Interfacial study of surface-modified ZrO2 nanoparticles with thioctic acid for the selective recovery of palladium and gold from electronic industrial wastewater

The stability and performance of surface-modified zirconia nanoparticles with thioctic acid to effectively and selectively capture Pd(II) and Au(III) ions from industrial electronic waste water were investigated. A particular interest laid in the robustness of the interface in highly acidic and oxidizing environments under real conditions. Surface modifications of commercial nano-ZrO2 were optimized from Bronsted acid-base surface reactions in aqueous solution, either by direct thioctic acid (TOA) grafting or by a two-step process: (i) alendronic acid (AA) grafting and (ii) amide coupling reaction between AA and TOA, giving inorganic-organic hybrid systems designated respectively as ZrO2-TOA and ZrO2-AA-TOA. All materials were thoroughly characterized and had a similar specific surface area (50 m(2).g(-1)). They also demonstrated similar characteristics in terms of thermodynamic and kinetic adsorption data (Langmuir adsorption model and pseudo-second order model). The ATR-FTIR, DRIFT-FTIR and P-31 solid state NMR data elucidated the bidentate bridging and tripodal bonding modes for TOA and AA coordination, respectively. In addition, the DRIFT technique was used to reveal the covalent amide bond between the -NH2 group of the AA group and the -COOH groups of the TOA. The adsorption capacities of ZrO2-TOA at Pd and Au of 44.6 mg/g and 6.3 mg/g respectively, showed that our systems had competitive adsorption capacities and selectivity compared to the literature. Nevertheless, ZrO2-TOA was too sensitive to hydrolysis in high acid concentration ([HCl] > 1 M) and the use of ZrO2-AA-TOA was then mandatory to overcome hydrolytic cleavage of M-OOC bonds in media with high acid concentration. The novel ZrO2-AA-TOA material demonstrated promising stability, efficiency and reusability in industrial applications for gold and palladium recovery.