Research Objectives and Approach: Use of desalinated water is rapidly increasing throughout the world, but potentially major unintended consequences of its introduction into existing drinking water distribution systems are unexplored. This study will determine mechanisms of corrosion of and metal release from iron, lead- and copper-containing materials exposed to desalinated water and its blends with conventionally treated water. The objective is to determine how gradual or abrupt changes of water chemistry (e.g., pH, concentrations of natural organic matter, NOM, phosphate) associated with the presence of desalinated water affect the formation and stability of corrosion scales and metal release from them. Another target is to determine conditions in which rapid destabilization of such scales can take place.
This project will examine these processes based on a consistent approach that addresses: i) transient state and endpoints of corrosion in desalinated water; ii) kinetics of relevant structural transformations; iii) role of colloidal phenomena and NOM and iv) specificities of action of corrosion inhibitors. Work will be carried out using two modes of corrosion exposures with copper, iron and lead-containing-materials. Experiments with scales pre-formed in conventionally treated and exposed to desalinated water, and representative individual solid phases will also be carried out. Methodologically, the study will rely of diverse methods such as XRD, SEM/EDX, colloidal (size distribution, æ-potential and sequential filtration) measurements coupled with analytical determinations.
Advanced experimental and interpretative methods will be used to generate knowledge that is indispensable for successful long-term use of desalinated water, a technology that plays an increasingly critical role in global sustainability. The intellectual merit of this project is based on 1) novel approaches to ascertain mechanisms and quantify contributions of physico-chemical processes (structural transformations, colloidal disaggregation and others) that are specific to desalinated water; 2) elucidation of critical indicators of the composition of corrosion scales that determine their instability and 3) development of approaches to prevent major metal release episodes in desalinated water. The applicants believe, to the best of their knowledge, this study is the first to address these issues based on fundamental science.
This study will provide insight into issues that are increasingly important and visible not only for environmental professionals but for the general public in this country and globally. Results of this project will ultimately help protect public health and ensure sustainable functioning of drinking water infrastructure, a matter of great societal and economic concern. This project will also contribute to training, mentoring and overall development of the UW students, including their exposure to international issues via cooperation with Australian partner institutions, notably Australian Water Quality Centre in Adelaide. Results and techniques of this study will be integrated into the engineering curriculum at the University of Washington, presented at UW Undergraduate Research Symposium and to the general community at the UW Engineering Open House and BRIDGE seminars. The graduate students will enhance their mentoring and teaching abilities by interacting with the laboratory undergraduates, and by presenting their research at seminars and conferences. Elements of this research will be incorporated in an international project-based course (jointly with Tohoku University in Japan) for engineering freshmen. To increase the participation of underrepresented minority and women students in Engineering at the University of Washington, they will leverage the resources of the College of Engineering Advising and Diversity Center with whom the group has partnered in the past.
Outcomes of the project are summarized below: 1. This project determined the extent of effects of desalinated water on iron release and corrosion of that metal. Our experiments showed that corrosion of iron in water blends with varying concentrations of natural organic matter results in markedly different rates of iron release. The latter parameter was determined to be primarily a function of the concentration of natural organic matter (NOM). The observed effects are especially prominent for NOM concentrations < 1 mg/L and reached a plateau at higher NOM concentrations. 2. Examination of exposed iron surfaces showed that the nature of corrosion scales changed drastically following the introduction of desalinated water. As the percentage of desalinated water increased, dispersed ferric hydroxides that were tightly bound to the surface in the presence of unblended conventionally treated water were replaced by crystalline lepidocrocite that can be relatively easily detached from the surface. This finding explains the nature of red water events observed in some cases upon the introduction of desalinated water in drinking water systems. 3. This project examined the retention of trace-level inorganic contaminants ranging from vanadium to uranium by corrosion solids formed in drinking water. Rates of retention of the inorganic contaminants by corrosion solids decreased as the concentration of organic carbon increased. This indicates that if source water contains trace-level inorganic contaminants, they are more likely to accumulate in drinking water systems that use desalinated water or its blends. The solids can be released via colloidal mobilization. 4. Alteration of NOM properties via its oxidation by ozone or chlorine results in a slight decrease of its effect on iron corrosion but NOM remains the dominant factor affecting release of soluble iron in all conditions and retention of surface scales. However, NOM alteration by processes typical for drinking water treatment causes significant changes in the case of release of metals other than iron. For copper, metal release from representative solids (tenorite, malachite) is correlated with the apparent molecular weight of NOM that decreases upon NOM treatment. 5. Experimental and modeling activities to interpret results of this project also resulted in the development of a new concept that can be used to quantify interactions between NOM and solution components such as sodium, calcium, magnesium, aluminum iron and copper in situ. Measurements carried out using this concept yield highly precise data that can be used to check the performance of and provide feedback for advanced formal models of metal complexation in aquatic systems. 6. Results of this project have been presented in seven papers published in Water Research, a refereed journal with one of the highest impact factors in the field of environmental engineering. The project’s results have been incorporated in two doctoral theses and presented at five international conferences. Several invited lectures discussing results and goals of this project as well as issues of sustainability, water reuse and desalination were given to general international audiences in Italy, Australia and China. 7. Results of this project address effects of water quality variations associated with the introduction of desalinated water on drinking water distribution systems that represent a very complex and important part of the infrastructure in the U.S. Our data demonstrate that in some situations, notably in water blends containing low concentrations of organic carbon, desalinated water blends introduced to drinking water systems may cause read water episodes. The data also indicate that particular caution must be exercised in introducing desalinated water into drinking water systems that contain high volumes of corrosion solids but for blending rates typically containing low enough fraction of desalinated water these effects can be minimized.
