Why clays? A background on my mini-project with Sarah

In order to get clean water, you have to play around in the dirt.

Many people are aware of the horrific rates of arsenic poisoning through drinking water in Southeast Asia. Professor Scott Fendorf and his team have studied this contamination process extensively in Bangladesh and Cambodia. Now my preliminary mentor Sarah is working under Scott studying a similar process closer to home,  in the Orange County Water District. Don't worry - arsenic levels in Southern California are only a tiny, tiny fraction of the levels in Bangladesh. However, a new plant in Orange County uses cleaned and purified wastewater to recharge subterranean aquifers (see the video and diagram below), and water district officials there need to be sure that the recharging process won't bring dissolved arsenic levels above the California standard maximum concentration.

Original video site here

Once wastewater is cleaned and purified, it re-enters the aquifers through two channels: infiltration basins (below, left side) and deep injection (below, right side). The infiltration basins simply allow the water to seep through the ground and into the higher-level aquifer. Below this first aquifer, however, lies a 'confining layer' made up of much less porous materials, particularly our phyllosilicate clays. 

A diagram of the different modes of the aquifer recharge process showing infiltration basins for subsurface aquifer recharge and deep injection for aquifer recharge past the denser confining layer and into the deeper aquifer.

Water must be injected through this confining layer, but the injection process has been shown to liberate some of the arsenic bound to the soil into the groundwater. Scientists at the Orange County Water Department, in collaboration with researchers like Sarah, have cleverly realized that the presence of certain ions in the injected water hampers the dissolution of arsenic into the water during recharge. In other words, if the optimal concentration of ions can be found, those ions can be put into the water during the purification process and the water can essentially shield itself against arsenic as it is injected through the clays.

The current hypothesis Sarah is working with is that positively-charged ions in the water bind weakly to the permanent negative charge on the clays (see this post of mine). This creates a "bridge" by which the negatively-charged arsenic compounds can bind weakly to the clays (called sorption).

Our project now is to figure out which ions are best at creating these bridges. We currently believe that divalent cations, i.e. alkaline earth metals (below - important because they have a 2+ charge rather than 1+) with high charge density (thus, smaller radii) will be best at helping the arsenic to sorb onto the clays and stay out of the water. When we get our data, hopefully we'll have a clue if we're right!

The periodic table helps us to think that beryllium, magnesium, and calcium will help the most at keeping arsenic out of the water.

Figure 1 & 2 Source: Fakhreddine S., Dittmar J., Phipps D., Dadakis J. and Fendorf, S. 2014. “Influence of Calcium and Magnesium on Arsenic Sorption to Phyllosilicate Clays,” In Proceedings of Goldschmidt Annual Meeting, Sacramento, CA.