J. P. G. Loch

Sorption of cadmium on suspended matter under estuarine conditions; competition and complexation with major sea-water ions

Sorption of Cd at low concentrations onto river Rhine suspended matter was examined in terms of sorption rate, reversibility and factors such as competition and complexation with major inorganic sea-water ions. More than 90% of the final amount of Cd is sorbed within the first few hours. Desorption experiments show that the process is virtually reversible. In experiments with diluted sea water the sorption of Cd strongly decreases even at low salinity. Sorption isotherms show that the sorption of Cd in NaNO3 solutions is regulated by the free Cd2+ activity. In a Ca(NO3)2 environment the Cd sorption decreases with increasing Ca2+ concentrations, which implies competition between Ca2+ and Cd2+ for the different sorption sites. In different electrolyte solutions of similar ionic strength the sorption of Cd decreases in the solution order NaNO3 > NaCl > NaCl+MgCl2+CaCl2 > diluted sea water. Although inorganic speciation calculations show that even at low salinities dissolved Cd is dominated by Cd-chloro complexes, chloride accounts for only about one third of the increased mobility of Cd. As a result of addition of Ca2+ and Mg2+ the sorption capacity of suspended matter for Cd is further reduced by a factor three.

Effect of Fluctuating Hydrological Conditions on the Mobility of Heavy Metals in Soils of a Freshwater Estuary in the Netherlands

The objective of this study was to measure the mobility of heavy metals in freshwater estuary soils that are seasonally inundated and to characterize the distribution of sulfide precipitates in these soils. Precipitation and dissolution of labile sulfides may account for changing pore water concentrations of heavy metals in freshwater wetland soils that are subject to temporary flooding or fluctuating groundwater levels. The presence of authigenic zinc- and iron-(mono)sulfide in this type of soils during periods with a high groundwater level was demonstrated by electron microprobe analyses. Because sulfide precipitates are strongly associated with root remnants, fresh soil organic material may be an important intermediary in the sulfur cycling and, consequently, in the behavior of heavy metals in these freshwater soils. Oxidation of labile sulfides may be partly responsible for the increased zinc and sulfate concentrations in the pore water during periods with low groundwater levels. Heavy metals may also be mobilized by degradation of their host phase organic matter during periods with high biodegradation.

Influence of pH and Zinc Concentration on Cadmium Sorption in Acid, Sandy Soils

Batch adsorption experiments were carried out with samples from an A-, Bh- and C-horizon of contaminated sandy soil of podzolic character from the Kempen region at the Dutch-Belgian border. Cadmium sorption was studied on 3 soil samples at 3 different pH-levels (3.6, 4.3 and soil buffered pH) and 3 different additions of zinc (0–40 mg l-1).Adsorption of cadmium by acid sandy soils can be fitted by a Freundlich adsorption isotherm. Although zinc competes with cadmium for the sorption sites, we observe a two to three times stronger competition effect of the proton cation, which is explained by the chemical properties of both ions. The cadmium adsorption coefficient KF decreases considerably by an increase of the proton activity used in the sorption experiments. Organic matter content explains for a large part the variation of KF of te three soil samples. Desorption data do not fit the proposed regression model for adssorption. Not all the cadmium, intitially present in the polluted soil, will fylly desorb reversibly. Thus, part of the cadmium may be irreversible bound.

ACCUMULATION OF CADMIUM AND ZINC FROM DIFFUSE IMMISSION ON ACID SANDY SOILS, AS A FUNCTION OF SOIL COMPOSITION

Sandy soils, in the border area of Belgium and the Netherlands (the Kempen region), are heavily contaminated by atmospheric deposition of cadmium and zinc from nearby smelters. Groundwater contamination by leaching from these low retention soils is subject of study. There are reports of high cadmium and zinc concentrations in groundwater in the area, but in most cases the direct sources are unknown. In an attempt to predict present or future risk of groundwater contamination by soil leaching, metal binding processes (retardation) were studied that are specific for these soil types under the existing acidifying conditions. From four fields nine contaminated profiles were sampled and analyzed for cadmium and zinc. Average concentrations of 131μg g−1 zinc and 1.6μg g−1 cadmium with maximum values of 2989μg g−1 respectively 16.3μg g−1 were found. In addition pH and contents of organic matter, aluminium, iron, and manganese were determined. The relative importance of these soil parameters for metal retardation is derived from the profiles. The data show that organic matter is the most important soil component for binding cadmium and zinc. Adsorption of cadmium and zinc on aluminium, iron and manganese (hydr) oxides appears to be of minor importance at low pH (<5.5).

Accumulation of HNO3-extractable tin in agricultural and non-agricultural soils by the use of triphenyltin acetate

The accumulation of extractable tin (Sn) due to use of the fungicide triphenyltin acetate has been studied on a field scale both in agricultural soils and adjacent and remote non-agricultural soils. Correlations between Sn, organic matter and soil-pH are evaluated. Removal routes of Sn are proposed and quantified where possible. In the soil samples Sn was analyzed after a HNO3-extraction using graphite furnace atomic absorption spectrometry. Results show that accumulation of Sn in the plough layer of the agricultural soils takes place. However the quantities found are lower than predicted with model calculations. The formation of immobile SnO2 might be partly responsible for the observed gap. Correlations between Sn and organic matter are positive for the plough layer, however large variations suggest a disproportional removal of Sn from the plough layer. Tin is also found to accumulate in the topsoil of surrounding non-agricultural soils. The organic rich O-horizon shows high levels, but Sn is also found at greater depth suggesting downward transport. The Sn at these locations probably originates from transport of tin and residues of the fungicide from the agricultural soils through the atmosphere either by spray drift or wind erosion.

DESORPTION BEHAVIOUR OF POLYCYCLIC AROMATIC HYDROCARBONS IN HARBOUR SLUDGE FROM THE PORT OF ROTTERDAM, THE NETHERLANDS

Desorption of eight polycyclic aromatic hydrocarbons (PAH) fromtwo harbour sludges from the Port of Rotterdam, the Netherlands,was studied by column elution experiments. When water moves in a sludge layer desorbing contaminants like PAH can be dispersedin the environment. Separation of liquid and solid phase inbatches of sludge, stored for more than three years, by ultra-centrifugation at 4 °C yielded equilibrium partition coefficients. Temperature effects could not account for the observed differences with reported literature values. The differences are attributed to the contact time of PAH in thesediment. Laboratory data reported in the literature were oftenobtained after short contact times and therefore may not represent equilibrium partitioning. Our values represent contacttimes in excess of three years and are therefore more representative for the field conditions. Partition coefficientsobtained from column elution experiments were slightly above those obtained from the batch experiments. During column elutionof the sludge from the Beneden Merwede sorption equilibriumwas absent for the lighter compounds. This is attributed to the presence of a large portion of immobile water in the columns. Elution in the Beerkanaal columns occurred at near sorption equilibrium although pore water velocities were higher. Assumingthat desorption is diffusion controlled, observed desorption inboth materials could be explained. The non-equilibrium desorptionin the Beneden Merwede sludge for phenanthrene, and to alesser extent for anthracene and fluoranthene, could be describedby a diffusion limited model assuming spherical particles.