Biogeochemical Cycling of Silver in Acidic, Weathering Environments

Abstract

Under acidic, weathering conditions, silver (Ag) is considered to be highly mobile and can be dispersed within near-surface environments. In this study, a range of regolith materials were sampled from three abandoned open pit mines located in the Iberian Pyrite Belt, Spain. Samples were analyzed for Ag mineralogy, content, and distribution using micro-analytical techniques and high-resolution electron microscopy. While Ag concentrations were variable within these materials, elevated Ag concentrations occurred in gossans. The detection of Ag within younger regolith materials, i.e., terrace iron formations and mine soils, indicated that Ag cycling was a continuous process. Microbial microfossils were observed within crevices of gossan and their presence highlights the preservation of mineralized cells and the potential for biogeochemical processes contributing to metal mobility in the rock record. An acidophilic, iron-oxidizing microbial consortium was enriched from terrace iron formations. When the microbial consortium was exposed to dissolved Ag, more than 90% of Ag precipitated out of solution as argentojarosite. In terms of biogeochemical Ag cycling, this demonstrates that Ag re-precipitation processes may occur rapidly in comparison to Ag dissolution processes. The kinetics of Ag mobility was estimated for each type of regolith material. Gossans represented 0.6-146.7 years of biogeochemical Ag cycling while terrace iron formation and mine soils represented 1.9-42.7 years and 0.7-1.6 years of Ag biogeochemical cycling, respectively. Biogeochemical processes were interpreted from the chemical and structural characterization of regolith material and demonstrated that Ag can be highly dispersed throughout an acidic, weathering environment.

Terraced Iron Formations: Biogeochemical Processes Contributing to Microbial Biomineralization and Microfossil Preservation

“…Acidic, iron-oxidizing bacteria can be enriched from TIFs [17,18]. Using a sterile mortar and pestle, a second TIF aliquot was crushed into coarse granules-i.e., 10-mesh sieve size.…”

“…Acidophilic, iron-oxidizing bacteria produce increased H + concentrations as well as schwertmannite and hydronium jarosite ((H 3 O)Fe 3 (SO 4 ) 2 (OH) 6 ) as by-products of their active metabolism. These IOM precipitates occur as pseudoacicular and euhedral crystal morphologies, respectively [11,17,18,29,30]. The average pH of the bacterial enrichments was determined, and IOM precipitates were collected, air dried for 24 h, and weighed to obtain an average mass.…”

“…Previous studies have suggested that TIF structure and development could be considered a modern analog for stromatolites or Martian iron oxyhydroxides [13,14]. More recently, these structures have been used to estimate the mobility of silver, since Ag + can form argentojarosite (AgFe 3 (SO 4 ) 2 (OH) 6 ) [15][16][17]. Additionally, gold biogeochemical cycling has been closely linked to iron cycling.…”

“…The preservation of microfossils with an ancient or modern iron-bearing matrix provides insight into the biogeochemical conditions under which microbial cells and biofilms were preserved [19][20][21][22]. Previous studies have primarily focused on TIFs that developed on gently sloping surfaces where quiescent water often forms meter-size pools-e.g., AMD-affected rivers, streams, and abandoned mine drainage channels [2][3][4]17]. Additionally, few studies have looked at the internal 'architecture' of TIFs at the micrometer scale as a means of interpreting past biogeochemical processes [2,14].…”

The geomicrobiology of mining environments

“…arsenic 13 ; silver 14 ; gold 15 ; rare earth elements 16 ; thiocyanate 17 ), the mechanisms and origins of pH, salt, and metal tolerances (e.g. acid and chloride tolerance 18 ; gold 19 ), and microbemineral interactions 20 .…”