Diurnal variations in, and influences on, concentrations of particulate and dissolved arsenic and metals in the mildly alkaline Wallkill River, New Jersey, USA

Arsenic release from arsenopyrite oxidative dissolution in the presence of citrate under UV irradiation

Arsenopyrite oxidative dissolution is one of the most important sources of arsenic (As) pollution in the soils and waters around sulfide mining areas. Sunlight and low-molecular-weight organic acids in the environment affect the redox behavior of sulfide minerals. In this work, the As release from arsenopyrite was studied in the presence of citrate under UV irradiation, and the effects of dissolved oxygen and citrate concentrations and pH on As release rate were also investigated. The results indicated that As release from the oxidative dissolution of arsenopyrite is affected by the complexation between citrate and dissolved iron ions. Under dark conditions in air atmosphere, dissolved oxygen, Fe(III)-citrate and the active intermediate product O₂^- facilitated the release of As at pH 7.0, and the As release rate increased first and then decreased with increasing pH from 5.0 to 9.0. Under UV irradiation in air atmosphere at pH 7.0, the reactive oxygen species (ROS) including O₂^- and OH generated by Fe(III)-citrate through the photo-Fenton reaction accelerated the As release and oxidation. However, Fe(III)-citrate photolysis led to the rapid flocculation and precipitation of dissolved iron ions, inhibiting the further oxidation of arsenopyrite. With increasing pH from 5.0 to 9.0, the As release rate gradually decreased under UV irradiation. Increases in the concentrations of citrate and dissolved oxygen promoted the formation of Fe(III)-citrate and ROS in the reaction system under both UV irradiation and dark conditions. The present work expands our understanding of the geochemical behavior of As in near-neutral pH environment.

The biogeochemistry of arsenic in a remote UK upland site: trends in rainfall and runoff, and comparisons with urban rivers

Ten years of monitoring of rainfall and streams in the remote acidic and acid sensitive moorland and afforested moorland of upland mid-Wales reveals concentrations of arsenic (As) typically <1 µg L(-1). On average, the lowest concentrations occur within rainfall and they have declined over time probably in response to reductions in global emissions. There is a corresponding reduction within the streams except for forested systems where concentrations up to doubled following clear-fell. Within the streams there are both annual cycling and diurnal cycling of As. The annual cycling gives maxima during the summer months and this probably reflects the importance of groundwater inputs and mineralisation/desorption from the surface soil layers. Correspondingly, the diurnal cycling occurs during the summer months at low flow periods with As concentrations highest in the afternoon/evening. For the urban/industrial basins of northern England with historically a much higher As deposition, land contamination and effluent discharges, comparative data indicate As concentrations around three fold higher: strong seasonal patterns are observed for the same reasons as with the uplands. Across the sites, the As concentrations are over an order of magnitude lower than that of environmental concern. Nonetheless, the results clearly show the effects of declining emissions on rainfall deposition and some indication of areas of historic contamination. Arsenic is mainly present in the <0.45 fraction, but cross-flow filtration indicates that approx. 43% is in the colloidal phase at the clean water sites, and 16% in the colloidal phase at the contaminated sites. Part of this colloidal component may well be associated with organic carbon.

Phototrophic biofilm activity and dynamics of diurnal Cd cycling in a freshwater stream

Diel cycles of dissolved cationic metal concentrations commonly occur in freshwater streams in apparent response to coincident cycles in water quality parameters (pH, O2, temperature). Hourly sampling of the Cd-contaminated Riou Mort (France) revealed large diel cycles in "total" dissolved Cd (232-357 nM; < 0.45 microm) and "truly" dissolved Cd (56-297 nM; < 0.02 microm) which were strongly correlated with changes in water pH. Using measured fluxes, a dissolved O2 model was constructed that indicated that benthic metabolic activities, respiration and photosynthesis, were responsible for the diel O2 (and thus, CO2 and pH) variation in the stream. However, microsensor measurements also showed that the pH changes occurred at the biofilm interface earlier than in the bulk water column. This difference in timing was reflected in the Cd dynamics, where pH-controlled sorption effects caused Cd partitioning from the truly dissolved pool onto the biofilm in the morning, and from the truly dissolved pool onto large colloids (0.02-0.45 microm) later in the day. Because this process causes large changes in the bioavailable Cd fraction, it has significant implications for Cd toxicity in freshwater streams. This study demonstrates the profound control of benthic microbiological processes on the cycling of heavy metals in aquatic systems.

Diel changes in water chemistry in an arsenic-rich stream and treatment-pond system

Arsenic concentrations are elevated in surface waters of the Warm Springs Ponds Operable Unit (WSPOU), located at the head of the upper Clark Fork River Superfund site, Montana, USA. Arsenic is derived from historical deposition of smelter emissions (Mill and Willow Creeks) and historical mining and milling wastes (Silver Bow Creek). Although long-term monitoring has characterized the general seasonal and flow-related trends in As concentrations in these streams and the pond system used to treat Silver Bow Creek water, little is known about solubility controls and sorption processes that influence diel cycles in As concentrations. Diel (24-h) sampling was conducted in July 2004 and August 2005 at the outlet of the treatment ponds, at two locations along a nearby reconstructed stream channel that diverts tributary water around the ponds, and at Silver Bow Creek 2 km below the ponds. Dissolved As concentration increased up to 51% during the day at most of the stream sites, whereas little or no diel change was displayed at the treatment-pond outlet. The strong cycle in streams is explained by pH- and temperature-dependent sorption of As onto hydrous metal oxides or biofilms on the streambed. Concentrations of dissolved Ca(2+) and HCO(3)(-) at the stream sites showed a diel temporal pattern opposite to that of As, and geochemical modeling supports the hypothesis that the concentrations of Ca(2+) and HCO(3)(-) were controlled by precipitation of calcite during the warm afternoon hours when pH rose above 9.0. Nightly increases in dissolved Mn and Fe(II) concentrations were out of phase with concentrations of other divalent cations and are more likely explained by redox phenomena.