Phosphorus is an essential macronutrient for plant growth and a key element in all biological systems. Zeolites—microporous, crystalline aluminosilicates amenable to modification through green chemical techniques—are one proposed class of material for the recovery of phosphate through adsorption. In this work, copper ZSM-5 zeolites with varying Si/Al ratios were synthesized using an ion exchange method and systematically studied for their performance in adsorbing inorganic dissolved phosphate from synthetic solutions. Maximum orthophosphate uptake of greater than 70% was reported at process-relevant conditions. Copper ZSM-5 with lower Si/Al ratios exhibited the greatest adsorption capacity, with higher pH enhancing uptake. On balance metal-exchanged zeolites were found to present a promising direction in the development of highly tunable synthetic sorbents for inorganic dissolved phosphate.
Metal-exchanged zeolites have long been of interest due to their catalytic activity. In particular, copper ZSM-5 has been studied intensely because of its activity in treating automotive exhaust gases. However, the exact nature of the catalytic sites—generally considered to be Cu dimers of the form [Cu–O–Cu]2+—has never been fully understood. Quantifying the active site structures in metal-exchanged zeolites has represented one particular challenge in overcoming the gaps in understanding between material properties and their fundamental structure. Traditional methods used to titrate these sites have relied on gaseous probes combined with complex spectroscopic analysis. In this work, liquid phase phosphate anion adsorption is proposed as a unique, simpler alternative to probe and quantify the Cu sites in copper ZSM-5. Equilibrium and kinetic data for phosphate adsorption was considered together with the copper loadings of Cu-ZSM-5 prepared at several Si/Al ratios to investigate the zeolite structure. Phosphate adsorption property parameters were found to show strong linear correlations with the Cu density, facilitating quantification of the copper dimer fraction through analysis of the adsorption site capacity at low concentrations. It was concluded that liquid phase phosphate anion adsorption could represent a simple tool for probing and quantifying the copper speciation in copper ZSM-5. This presents a new and unique direction for understanding the active site structures in metal-exchanged zeolites
