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Minimizing Dissolved Silica to Reduce Ash Content in Reconstituted Waters Used in Disinfection Byproduct Health Effects Research
Citation:
Bollman, J., A. Kennicutt, P. Rossman, D. Wahman, AND J. Pressman. Minimizing Dissolved Silica to Reduce Ash Content in Reconstituted Waters Used in Disinfection Byproduct Health Effects Research. American Water Works Association Water Quality Technology Conference, Portland, OR, November 12 - 15, 2017.
Impact/Purpose:
This research will be of interest to engineers, scientists, operators, and managers of drinking water systems
Description:
Previous health effects research used chlorinated, concentrated natural organic matter (NOM) solutions to create whole mixtures of disinfection byproducts (DBPs). Ohio River water was used as the source water to provide the background NOM matrix. Concentrated river water was collected as the retentate in a reverse osmosis (RO) system, some of which was subsequently lyophilized (freeze-dried) to produce a dry, solid form of the NOM that could later be reconstituted at an alternate desired total organic carbon (TOC) concentration. During previous health effects studies with concentrated solutions, experimental apparatuses were clogged with what appeared to be silica gel formed from exceeding silica solubility. It was hypothesized that the high ash content (much of which is silica) of these concentrated waters resulted in issues related to the clogging; therefore, a low-ash alternative was desirable. “Ash content” is identified as the total amount of minerals present in a sample (i.e., the inorganics that remain post-combustion). Comparing the results from elemental analysis, there was a correlation between the dry ash content in the lyophilized NOM solid and the concentration of silica in the RO liquid concentrate (as measured by ICP-AES, inductively coupled plasma atomic emission spectroscopy). Reconstituted waters are meant to be reliable representations of real-world water matrices, which are not typically found saturated with dissolved silica. Additionally, silica has the potential to create interference with analytical detection methods. The goal of this work was to evaluate several ways to minimize the ash content in reconstituted NOM water matrices by minimizing the silica concentration, using silica’s known solubility. Dissolved silica concentration is a function of pH; therefore, NOM was reconstituted at pH 7, 8, 9, and 10. In addition to pH, reconstitution temperature was also evaluated. Any undissolved particles (i.e., silica) were filtered using a 0.45 µm or a 0.20 µm polyethersulfone (PES) membrane. NOM concentration factor was also evaluated for silica solubility limitations where samples of high NOM concentrations were diluted post-filtering to target lower silica content. The ash content in samples of solid NOM were measured using thermogravimetric analysis (TGA), and dry ashing was done in an air-nitrogen mixture by ramping 2°C/min to 750°C and holding for an additional 8 hours. Hach Method 8185 was used for dissolved silica sample analysis. Reconstituted (liquid) samples were oven-dried post-filtering and also measured via TGA for ash content. Results showed decreases in dissolved silica and ash content for the various reconstitution methods. Overall, this research provides a standard reconstitution methodology to minimize silica concentration when using freeze-dried NOM, creating a more representative water matrix that can be reproduced for experimental purposes.