groups (Fe-OH2+0.5) on goethite surface. The high negative charge density at the surface of the H2AsO4- tetrahedron favored its ISA. On the other hand, the lower the level of ISA, the greater binding energy on goethite or adsorption stability of H2AsO4-. The highest ISA of Pb occurred at the highest pH, when the hydroxylated species PbOH+ preferentially binds to oxygen from the deprotonated ferrol group (Fe-O-1.5), without the need for ligand exchange. The proposal mechanisms of inner sphere adsorption allow better understanding of the recovery actions of contaminated soils rich in Fe and Al oxides.