Bert Allard

Abiotic reduction of mercury by humic substances in aquatic system-an important process for the mercury cycle

The potential ability of humic substances to reduce Hg(II) to Hg(0) in aqueous systems and, consequently, strongly influence Hg speciation and mobility in the environment is known but has not been studied in detail. A demonstration of the redox behavior of Hg in the presence of humic substances is made in the present work. Calculations show that the reduction is thermodynamically possible. The effects of some chemical parameters (pH, aerobic/anaerobic conditions, presence of chloride) on the process were studied experimentally. Hg(0) production was highest in O2-free systems in the absence of chloride at pH ca 4.5, when ca 25% of initially 2x10−6 M Hg(II) was reduced to Hg(0) in 50 hr. The presence of a competing ion (10−4 M Eu) in the system as well as methylation of the carboxyl groups in the humic substance considerably reduced the Hg(0) production. The practical importance of the abiotic reduction of Hg in the environment is pointed out.

EFFECTS OF ACIDIFICATION AND NATURAL ORGANIC MATERIALS ON THE MOBILITY OF ARSENIC IN THE ENVIRONMENT

Effects of acidification on adsorption and potential mobility of various As forms have been studied in batch-type distribution experiments. The adsorption of As on alumina decreased in the order As(V) > MMAA = DMAA > As(III) at pH below 6 and As(V) > As(III) > MMAA = DMAA at pH above 6. The adsorption reached a maximum around pH 5 for As(V), pH 7 for As(III) and pH 4 for MMAA and DMAA. The presence of a fulvic acid at concentration levels of 10 mg L−1 or higher generally reduced the As adsorption in the pH range 5 to 7. In light of both laboratory and field observations environmental acidification would increase the leaching of As from soils or sediments to surface and groundwaters under reducing conditions, but could also reduce the mobility due to enhanced adsorption under oxidizing conditions.

Influence of pH and organic substance on the adsorption of As(V) on geologic materials

The adsorption of As(V) on alumina, hematite, kaolin and quartz has been measured as a function of pH (2 to 10), and As concentrations (10−4 to 10 −8 M; in the alumina and kaolin systems only). The effects of sulfate (0 to 80 mg L−1) and fulvic acid (0 to 25 mg L−1) were studied. The charge of the solid surface and the As speciation in solution (determined by pH) were the most important chemical parameters affecting the sorption behavior. At pH below PZC of the solid, there was a qualitative correlation between the adsorption and the anion exchange capacity of the solid. For hematite at low pH (below 5) there was a reduction of the adsorption possibly related to the formation of positively charged species. The presence of sulfate or fulvic acid reduced the adsorption.

Chemical composition of cement pore solutions

Abstract The chemical compositions of the pore solutions extracted from seven different cement pastes (one Swedish and one French standard Portland cement, sulfate resistant, blast-furnace slag, fly ash, silica, and high alumina cement) have been determined. Analyses covered Na, K, Ca, Mg, Al, Fe, and Si as well as pH and Eh (redox potential). Ionic strengths in the range 0.03–0.29 M and pH-values in the range 12.4–13.5 were obtained. Only the pore solutions from the slag cement and the French Portland cement were reducing (negative Eh). The dominating cations of standard Portland, sulfate resistant, slag, silica and fly ash cement pore solutions were Na and K, and in sulfate resistant cement also Ca. The main components in the pore solutions of aluminate cement were Na and Al. The Si and Fe concentrations were low in all pore solutions.

Terrestrial Sources of Methylmercury in Surface Waters: The Importance of the Riparian Zone on the Svartberget Catchment

The runoff of methylmercury (MeHg) from forested catchments to surface waters has been identified as a potentially significant input of MeHg to the aquatic ecosystem. Little, however, is known of the processes which control the transfer of MeHg from soils to surface waters. This study investigated the potential terrestrial sources of MeHg in runoff by sampling profiles of soil solution chemistry and determining the flux of water through those profiles into two tributaries on the Svartberget Catchment in northern Sweden. One study profile was from the podzol soil that covers most of the catchment area. The other profiles were taken in the riparian zone of each of the two tributaries. Soil solution was extracted from the soils by centrifugation.High catchment soil solution concentrations of MeHg (>1 ng l−1) occur in the surface layers of the soil, but overland flow on the catchment is rare. MeHg concentrations in the podzol profile dropped to less than 0.2 ng l−1 in the mineral soil just 5 cm below the mor layer. In the riparian soil profiles sampled in October, MeHg concentrations were higher (ca. 0.4 ng l−1), but in a July sampling the concentrations in the riparian profiles were comparable to those in the podzol (i.e. < 0.2 ng l−1). Very high concentrations of MeHg were found in the streambank sphagnum mosses (>2 ng l−1) partially submerged within the stream.The concentrations of MeHg observed under the podzol soil were insufficient to sustain the concentration of MeHg in runoff from the forested subcatchments where podzol profiles cover more than 70% of the surface area. The only sources of additional MeHg that lie along major runoff flow pathways are the riparian soils and mosses on the streambanks. It is therefore hypothesized that output of MeHg from the forest areas of the catchment is controlled by the biogeochemical processes in the riparian zone.

Speciation of heavy metals in road runoff and roadside total deposition

The concentrations of Cd, Co, Cu, Pb, W and Zn were measured in road runoff and total deposition at two Swedish field sites during one year. It was found that the concentrations of most elements increased significantly during the winter, up to one order of magnitude. For cobalt and tungsten, it was found that around 90% of the total mass transport occurred during the winter, whereas for Cu, Pb, Cd, Zn and Na, the corresponding figures were 70–90, 40–80, 60–90, 50–70 and ≥99% depending on site specific conditions. The deicing salts (rock salts) did not significantly contribute to the increase in trace element concentrations. Instead, the increased concentrations were due to more intense wearing of the pavement during the winter because of the use of studded tires in combination with the chemical effects caused by the use of deicing salts. New potential elemental markers for roads and traffic are suggested.

Degradation of humic substances by UV irradiation

Abstract Aqueous solutions of humic substances (humic and fulvic acids) were irradiated with light at λ = 254 nm followed by analyses of total organic carbon (TOC), ultraviolet (UV) absorbance and molecular weight distribution. A degradation of the dissolved humic compounds by up to 50% after 6 h was observed. The average molecular weight decreased (from 1800 to approx. 300) as well as the UV absorbance of the remaining organic fraction. Low molecular weight organic acids were formed during the irradiation and tentatively identified by capillary electrophoresis (formic, acetic, succinic, and oxalic acids). The low molecular organic acids continued to degrade by further irradiation; the loss of TOC is assumed to be due to CO 2 formation. Indications of a large-scale photo degradation process in nature was observed as a seasonal variation of the ratio humic substances/TOC related to the length of the day. Weekly sampling and analysis of UV absorbance and TOC in the Kalix River in northern Sweden indicate a higher humic fraction of the TOC during winter (dark period) than during summer.

Determination of low molecular weight organic acids in soil solution by HPLC

An HPLC method employing an ion exclusion column was developed for the determination of low molecular weight organic acids in soil solution. The method includes extensive sample pretreatment using ultrafiltration and cation exchange. The method showed linear calibration graphs (r>0.99) and the limits of detection in the range 0.1-26 muM. The recovery of eleven added acids ranged from 89 to 102%. Soil solutions of five horizons of a podzolised soil were analysed. The results showed that these compounds made up 1-3% of the dissolved organic carbon and 0-14% of the acidity. Identification of the major acids was also carried out by capillary zone electrophoresis.

Identification of the riparian sources of aquatic dissolved organic carbon

Abstract The riparian zone has been suggested by several researchers as the place where organic-rich runoff acquires much of its dissolved organic carbon (DOC). The great variation in the hydrochemical environments within the riparian zone, however, makes quantification of the contribution from specific source areas within the riparian zone desirable. This paper explores the feasibility of identifying those sources by comparing the quality of DOC and other chemical parameters of runoff with soil solutions from different locations within the catchment. The study was located on the Svarberget Catchment in northern Sweden. Two soil profiles, one a riparian peat and the other an iron podzol, as well as streambank moss, were sampled as potential sources of aquatic DOC. Soil solutions in these samples and runoff were characterized by their inorganic chemistry, separation of the humic/fulvic component of DOC, and gel filtration to determine the molecular size distribution of DOC. The results show that the superficial organic horizons differ markedly in a variety of ways from the chemistry of runoff. The most likely origin of runoff is a mixture of upslope water from the B horizon with subsurface soil solution from the riparian zone. This is in agreement with what is known about flow pathways at Svartberget, but the information collected in this initial study was insufficient to quantify the contribbution from specific sources. Potential, however, is seen to increase the power of this approach by using more techniques to differentiate the chemistry of specific source areas and exploit information provided by both seasonal and flow-related variations in the chemistry of runoff.

Microorganisms as metal sorbents : comparison with other soil constituents in multi-compartment systems

Microorganisms as metal sorbents : comparison with other soil constituents in multi-compartment systems