This Small Business Innovation Research Program (SBIR) Phase I project is aimed at development of a conductometric cell for water quality monitoring that employs properties of polyxoxmetalats (POMs) and their so called lacunary derivatives. The proposed approach is based on measurements of electric conductivities of solutions of lacunary POMs and water samples under investigation before and after mixing in the conductometric cell consisting of two chambers. The lacunary POMs are very soluble in water and highly charged anions capable of incorporation of multi-valent cations, such as Ca2+ and Mg2+, which are primarily responsible for total water hardness. Changes in solutions' conductivities due to the encapsulation of these cations into lacunary POMs will be used for their quantitative determination in water samples. Research objectives of this project will include development of the conductometric monitor and semi-empirical calibration models for determination of water hardness. Performance of the cell will be compared with potentiometric and colorimetric techniques currently used on the market for water hardness determination. The anticipated technical results of this research will lead to implementation of this technology to produce a series of water hardness monitors ranging from portable instruments to in-line industrial grade monitoring systems.
The broader impact/commercial potential of the proposed innovation relates to application of this instrumentation to reliable water analysis. This research project will enhance better understanding of encapsulation of multi-valent cations into lacunary POMs under moderate conditions and its effect on electric conductivities. The main area of application of the proposed approach is in water quality control and environmental monitoring. Development of this technology will be beneficial for water distribution and monitoring infrastructure. It will be of interest to a wide spectrum of customers: municipal facilities, beverage producers, hospitals, hotels, laundry facilities, and industrial plants. Water hardness creates a variety of problems negatively affecting efficiency of heat transfer and water distribution systems, and inexpensive and reliable in-line instrumentation is of great demand on the market. The most prospective application of this monitor is anticipated to be in advanced municipal water treatment. In the United States, this market is growing at a compound annual growth rate of 8.2% to reach $2 billion in 2016. POMs are very diverse, and their lacunary forms are capable of incorporation of various cations. If proven successful, this approach can be extended to monitoring of other species and toxic pollutants in drinking water or wastewater.
The major objective of this Small Business Innovation Research Phase I project was investigation of a new approach for detection of multi-valent cationic species ("hard ions") in water samples based on conductometric monitoring of selective encapsulation of cations of interest into polyoxometalate anions (POMs). This research included proof-of-concept feasibility study of application of POMs for conductometric analysis of water samples, development of algorithms and optimization of data acquisition and analysis for reliable total hardness (calcium and magnesium ion content) measurements of fresh water samples, preparation and experimental verification of prototypes of polyoxometalate-based conductometric cell. The proposed approach was tested in a series of electric conductivity measurements and conductometric and potentiometric titrations of solutions of so called lacunary POMs (or "defective," in which one or more tungsten atom are missing) and water samples under investigation. In this research project, the most commonly used POM – phosphotungstate PW12O403- and its monovacant lacunary form PW11O397- were used. The lacunary POMs are capable of incorporating multi-valent cations, which affects conductivities of the solutions containing initial POMs and multi-valent cations. Very attractive features of POMs for conductometric measurements are that POMs are very highly charged anions and are highly soluble in water. It was postulated that different behavior of multi-valent cations, as opposed to that of mono-valent cations, toward POMs could provide quantitative outlook for the ion composition of the water samples under investigation. The proposed approach involved building semi-empirical and empirical calibration models. In implementation of this approach, it was also assumed that the number of major ions (K+, Na+, Ca2+, Mg2+, HCO3-, Cl-, SO42-) and common inorganic contaminants (Fe2+, Fe3+, Cu2+, Ni2+, Zn2+) in natural fresh waters is limited. This feasibility study showed that in aqueous solutions, lacunary POMs form stable complexes with multi-valent cations, with composition depending on stoichiometric ratio of POM to metal content. The observed effect can be monitored using conductometry and used for determination of concentrations of multi-valent cations in water samples. In addition to initially stated goals, formation of new 2 : 1 (POM to metal molar ratio) complexes of lacunary POMs with two-valent cations in solution was discovered. We have established that the major phenomenon that determines conductometric and potentiometric behavior in reactions of monovacant lacunary POM with major "hard ions" in fresh waters is formation of 1 : 1 and 2 : 1 complexes. Outcomes of the Phase I research can be summarized as follows: Methodology was developed for application of lacunary POMs for determination of concentrations of multi-valent cations in water samples; Optimal parameters for implementation of this method for water hardness determination were formulated. It was established that encapsulation of two-valent cations into lacunary POMs takes place at moderate temperatures, is favored by lower pH, and requires at least two-fold molar excess of POM to metal content; Formation of 2 : 1 complexes of POM with two-valent cations in aqueous solutions was confirmed. It was discovered that stoichiometric ratio of POM to metal content in water samples determines formation of 2 : 1 or 1 : 1 complexes, which has strong effect on conductometric properties of solutions. We observed formation of 2 : 1 complexes of the anion PW11O397- with Mg2+, Cu2+, Ni2+, and Zn2+ that had never been reported; The prototype of the conductometric cell was assembled and tested for in-line water hardness analyses, and the hardware performance was evaluated. Theoretical findings from this project contribute to understanding reactivity of lacunary POMs, their stability and selectivity toward cations present in fresh waters. This feasibility study showed that conductivity can serve as a reliable tool in analysis of behavior of POMs in solutions. Conductometric instrumentation is inexpensive, and its use can be extended both in laboratory and industrial settings. Practical implication of these findings can be used for analytical purposes, as increasing importance of water quality require reliable instrumentation and development of new monitors for municipal, industrial, or household water purification and treatment systems. The most prospective application of this approach is anticipated to be in determination of water hardness to monitor efficiency of advanced municipal water treatment and desalination systems. The developed approach can be extended for analysis of a variety of multi-valent cations. Findings from this project are likely to make an impact on expansion of knowledge on new aspects of chemistry of POMs and their applications for analytical purposes, new approaches in environmental monitoring and analytical instrumentation.
