Here is a recent study that investigates the mechanisms of arsenic partitioning into, or out of, streambed sediments downstream of the Porgera Gold Mine in Papua New Guinea. [1]
Arsenic (As) is a metalloid element with atomic number 33. Arsenic is known to be released in the environment during gold mining activities. Preventing toxic contamination of soil and water in mine sites is important in a mining operation. This is possible with the application of various techniques on effective arsenic sequestration.
A recent study titled “Arsenic sequestration in gold mine wastes under changing pH and experimental rewetting cycles” was done by Beth Hoagland, Luke Mosley, Tess Russo, Jason Kirby, Cecilia Cullen, Matthew S. Fantle, Mark Raven and Joshua Fisher. Their study was featured by Science Digest website last January 2021.
Why is this study important in protecting the environment and in assuring health and safety to humans?
“The discharge of mine-derived hard rock and liquid tailings waste can alter water and sediment chemistry and release contaminants that pose risks to the functioning of aquatic ecosystems and human health (Nordstrom, 2011; Hudson-Edwards, 2016). Mining companies typically manage these risks by adding lime (Ca(OH)2, CaO) to tailings waste to increase alkalinity and pH and precipitate or co-precipitate metals out of solution. However, such approaches may not be as effective for metalloids (oxyanions), such as arsenic (As), whose response to liming is different than metals such as iron (Fe) (Jones et al., 1997; Smedley and Kinniburgh, 2002). Arsenic, a naturally-occurring metalloid associated with gold-bearing sulfide deposits (Corkhill and Vaughan, 2009; Nordstrom, 2011), is less strongly sorbed to minerals such as Fe-oxides at neutral-alkaline pH compared to metals such as zinc (Zn) and lead (Pb) (Jones et al., 1997). Thus, the efficacy of using lime to remove As from wastewaters is potentially limited as an As remediation strategy and depends on geochemical conditions in addition to pH (Moon et al., 2004; De Andrade et al., 2008; De Klerk et al., 2012)” [1]
How do we determine the treatment options for the removal of arsenic from mine wastes? For this, the geochemical conditions of the receiving environment must be considered.
“The background conditions of the environment receiving the wastes can influence the aqueous concentration, speciation (e.g. arsenite (As(III)) and arsenate (As(V)), and partitioning of As between water and sediment phases (Smedley and Kinnibrugh, 2002; Cheng et al., 2009). Such conditions include climate, the mineralogy of interacting sediments, and the chemistry of interacting waters (e.g. pH, Eh, and other chemical constituents present in solution).” [1]
With regards to the location of the study, its geographic and climatic features, the Porgera catchment is located in the headwaters of the Strickland Watershed in the Enga Province of the Papua New Guinea highlands (5◦27′ 47.83′′S, 143◦ 8′ 45.62′′E).
“Mean annual temperature in Porgera is 15.5 ◦C. The long-term mean annual precipitation is 3750 mm and the long-term daily mean precipitation is 10.4 mm, where precipitation events occur more than 300 days per year (Ross, 2012).” With the tropical rainfall that happens almost daily in Porgera, the mine-derived sediments then interact with dilute rainwaters.
The kind of rocks present in the highlands are underlain by igneous and sedimentary rocks, which host a suite of sulfide minerals such as pyrite (FeS2), sphalerite ((Zn,Fe)S), and galena (PbS).”
These sulfides have submicroscopic gold content, (pyrite and arsenical pyrite). These are extracted using acid-pressure oxidation and recovers using conventional cyanidation techniques (Fleming et al., 1986; King and Knight, 1992).
“After extraction, the mine separates the slurry from the gold-bearing solids and neutralizes associated wastewaters and waste sediments with lime (Ca(OH)2, CaO). The treated waste is discharged into the watershed and creates a braided, rocky channel known colloquially as the Red River.”
From the initial part of the study, they reported that “arsenic release related to gold mining activity can alter surface water and sediment chemistry. However, the toxicity of As in mine wastes, which is controlled by the speciation, concentration, and bioavailability of As, depends on the geochemical conditions of the impacted environment (e.g., pH, climate, mineralogy, etc).” [1]
“This study investigates the mechanisms of As partitioning into, or out of, streambed sediments downstream of the Porgera Gold Mine in Papua New Guinea.”
The use of lime as treatment for mine tailings and then discharged directly into the watershed makes the interaction prone to interaction with rain water. This reduces groundwater or acid rock drainage if it were to develop post mine-closure. [1]
One of the things to consider about the use of lime in the treatment of the mine tailings is that this practice increases the pH. This in effect triggers the precipitation of some trace metals that were derived from wastewaters. With an overall spike in pH, arsenic can now become more soluble.
For the study, the group conducted batch reactor experiments to bring out the effects of changing pH (ranging from 4 to 10) and wetting/drying cycles on arsenic interactions with lime-treated tailings. Another objective of the experiment is to understand the potential arsenic behavior following mine closure. [1]
“Across the pH range investigated, lime-treated waste sediments and streambed sediments located downstream of the open pit mine effectively scavenged As from the water column,” according to the study.
More specifically, tailings that were treated with lime buffered the pH. This reaction enhanced the interactions between dissolved arsenic and sediment surfaces via surface complexation reactions on amorphous iron oxides, “ as suggested by surface complexation modeling and batch reactor experimental results.” This “arsenic scavenging mechanism” further controlled and counteracted the increased solubility of arsenic at high pH.
Another aspect of the study was conducting a wetting/drying cycle experiments. From this, the group was able to infer that lime-treated tailings that are subjected to repeated wetting/drying cycles rapidly desorbed arsenic during the onset of rewetting, “but sorbed arsenic via an aluminum-bridging mechanism in subsequent wetting/drying cycles.” [1]
What useful information can we derive from these results?
In general, these results highlight the importance of continued lime treatment in order to lower the arsenic mobility in mine wastes following mine closure. This is applicable particularly for mine sites where wastes are released directly to the watersheds with no containment infrastructure to gather or filter out the outflow.
These results highlight the importance of continued lime treatment to reduce As mobility in mine wastes following mine closure, particularly for mine sites where wastes are released directly into watersheds with no containment infrastructure.
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In conclusion:
- Gold mine wastes treated with lime facilitated As removal via surface complexation.
- Lime-treatment enhances As sequestration by mine tailings from pH 4 to 10.
- Mineral saturation and Al-bridging influence [As] during wetting/drying cycles.
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Reference:
[1] Hoagland, Beth et. al. (January 2021). Science Direct - Applied Geochemistry. Volume 124. "Arsenic sequestration in gold mine wastes under changing pH and experimental rewetting cycles". Retrieved from - https://www.sciencedirect.com/science/article/pii/S088329272030281X
Other references:
- Department of Geosciences, The Pennsylvania State University, University Park, PA, USA
- Acid Sulfate Soils Centre, The University of Adelaide, Adelaide, SA, Australia
- Earth and Environmental Systems Institute, The Pennsylvania State University, University Park, PA, USA
- CSIRO Land and Water, Contaminants and Biotechnology Program, Urrbrae, SA, Australia
- The Earth Institute, Columbia University, NY, NY, USA
About the map
Map of stream water and streambed sediment sampling sites in the Porgera Watershed. Shaded areas represent areas of concentrated gold mining activity mapped using Google Earth. Background colors correspond to surface elevation above mean sea level (m) determined from the SRTM 30m digital elevation model. The inset map highlights major watersheds of Papua New Guinea and the location of the Porgera catchment in the greater Strickland River watershed. The Anj-Kai label represents the confluence of the Anjolek and Kaiya Rivers. The Red-Up, Red-Mid, and Red-Pog represent sampling sites at the upstream and mid-stream locations in the treated wastes, and the confluence point of the treated waste stream and Pongema Rivers. Note that Yakatabari Creek, or the open pit runoff site, is a small tributary of Kakai River and the stream reach is not depicted on this map. The Yakatabari Creek sampling location is marked adjacent to Kakai River and upstream of the confluence point of these two reaches.
