Anna Ledin

Measurements in situ of concentration and size distribution of colloidal matter in deep groundwaters by photon correlation spectroscopy

Abstract Photon correlation spectroscopy (PCS) was applied for the characterization (concentration and size distribution) of colloidal matter in a deep groundwater. The measurements were performed on-line and in situ at the Aspo underground Hard Rock Laboratory, Oskarshamn, Sweden. The colloidal concentration in the rather saline groundwater was not above 0.1 mg/l and probably below 0.03 mg/l, according to the field measurements and with calibrations of the PCS-instrument with well-defined reference colloids of α-Fe2O3, γ-Al(OH)3 and SiO2. The results clearly demonstrated that the stability and concentration of a colloidal-size suspended phase in an anoxic groundwater with high Fe(II) content, like the one in Aspo (0.3 mg/l), is sensitive to exposure to atmospheric conditions during the handling of the sample. Diffusion of air into the closed measuring cuvette was enough to alter the colloidal content significantly within 6 h. A particle fraction with a size distribution range of 170–700 nm was formed within 45 min when air was allowed to diffuse into the aqueous phase from the air-filled upper part of the cuvette. The corresponding time to generate a significant colloidal precipitate was less than 1 min (size distribution range 100–600 nm) when a stream of air (1.5 ml) was bubbled through the water samples. The precipitating colloidal phase was a mixture of ferric hydroxide and calcium carbonate in all three cases.

Applicability of photon correlation spectroscopy for measurement of concentration and size distribution of colloids in natural waters

Abstract The suitability of proton correlation spectrosocpy (PCS) for the determination of concentration and size distribution of colloidal matter was tested in a surface water and in a deep groundwater. Well-defined colloids (sizes predominately in the range 50–500 nm) of α-Fe2O3, γ-Al(OH)3, SiO2, kaolinite, illite and humic acid in aqueous solutions were used as references for calibration of the PCS-instrument (signal vs. concentration). The intensity of the scattered light was dependent on the composition of the colloid. The concentration ranges, where a quantitative determination of the colloid could be achieved, were 0.03–2 mg/1, 0.1–2 mg/1, 0.1–7 mg/1, 0.5–10 mg/1, 0.5–50 mg/1 and 0.5–75 mg/1 for α-Fe2O3, γ-Al(OH)3, SiO2, kaolinite, illite and humic acid, respectively (in homogeneous systems). The colloidal populations in the surface waters (inlet and outlet of a lake) had a size distribution in the range 100–500 nm. The measured count rates were around 330 kcps and 30 kcps at the inlet and outlet, respectively, which correspond to totally 2.8 mg/1 and 0.9 mg/1 of colloidal matter according to gravimetric determinations (collected on clogged 0.015 μm filters). Comparisons of concentrations estimated from measurements of reference colloids with PCS and gravimetric determinations of the colloidal showed that a combination of the two methods would be needed. The measurements of a deep groundwater were performed on-line and in situ with a PCS technique. The content of colloidal matter in the groundwater was below the detection limit for the PCS-equipment, which corresponds to a concentration not above 0.5 mg/1 and probably below 0.1 mg/1 in the present system. The feasibility and advantages of using PCS for non-disturbing size characterisation and concentration measurements of colloids in aquatic systems have been demonstrated, as well as some limitations of the method.

Variations in hydrochemistry, trace metal concentration and transport during a rain storm event in a small catchment

General hydrochemistry and trace metal concentrations were monitored during a runoff event in a small catchment (0.9 km2) with an old abandoned copper mine and covered sulphidic waste. Five hours of rising hydrograph was sampled manually every 10 minutes while an automatic sampler was used during the recession phase. The hydrograph started to rise about two hours after the commencement of rain, but the hydrochemistry showed only small changes for yet another hour and a half. At this time the concentration of total organic C (TOC), Cu and Zn decreased rapidly, while most other elements exhibited a peak in concentration. Different origins and varying hydrological response could explain these observations. Chemical redistribution might, however, be important for some elements, notably Al and Fe. After this peak the concentrations of most elements decreased, probably because of dilution with precipitation water whereafter a secondary concentration maximum was observed. This behaviour was most evident for K but also for Na, Mg and Ca. A noteworthy exception is pH that increased during the event and remained at high levels, while concentrations of Mn and Cd were lowered possibly due to depletion of the mobile pool. Increased pH during the event is probably caused by liming of soils surrounding the creek, in combination with soil buffering processes. The increase of alkaline and alkaline earths (notably K) concentrations could be explained by transport of accumulated elements as the groundwater level rose.

Effects of pH, ionic strength and a fulvic acid on size distribution and surface charge of colloidal quartz and hematite

Size distribution of colloidal quartz and hematite (initial diameter 140–190 nm and 70–360 nm, respectively) were determined by Photon Correlation Spectroscopy (PCS) in well-defined laboratory systems. Effects on the mean diameter and the size distribution were measured at various values of pH (2–12) and ionic strength (0.001–0.7 mol/l) in an inert medium (NaClO4). The effect of an aquatic fulvic acid on the stability of the colloids was included in the study. In the absence of fulvic acid the colloids flocculate at a pH close to pHzpc of the respective mineral. This increase in mean diameter is due to electrostatic interactions. Hematite colloids are also destabilised at a pH much below pHzpc. The quartz colloids are stabilised in the presence of fulvic acid. The hematite/fulvic acid systems are stable at high pH, but flocculate at low pH. Increasing ionic strength induces agglomeration of the quartz colloids at 0.1 mol/l, independently of the presence of fulvic acid (2 mg/l). Hematite is destabilised at 0.02 mg/l in the absence of fulvic acid, while in the presence of fulvic acid the colloids aggregate at ionic strength 0.07mol/l.

Effects of fulvic acid on the adsorption of Cd(II) on alumina

Abstract The effects of fulvic acids on the adsorption of cadmium on alumina has been studied by a batch distribution technique and with variation of pH (3 – 10), cadmium concentration (10 −4 – 10 −8 M) and fulvic acid concentration (0, 1, 10 and 50 mg/l). Two fulvic acids of different origin (surface water and deep groundwater) were used. The presence of fulvic acid generally enhanced the adsorption at pH below PZC (pH 7.8) and reduced the adsorption above this pH. Linear adsorption isotherms were obtained at cadmium concentration below 10 −6 M. The presence of fulvic acids at concentration levels representative of surface waters would be expected to have a significant impact on the distribution and mobility of cadmium in the environment.

Characterization of the submicrometre phase in surface waters : a review

The need for improved knowledge about the composition and behaviour of solid matter in the colloidal size range (1 nm to 1 µm) in natural waters has been identified by several authors in the literature. The increased interest in these colloids is due to their large impact on the transport, redistribution and bioavailability of numerous chemical compounds (such as trace metals, nutrients and organic pollutants). This paper presents a review of different analytical techniques available for determination of some of the properties of interest (e.g., colloid concentration, size distribution, mineral composition and surface properties). It is obvious that most methods suffer from limitations and that the only feasible way to overcome these problems is by using a combination of different techniques. Sampling in a Swedish lake, as an example, illustrates one promising combination including light scattering and gravimetric analysis for determination of colloid concentration, photon correlation spectroscopy and scanning electron microscopy for size distribution measurements and energy dispersive microscopy, powder X-ray diffraction and infrared spectroscopy in order to obtain information about mineral constituents and general surface properties. Size fractionations were performed by filtration and centrifugation to gain information regarding the distribution of metals and organic matter between dissolved and solid phases. The study indicated, for instance, a seasonality of the size distribution of the colloidal matter, with smallest colloids (range 120–340 nm) found in spring, while the colloid samples during summer and autumn have a size distribution in the range 280–700 nm. This result also illustrates the limitations when using filters with, e.g., a pore size of 0.45 µm for the distinction between particulate and dissolved substances.

Light induced changes of Fe(II)Fe(III) and their implications for colloidal forms of Al, Mn, Cu, Zn and Cd in an acidic lake polluted with mine waste effluents

Abstract The photo induced reduction of ferric (hydr)oxides stabilises the ferrous state in acidic (pH 4) lake water, at light intensities higher than 7 W/m2 and despite equilibrium with atmospheric oxygen. Precipitation of ferric (hydr)oxides occurred at lower light intensities, particularly at water depths constantly below this light intensity and with contributions of Fe(II) from the bottom sediments. No specific reducing agent could be identified, but the low concentrations of dissolved organic carbon and humic substances indicate that inorganic components could be involved in the reduction. A maximum of 20% of the copper and zinc concentrations and up to 50% of the cadmium were associated with colloidal matter. These distributions were observed at depths in the lake where the ferrous state was stable and with a fairly constant solid/solution distribution of the trace metals.

Fractionation of trace metals in surface water with screen filters

Abstract Size fractionation of solid particulate phases serving as carriers of trace elements (e.g. Al, Cd, Cu, Fe, Mn and Zn) in surface waters was conducted by pressure filtration using screen filters with defined pore sizes in the range 5-0.015 μm. The particle size distribution in solution was determined by Photon Correlation Spectroscopy. The accumulation of solids on the filter surface was demonstrated, leading to a successive reduction of the efficient pore size (‘clogging’). Thus, filters with a defined pore size of 0.40 μm and a diameter of 47 mm had an apparent separation efficiency corresponding to a pore size of 0.015 μm after filtration of 100 ml containing approximately 100 mg/l of suspended solids. Aluminium and iron were almost exclusively present in the particle size range 5-0.20 μm. Copper and manganese were retained to 45% and zinc and cadmium to 30% by the 0.015-μm pore size filter.

PROPERTIES OF THE PARTICULATE PHASE IN DEEP SALINE GROUNDWATER IN LAXEMAR, SWEDEN

Composition, concentration and size distribution of the particulate phase in saline groundwater from the Laxemar borehole, Sweden, were measured at three different sampling sections sealed off by rubber packers (315.0–321.5, 1090.0–1096.3 and 1420.0–1705.0 m). Concentration and size distribution analysis were performed on-line in a field laboratory using measurements of light scattering in combination with photon correlation spectroscopy (PCS). Estimates of the chemical composition of the solid phase were obtained from scanning electron microscopy (SEM) coupled to energy dispersive spectroscopy (EDS), and saturation calculations were made with the geochemical code PHREEQE, based on hydrochemical analysis. The results indicate that the particulate phase consisted of aluminosilicates, SiO2, CaF, and probably CaCO3. The concentration of particulates varied between the different depths with the lowest content at the 1090.0–1096.3 m level, where it was estimated to be <0.5 mg/litre. Estimated concentration range in the 315.0–321.5 m section was 0.5–0.7 mg/litre (depending on the assumed mineral composition), while the highest concentrations were found in the samples from the deepest level (concentration range 1.4–3.0 mg/litre). The average diameters increased with increasing depth, (540, 640 and 1840 nm at the depths 315.0–321.5, 1090.0–1096.3 and 1420.0–1705.0 m, respectively) which probably was related to the increasing salinity and corresponding increase in the Ca concentration. The study clearly illustrates that limiting the changes in the activities of CO2 and O2 during sampling and measurements are of utmost importance in order not to disturb the solid phase in terms of composition, concentration and size distribution.

Temporal changes in concentration, composition and size distribution of the colloidal phase in Lake Risten, Sweden

The concentration and size distribution of colloidal matter ( < 1.2 μm) at different depths in the oligotrophic Lake Risten, Sweden, as well as in its main tributary were studied with light scattering in combination with photon correlation spectroscopy at least 4 times/year during 3 consecutive years. Qualitative information on the composition of the colloidal phase was obtained from estimations of the colloidal fraction of Al, Fe, Mn and organic carbon as well as by a sequential leaching procedure. The concentration of colloidal matter varied within a broad range (1.4–100 mg/l) in the stream, while the measurements in the lake indicated minor changes (range 0.8-3.3 mg/l) between the different sampling occasions as well as locations (both vertically and longitudinally). Annual variations in the size ranges of the colloids (spring 120–340 nm; summer 280–700 nm; winter 200–500 nm) were observed. The changes in the average diameters of the colloidal population showed no correlation with changes in any general hydrochemical parameters, but the estimates of the fraction of colloidal organic carbon (COC) indicated increased concentrations with increasing diameters.