Specific scavenging of polluting metals by prolate particles of phosphonate functionalized mesoporous silica

Phosphonate and/or bisphosphonate-functionalized mesoporous silica (MS) sorbents were developed and studied using 31P-MAS-NMR, SEM-EDS, TEM, BET, and batch sorption experiments before and/or after metal ion scavenging (Bi+3, Ba+2, Cd+2, U+6, Ag+, Pd+2 and Zn+2). The amount of phosphorus inside the sorbents was ascertained by SEM-EDS, and the phosphonate vs bisphosphonate content derived from NMR. The organic matter loading was between 15 and 38 %w/w and inversely correlated with BET. Pore size (4.9–6.6 nm) and surface areas (219–473 m2/g) were derived from nitrogen sorption isotherms. The NMR signals of the phosphonates are shifted downfield by the metal cations while the bisphosphonate signals are shifted upfield for urea-derived sorbents and downfield for the amino sorbent. The sorbents are composed of prolate particles of micrometric dimensions. The prolate particles of the urea-derived sorbents are stable following the adsorption of metal cations, while they are disrupted in the case of the amino sorbent, due to the repulsion between negative charges at basic pH. Only metal cations with relatively large ionic diameters were adsorbed revealing the need for multi-point interactions inside the sorbent, which is not possible for smaller cations. For all the sorbents, the sorption data of uranyl fit the Freundlich isotherm model, which predicts a heterogeneous surface. The K and Kd values derived from this model show good sorption capabilities. Kds for metal cation solutions (20 μg/mg) on the amino sorbent were between 500 and 35000 mL/g. Both selectivity and sorption capabilities of the sorbents allow their use in environmental analysis and remediation.