A new view on Arsenic in groundwater

Research with global impact – 469 downloads since publication in late 2015!

Impact

Up to 50 readers may access a free online version of our ES&T research paper: Arsenic and phosphorus association with iron nanoparticles between streams and aquifers: implications for arsenic mobility. Just click here to access.

Arsenic is a hugely important contaminant in aquifer systems. In the sedimentary aquifers of South East Asia, arsenic (As) accumulates due to natural processes, driven by the microbial consumption of organic material under low-oxygen conditions. The resulting effects on human health can be truly appalling. Termed arsenicosis, it affects over 130 million people in 30 countries (Ref 1).

We have lots of arsenic here in New Zealand too, as a result of geothermal activity, but thankfully we have sufficiently advanced drinking water treatment processes in place that no case of arsenicosis has been reported in NZ (to my knowledge)!

Arsenic contamination areas.jpg

Arsenic poisoning is a global problem arising from naturally occurring arsenic in ground water (Source: Wikipedia.org).

Back in 2012 I did some work on arsenic in groundwater with colleagues at the Connected Waters Initiative Research Centre, University of New South Wales, Australia. This was during my postdoc and was really a side project in collaboration with Josh Larsen, at the time a fellow post doc at UNSW. Josh was working at a site in the Namoi region of NSW, part of the Murray-Darling Basin. The site, Maules Creek, has been developed by Martin Andersen at UNSW to study groundwater-surface water interactions: literally the exchange of water between streams and aquifers. We headed to Maules Creek and over the course of two hot and sunny days, did some pretty interesting work on As, focusing on its interaction with natural nanoparticles (tiny particles with dimensions 1000 times smaller than the thickness of a human hair). For a primer on nanoparticles you could do worse than checking out my article on it here. It’s free and open access, and written for a lay audience.

Following this work, we knew we had something pretty good, but with Josh and I going separate ways following our postdocs and with a multitude of other distractions taking precedence, it took until this year for us to write the work up. I’m pleased to say that it’s just been published in the American Chemical Society publication Environmental Science and Technology. Those of you with institutional access can read the full article here.

What was novel about this work?

We took laboratory equipment into the field, and separated nanoparticles under an inert atmosphere (high purity nitrogen) in order to measure the degree of association of As with the particles.

IMG_4375IMG_4405IMG_4397

Fieldwork by the Connected Waters research team at Maules Creek (left to right: Denis O’Carroll pumping shallow groundwater using a peristaltic pump and monitoring chemistry in a flow cell, Denis and Adam Hartland ultra-filtering groundwater under pure nitrogen, Josh Larsen spear fishing).

What did we find?

We discovered that the concentration of As in the size range occupied by nanoparticles (1-100 nanometres) was strongly controlled by the concentration of iron in the same size range. Most significantly, we found that the amount of As present as truly dissolved ions was dictated by the presence of phosphate ions. Phosphate forms an anion (a negatively charged ion), which is very similar to the ionic form of arsenic. Both these species will adsorb (chemically bind, or adhere) to the surface or iron oxides. The nanoparticles provided surfaces for the phosphate and arsenite ions to associate with, but it seems phosphate binds more strongly, causing displacement of As.

Fig 4 revised

Evidence for competitive adsorption between P and As in the presence of iron-bearing nanoparticles in Maules Creek surface water and groundwaters. Data presented are solution concentrations in ultrafiltered fractions.

What does this mean for our understanding of arsenic in groundwater?

Well, this work shows that arsenic has a larger chemical repertoire than simply being dissolved- or in the solid-phase. Arsenic can be carried by nanoparticles in aquifers, meaning that it can potentially travel further than we might expect if As were purely present in a dissolved form. Furthermore, the amount of As in the nanoparticulate form, depends on the concentration of phosphate or other competing anions. More work needs to be done from more contaminated systems, but this study may provide the answer to why As is able to migrate further in aquifers than would be expected based on its partitioning between being purely dissolved and/or being adsorbed to the aquifer matrix (the sand grains and particles from which aquifers are made).

AH- Cambridge, NZ.

17th November 2015

 

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