The present study focuses on innovative approaches through use of novel sorbent materials for detection of toxic metals dissolved in water. Two environmentally sustainable, simple-to-apply processes using different inorganic and polymeric sorbent materials are detailed in this investigation.
The first study reports a new approach for sensing of toxic metals such as lead, copper, zinc and nickel in water using pH as the sole surrogate indicator. An environmentally benign, inexpensive hybrid inorganic sorbent material (HIM) forms the heart of the process. A sharp change of pH in the exit solution signals the presence of toxic metal(s) in the feed which can be even understood through change of color using an appropriate indicator solution. At this point, upon withdrawal of toxic metal from the feed, the exit pH again rises to the alkaline domain, which confirms that exit pH is responsive to fluctuations of toxic metal concentration in the feed. The slope of the pH curve (i.e -dpH/dBV where BV is the bed volume of solution passed) shows a sharp and distinctive peak for different toxic metals.
Two types of hybrid inorganic sorbents have been synthesized from a mixture of (i) akermanite rock (primary constituent Ca2MgSi2O 7) and HFO; and (ii) calcium oxide, magnesium oxide and silica, through a rapid thermal fusion technique. Subsequently, these materials have been characterized, and used for investigations to validate the concept of sensing of trace concentrations of toxic metals in water through pH changes. Since pH is the sole detecting parameter, the presence of commonly encountered buffering solutes such as carbonate, phosphate and natural organic matter (NOM) is likely to interfere with the detection process. The present study overcomes the interference of these buffering species. It validates detection of trace lead and zinc in μg/L through pH swings in synthesized samples, and in Lehigh River water in the presence of phosphate and natural organic matter (NOM) using a hybrid anion exchanger (HAIX) ahead of HIM. The sensitivity of this technique for detection of an ultra-low concentration (<10 μg/L) of lead is verified with the aid of a pre-concentration analytical approach.
The second inquiry reports a unique sorption based colorimetric detection of trace copper in the presence of the strong chelating agent ethylene di-amine tetra acetic acid (EDTA) and other competing metals (e.g. nickel, zinc, iron, lead) through complementary use of two chelating polymeric ion-exchangers. Of all the environmentally regulated toxic metals, copper is unique for it is exceedingly more toxic to fish and other aquatic biota than to human beings. The present study exhibits selective copper sorption onto Dow-3N from the background of much higher concentration of EDTA, other common electrolytes and competing metals through adjustment of influent pH to about 1.5. Selective, interference-free copper sorption onto Dow-3N, followed by desorption through 4% NH3 produces copper-rich regenerant which is subsequently held onto immino di-acetate (IDA) chelating fiber demonstrating a characteristic turquoise blue color of the copper-IDA complex. This technique validates identification of trace copper in different synthetic samples, wastewater (after secondary clarifier) spiked with copper in the presence of EDTA and competing metals. In essence, the ability of Dow-3N to overcome the interference of EDTA and other metals at low pH (∼1.5) while maintaining high copper sorption, and the attribute of IDA fiber to form copper-IDA complex at an alkaline pH complemented by its low intra-particle diffusional resistance result in interference-free, colorimetric detection of trace copper.
The outcome of the present investigation is likely to be attractive in many diverse applications of water quality control. For the first technique, since pH is the sole surrogate indicator for the diagnostic tests of toxic metals in water, no major instrumentation, besides a pH meter, is necessary. The process is free from interference of relatively high concentration of common electrolytes; it easily overcomes interferences of phosphate and NOM through slight modification of the bed, and validates sensing of ultra-low metal concentration (<10 μg/L) using a pre-concentration method. The second technique describes a robust, confirmatory colorimetric detection process of trace copper in the presence of EDTA and competing metals in water. These easy-to-apply sensing techniques seem to have strong potential for applications beyond laboratory set-up, particularly for in-situ detection in remote locations, both in developing and developed countries. (Abstract shortened by UMI.)
