Water Treatment

Utilization of the many thousands of acres of flat and rolling land that were created in Appalachia through the coal mining reclamation process promises to expand the size and extent of agricultural production in the region. Indeed, while these poorly utilized lands present economic and ecological challenges, they may also present an opportunity to diversify regional agricultural programs and, at the same time, mitigate some of the negative impacts of surface mining. Unknown, however, is the extent to which past resource extraction and reclamation practices in Appalachia will impact food security and whether crops grown on these mine spoils – which are not reclaimed to prime farmland standards as is done in the Illinois coal basin [1] – will phytoaccumulate harmful substances such as heavy metals. The U.S. National Academy of Sciences, Engineering, and Medicine has identified seven elements of greatest concern to human and environmental health in relation to coal extraction, cleaning, and combustion: As, B, Cd, Pb, Hg, Mo, and Se. These constituents can enter the food chain through several processes: 1) from soil to plants via uptake, 2) from air to plants via direct deposition, 3) from water to plants via uptake or irrigation, 4) from water to animal and humans via drinking water, and 5) from animals to humans via meat, milk, and egg consumption [2-4]. While some elements such as Cu, Ni, and Zn result in phytotoxicity, others including Cd, Mo, Se, and B could pose food chain risks via bioaccumulation [5,6]. Unfortunately, little is known about bioaccumulation of heavy metals in agricultural products produced on Appalachian reclaimed mined lands, or from using water draining these lands for irrigation or hydroponics. Therefore, we propose to use our existing and extensive network established through decades of research and outreach on mine land reclamation [7] to 1) compare heavy metal concentrations in select agricultural products (e.g., tomatoes, apples, honey, nut crops) grown on reclaimed mine lands in Appalachia; and 2) compare heavy metal concentrations both in select agricultural products grown hydroponically using water draining from reclaimed mine lands in Appalachia, as well as the water itself.

Beyond the evaluation of heavy metal bioaccumulation, an assessment of the current infrastructure in the region to develop a viable food production system in the region is needed. The Community Economic Development Initiative of Kentucky has conducted initial investigations of farmers markets in this region and county data profiles. This is a starting point for the analysis, but additional techno-economic modeling is needed to determine the economic viability of non-bioaccumulating crops identified by NRT participants. Key variables that will need to be assessed are the yield potential of these sites, estimates of production cost, land availability, local demand, and transportation costs. There are expected to be lower environmental impacts from producing food closer to the consumer [8,9]. Assuming that crops do not bioaccumulate above hazardous levels it will be imperative to evaluate the sustainability of crop production in this region. LCA will be employed to evaluate at least the first three levels of food production including production and transportation of inputs to the farm, cultivation and processing.

From the foregoing it is likely that some degree of water treatment – especially distributed water treatment – will be needed to render remote growing locations viable for agriculture. In Appalachia, these locations comprise reclaimed mine land, as well as greenhouses constructed on such land. Water treatment can encompass a variety of approaches including methods to curb harmful organic material, sediment and particle filters for suspended solids, ultraviolet for biological contamination, and high-pressure membranes to remove total dissolved solids (TDS). In Appalachia, mining activities, mountaintop removal, and valley filling conducted over many decades have resulted in TDS contamination of the local water sources that would normally be used for food production. The pH of the water can range from acidic to basic, depending on the local chemistry, with high levels of calcium, magnesium, sulfur, manganese, and iron being common [10-14], in addition to possible heavy metals. Therefore, developing and/or tailoring water treatment technology to efficiently remove these contaminants would be highly beneficial.

To accomplish this, the use of capacitive deionization (CDI) – an emerging water treatment process to remove TDS from water – is promising. CDI operates via electrostatic adsorption of ions onto highly porous carbon surfaces using low applied voltages (<1.2 V). Ions concentrated into a waste stream are removed from the environment and beneficially reused. Due to the use of low voltages, separation can be driven by solar panels, eliminating the need for high pressure pumps, distillation equipment, or utility-fed electrical lines, which is important due to the remoteness of certain mining locations. Since solar panels provide DC electricity, they are used directly – without an inverter – by the CDI process. The use of selective carbon chemistry and materials design can also be used to further increase the efficiency, performance, and lifetime of the CDI system [15-17]. The combination of these properties and the salt content common to Appalachian locations makes CDI an ideal process. In this NRT, CDI separation cells will be modified to match the water streams present at site locations (including carbon chemistry optimization depending on observed pH), and integrated solar cell-CDI electrical installations will be explored. The use of functionalized carbon materials with native surface charges will be used to adjust the pH of the treated water streams while removing unwanted TDS. Ultimately, this NRT will answer the question of whether the quality of the plants grown with the water produced from this novel water treatment technology can be competitive with conventional water treatment options such as nanofiltration and reverse osmosis.

For the treatment of high concentration salt streams (>5000 ppm) and/or specific contaminant removal, alternative membrane and electrochemical technologies to CDI are preferred. Technologically, reverse osmosis (RO) is often not capable of high-water recovery due to limitations placed by the osmotic pressure of the solution. Membrane electrochemical processes, such as electrodialysis (ED) and electrodeionization (EDI), are not limited by osmotic pressure, and thus using these membrane technologies has the potential to lead to a low-energy, low-cost alternative to RO under certain conditions, such as dilute mining wastewaters. The power reduction is significant, RO having a specific energy consumption of 3-7 kWh/m³ compared to 0.2-1.5 kWh/m³ for EDI in the case of dilute solutions [18]. Another example of an electrochemical membrane technology is electrocoagulation, which can provide highly specific metals removal through the creation of complex oxides – such as green rust – that can target heavy metals.

The sustainability of deploying these technologies will depend on the cost and the number of units needed for a given region. Thus, utilizing the knowledge of the region of the NRT participants at least two case study areas will be selected to perform a TEA. These two areas should be similar in size but have differing contaminants in the water sources. A TEA will allow for the use of CDI to be compared with alternative technologies such as nanofiltration and reverse osmosis. To conduct this study data will need to be collected on the cost of the technology and the installation. Furthermore, information will need to be collected on the number of units that will be required based on the severity of the contamination.

This multidisciplinary work will span agriculture, forestry, hydrology, plant and soil science, chemistry and engineering and economics.

References

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