This study explores interactions between As and Fe(III) minerals, predominantly schwertmannite and jarosite, in acid mine drainage (AMD) via observations at a former mine site combined with mineral formation and transformation experiments. Our objectives were to examine the effect of As on Fe(III) mineralogy in strongly acidic AMD while also considering associated controls on As mobility. AMD at the former mine site was strongly acidic (pH 2.4 to 2.8), with total aqueous Fe and As decreasing down the flow-path from ∼400 to ∼20 mg L–1 and ∼33,000 to ∼150 μg L–1, respectively. This trend was interrupted by a sharp rise in aqueous As(III) and Fe(II) caused by reductive dissolution of As-bearing Fe(III) phases in a sediment retention pond. Attenuation of Fe and As mobility occurred via formation of As(V)-rich schwertmannite, As(V)-rich jarosite, and amorphous ferric arsenate (AFA), resulting in solid-phase As concentrations spanning ∼13 to ∼208 g kg–1. Schwertmannite and jarosite retained As(V) predominantly by structural incorporation involving AsO4-for-SO4 substitution at up to ∼40 and ∼22 mol %, respectively. Arsenic strongly influenced Fe(III) mineral formation, with high As(V) concentrations causing formation of AFA over schwertmannite. Arsenic also strongly influenced Fe(III) mineral evolution over time. In particular, increasing levels of As(V) incorporation within schwertmannite were shown, for the first time, to enhance the transformation of schwertmannite to jarosite. This significant discovery necessitates a re-evaluation of the prevailing paradigm that As(V) retards schwertmannite transformation.
Abstract Our understanding of tree stem methane (CH 4 ) emissions is evolving rapidly. Few studies have combined seasonal measurements of soil, water and tree stem CH 4 emissions from forested wetlands, inhibiting our capacity to constrain the tree stem CH 4 flux contribution to the total wetland CH 4 flux. Here we present annual data from a subtropical freshwater Melaleuca quinquenervia wetland forest, spanning an elevational topo‐gradient (Lower, Transitional and Upper zones). Eight field campaigns captured an annual hydrological flood‐dry‐flood cycle, measuring stem fluxes on 30 trees, from four stem heights, and up to 30 adjacent soil or water CH 4 fluxes per campaign. Tree stem CH 4 fluxes ranged several orders of magnitude between hydrological seasons and topo‐gradient zones, spanning from small CH 4 uptake to an emission of ∼203 mmol m −2 d −1 . Soil CH 4 fluxes were similarly dynamic and shifted from CH 4 emission (saturated soil) to uptake (dry soil). In Lower and Transitional zones respectively, tree stem CH 4 contribution to the net CH 4 ecosystem flux was greatest during flooded conditions (49.9% and 70.2%) but less important during dry periods (3.1% and 28.2%). Minor tree stem emissions from the Upper elevation zone still offset the Upper zone CH 4 soil sink capacity by ∼51% during dry conditions. Water table height was the strongest driver of tree stem CH 4 fluxes, however tree emissions peaked once the soil was inundated and did not increase with further water depth. This study highlights the importance of quantifying the wetland tree stem CH 4 emissions pathway as an important and seasonally oscillating component of wetland CH 4 budgets.
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