Toxic Metal Sensing through Novel Use of Hybrid Inorganic and Polymeric Ion-Exchangers

A new sorption-based technique for sensing trace toxic metals (e.g., lead, copper, zinc) in water is described in the article. The methodology uses a novel, environmentally benign, inorganic hybrid sorbent material (HSM) and the pH is the sole surrogate parameter for the detection of toxic metals. HSM is a granular composite of calcium magnesium silicate (Ca2MgSi2O7), sparingly soluble, and maintains a near-constant alkaline pH in contact with water due to steady release of hydroxyl ions (OH-) caused by slow hydrolysis of HSM. When water containing common electrolytes (Na+, Ca2+, Cl-, SO42-) is passed through the HSM bed in a mini-column, the exiting pH consistently remains around 9.0. In contrast, in the presence of trace concentration of toxic metals (say lead or zinc) in water, there is a significant drop in pH (2 units) under otherwise identical conditions. The drop in pH and breakthrough of toxic metal at the column exit occur concurrently. Since the pH is the sole indicating parameter, the presence of commonly occurring buffering species is likely to interfere with the sensing process. While interference of bicarbonate is avoided through adjustment of the inlet pH, the effect of phosphate and natural organic matter (NOM) is overcome using a hybrid anion exchanger (HAIX) prior to HSM. In HAIX, hydrated ferric oxide nanoparticles, dispersed within the polymeric phase, selectively remove phosphate or other weak acid anionic ligands (e.g., fulvate in NOM). This technique validates the detection of lead and zinc in synthetic water, tap water, and river water. The use of HAIX in conjunction with HSM allows the selective removal of phosphate/NOM, while HSM offers selective binding of toxic metal at slightly alkaline pH (9.0) resulting from its unique dissolution behavior. Also, different toxic metals, such as Pb, Cu, and Zn, are apt to form labile metal-hydroxy complexes at this pH condition causing dissipation of OH- in the aqueous phase. In essence, all these phenomena synergistically work for interference-free sensing of toxic metals through pH change. Except the pH meter or the indicator solution, no other instrument or chemical is required. This technique has immense potential for field applications in both the developed and the developing world.