Sensing of toxic metals through pH changes using a hybrid sorbent material: Concept and experimental validation

Abstract

AbstractThis article reports a new hybrid sorbent material that is capable of detecting trace concentration of toxic metals, such as zinc, lead, copper, nickel, etc., through pH changes only. The material is essentially a composite granular material synthesized through rapid fusion of a mixture of amorphous hydrated ferric oxide (HFO) and akermanite or calcium magnesium silicate (Ca2MgSi2O7). When a water sample is rapidly passed through a mini‐column containing this hybrid material, effluent pH at the exit always remains alkaline (≈9.0) because of slow hydrolysis of akermanite and steady release of hydroxyl (OH−) ions. This exit solution turns pink through the addition of a phenolphthalein indicator. Commonly encountered electrolytes containing sodium, calcium, chloride, and sulfate have no impact on the exit pH from the mini‐column. However, when trace concentration of a heavy metal (say lead) is present in the sample water, a considerable drop in pH (>2 units) is observed for the exiting solution. At this point, the solution turns colorless through the addition of a phenolphthalein indicator. Moreover, the change in the slope of pH, i.e., −dpH/dBV, provides a sharp, noticeable peak for each toxic metal where BV is the bed volumes of solution fed. The technique allowed detection of zinc and lead through pH swings in synthesized samples, spiked Bethlehem City water, and also in Lehigh River water in the presence of phosphate and natural organic matter (NOM). Using a simple preconcentration technique, lower than 10 μg/l of lead was detected with a significant peak. From a mechanistic viewpoint, high sorption affinity of HFO surface sites toward toxic metal cations, ability of akermanite to maintain near‐constant alkaline pH for a prolonged period through slow hydrolysis and labile metal‐hydroxy complex formation causing dissipation of OH− ions from the aqueous phase provide a synergy that allows detection of toxic metals at concentrations well below 100 μg/l through pH changes. Nearly all previous investigations pertaining to toxic metals sensing use metal‐selective enzymes or organic chromophores. This simple‐to‐operate technique using an inexpensive hybrid material may find widespread applications in the developing world for rapid detection of toxic metals through pH changes. © 2009 American Institute of Chemical Engineers AIChE J, 2009