Cleaning water with mud: clay minerals producing reactive oxidizing species

We take for granted that high quality drinking water is delivered directly to our home and that the wastewater we produce is treated to a level where it is safe to be released into the environment. Water treatment involves, however, intense inputs in the form of chemicals and energy to transform organic contaminants into a harmless form and to destroy harmful microbes, making water treatment a financially and environmentally costly process. In the proposed research, we will explore whether an abundant and low-cost natural material, clay minerals, can sustainably generate reactive oxidizing species for advanced oxidation in water treatment. This approach would need no chemical or energy input and would therefore minimize negative environmental impacts as well as make advanced oxidation affordable also for low-income countries.

We suggest that ferrous iron-containing clay minerals can produce a series of reactive oxidizing species during the reaction with oxygen, similar to what is known for dissolved ferrous iron. Using iron in the structure of clay minerals is, however, key to reliable and repeated regeneration of oxidizing reactivity at the site where oxidation is needed and thus to designing a sustainable advanced oxidation process. In this research, we will provide a proof of concept for advanced oxidation based on iron-bearing clay minerals and will specifically investigate
(1) whether ferrous iron-containing clay minerals oxidize contaminants and inactivate microbes in the presence of oxygen;
(2) which reactive oxidizing species is produced during the reaction and whether any species bound to the clay mineral surface are formed, and
(3) how the oxidative reactivity of clay minerals can be sustainably regenerated.

In a first step, we will thus probe for oxidative reactivity in our clay mineral systems by monitoring the oxidation of model organic contaminants and the viability of microbial organisms. Different clay minerals will be screened to determine the effect of clay mineral properties on their oxidative reactivity and to identify the most promising clay mineral for further investigation. To identify which reactive oxidizing species is produced during the reaction, we will then use specific probe compounds that will react with only one of the different reactive oxidizing species that could potentially form. In additional experiments, clay minerals will be separated from the specific probe compounds by means of semipermeable membranes, allowing us to identify whether reactive oxidizing species are bound to the clay mineral surface. The results from these experiments will demonstrate the effectiveness of iron-bearing clay mineral-based advanced oxidation for disinfection and contaminant oxidation. In a last series of experiments, we will assess how we can reliably and repeatedly regenerate the oxidative reactivity of iron-bearing clay minerals. To this end, we will start with batch experiments in which the re-cycling of clay mineral reactivity will be achieved with chemicals and we will then apply the same approach to column experiments. Next, the activity of specific microbes will be used to regenerate the oxidative reactivity of clay minerals and finally only water flow and column saturation will be managed to show how iron-bearing clay minerals can be used for sustainable advanced oxidation.

In this research, we aim to demonstrate that advanced oxidation for water treatment can be implemented using a natural material often referred to as "mud" and how this advanced oxidation process can be managed and applied sustainably for water treatment or soil and sediment remediation. The expected results will illustrate a way forward to identifying new and intrinsically sustainable water treatment processes that could be applied in high-, middle, and low-income countries.

Access to clean water was identified as one of the UN Global Issues and is a major component of the UN Millennium development goals. In places where water treatment is being carried out, treatment often relies on energy and cost intensive technologies that are therefore not sustainable and have only limited potential for application in low income countries. In the UK, the water industry consumes about 1% of the energy produced in the UK and energy and chemical costs for water and wastewater treatment amount to about 1 billion £ each year.

The proposed project will provide a proof of concept for how to sustainably generate reactive oxidizing species for advanced oxidation in water and wastewater treatment by exploiting the reaction of the natural material Fe-bearing clay minerals with oxygen. Because this novel approach would require neither additional chemicals nor energy, it would help minimize energy consumption, carbon emissions, and chemical costs of water and wastewater treatment. Demonstrating the potential of a natural abundant material to produce reactive oxidizing species may change how and where we look for next generation, sustainable water treatment processes and also suggest a way forward to making advanced oxidation an available and affordable option for developing countries, where most of the roughly 600 million people still lacking access to clean water live. Our research results could therefore potentially benefit the water industry nationally and internationally by reducing energy and chemical costs for water and wastewater treatment as well as water consumers in high, middle, and low-income countries by providing reliable and low cost water services.

Similarly, our approach could change current practices in soil and sediment remediation and transform high-input technologies such as in-situ chemical oxidation into inherently sustainable applications. Reducing chemical and energy inputs in remediation activities will render them more affordable and thus more likely to happen, which will be beneficial for maintaining or increasing the environmental health of entire ecosystems.

Also other engineered water applications such aquifer recharge or aquifer storage and recovery systems could benefit from this project. Generally, oxygen containing and potentially contaminated surface water is the source water for these applications. Our approach could be used to efficiently clean the water on its way to the aquifer and to prevent release of contaminants from reducing aquifers or sediments. The same mechanisms could also be applied in sustainable urban drainage systems (SUDs) in urban areas, where surface water takes up contaminants during run-off and could be cleaned during its passage within the SUDs. Urban environments would thus be protected from potential surface water contamination and urban population could potentially benefit from re-use and recycling of these and other urban water streams such as grey water with low-cost, sustainable approaches as presented here.

The ubiquity, abundance, and low costs of clay minerals suggest that the here proposed new advanced oxidation process could be applied globally in high, middle, and low-income countries in a diverse range of applications.