Helen Farrah

The sorption of mercury species by clay minerals

The amount of Hg sorbed by kaolinite and illite, in the absence of ligands, changes little with pH; with montmorillonite, Hg uptake decreases with increasing pH.An overall decrease in the amounts sorbed by these clays occurs in solutions which contain ligands such as cyanide (2:1); acetate (5:1); and nitrilotriacetate (6:5). [Values in parentheses are mole ratio of ligand: Hg.]In the presence of chloride (2:1), the order for the uptake of Hg by the three clays is illite > montmorillonite > kaolin.The addition of thiourea (2:1) results in total precipitation of mercury at pH > 4; in the presence of sulfate or phosphate (> 1:1) Hg is lost from solution by precipitation/sorption at pH 4 but the amount decreases to near zero at pH > 8.Solution processes (i.e. complex formation, precipitation) appear to have a dominating influence on mercury distribution.

The affinity of metal ions for clay surfaces.

Abstract An investigation of the adsorption, at 25°C and pH 5, of Pb, Cu, Zn, Cd, Mg and Ca ions by Na+-form kaolin, illite and montmorillonite, has shown that the effect of concentration can be described by a Langmuir isotherm. The derived values for the constant, k, were all of the order 105 l mol−1 with the pre-power terms varying between cations within the range of 1–5 (kaolin and illite) or 3–10 (montmorillonite). The capacity values varied with the type of clay (montmorillonite > illite > kaolin), species being adsorbed, and pH. The relative affinity of the metal ions for the clay surfaces was examined by adding Na+-form clay suspensions to binary mixtures of divalent cations and subsequently treating the adsorption data by means of a competitive form of the Langmuir equation. This yielded values for bond constant ratios k A k B , and capacity values for the component cations. As in the previous series, the derived capacities varied with clay type and cation being adsorbed, and k A k B ratios were small (e.g., between 1 and 3). Affinity sequences, which can be defined in terms of decreasing k-values, decreasing capacity values, or decreasing uptake under specified conditions (e.g., equal initial or equilibrium concentration of divalent cation), were found to vary with clay type and sequence parameter chosen. The adsorption of the divalent cations was accompanied by proton displacement. Most of the observed solution pH changes can be explained in terms of competitive adsorption, but with kaolin and illite, there was a small residual effect which has been attributed to specific interactions at edge sites. The observed adsorption behaviour, and H+ release patterns, indicate that sites of different nature and accessibility are present, and the study clearly demonstrates that the equilibrium position is sensitive to the total chemical environment. The mechanism of the adsorption process has been considered and extension to natural clays, e.g. in soils, is discussed briefly.

Extraction of heavy metal ions sorbed on clays

The ability of seventeen different chemical solutions to displace heavy metal ions (Pb, Zn, Cu, Cd), pre-adsorbed on clay (kaolinite, illite and montmorillonite) at either pH 5 or 7, has been examined and the relative efficiency of each extractant ascertained. Of the reagents used, only EDTA (0.001 M, pH 7) quantitatively released all four ions from the three clays; oxalic acid (0.1 M, pH 3.3), totally displaced at least three ions from each clay. Other reagents, for example ammonium oxalate (0.1 M), ammonium nitrate (0.01 M), nitric acid (0.1 M) and sodium citrate (0.01 M) effectively displaced one or more heavy metal ions from individual clays. Near quantitative displacement by an excess of Na (0.1 M) or Ca (0.05 M) ions was observed only on montmorillonite. Pre-adsorption at pH 7 was accompanied by precipitation of excess metal ion, and the extraction efficiency in these systems was determined by the ability of the reagent to both dissolve hydrous oxide species and displace sorbed metal ions.The implications of the results with respect to the nature of the adsorption process and relevance to environmental systems have been considered.

pH effects in the adsorption of heavy metal ions by clays

Abstract The amount of hydroxonium or hydroxyl ion sorbed by the sodium form of clays (kaolin, illite or montmorillonite) has been found to increase in proportion with the amount added (i.e., data fits a Boedeker type equation). The adsorptive capacities, and bonding strengths (as evaluated from Langmuir isotherm plots) varied with clay type and nature of the adsorbing species (values for OH− being greater than for H+). The presence of clay suspensions reduced the pH required for total precipitation of heavy metal ions (e.g., Cu, Pb, Cd, Zn), as hydroxy species, and this behaviour has been re-examined in terms of observed pM, pOH relationships and the effect of ligand additions.

Factors influencing the potential mobility and bioavailability of metals in dried lake sediments

AbstractSamples taken from polluted creek sediments were dried at temperatures between 20°C and 100°C (either in air or under a nitrogen atmosphere) and selective chemical extraction procedures were then used to examine the effect of drying temperature and oxidation on the bonding mode distribution patterns of Zn, Pb, Cd and Cu. Exposure to air during the drying stage tended to increase the fraction of total metal which was less firmly bound (i.e. ion exchangeable, weakly sorbed). Less metal was present in these categories when higher drying temperatures were used, and temperature changes had a larger effect when the samples were dried under nitrogen. The drying process appears to promote metal migration to phases having a stronger bonding power. In most of the samples studied, heating caused Pb to migrate to the carbonate phase while accumulation in the organic/sulfide phases was the dominant process for Cu and Cd. In the case of Zn, migration to the carbonate phase was favoured in some studies (e.g. usi...

Interaction of clays with dilute fluoride solutions

Interaction between dilute (mg L−1) NaF solutions and clay suspensions (0.08 % w/v) has been examined as a function of pH (range 3 to 8), clay type (Na+- or Ca2+-kaolinite, illite, montmorillonite) and NaF concentration. No F loss from solution was detected at pH > 6.5, while enhanced uptake was found on decreasing the pH, especially in the 4 to 3 region. Removal of F from 1 to 6 × 10−4 M NaF was only slightly dependent on weight of solid, but did increase with [F−]. It is proposed that F losses are due to the formation of sparingly soluble Al species (e.g. cryolite, Na fluoro silicate), occasionally augmented with CaF2 formation (Ca2+-clays). The Al is released by proton attack on the lattice, following conversion of the suspended solids into the unstable H+-form, either through acid addition (pH < 5) or through hydrolysis of the Na+-form material. The latter process was most pronounced with the illite and montmorillonite samples. The amount of F fixed by montmorillonite was roughly double that held by the other two clays, and had a maximum value (pH 3) of ∼ 4 mg g−1, using 11 mg L−1 NaF solutions. Soluble fluoro-complexes, similar in quantity to the retained F, were detected, in many of the studies. It was concluded that contact of the clay components of soils or sediments with mg L−1 levels of F in adjacent aqueous phases would result in only a minor proportion being retained.

Efficiency of a small artificial wetland with an industrial urban catchment

Abstract The efficiency of a small artificial wetland in Walter Park, in an industrial urban area in the City of Lake Macquarie, Australia, was examined by monitoring lead levels and suspended solids contents in the water. The location was adjacent to a main road and near a lead–zinc smelter. Samples taken from the sediment were subjected to selective chemical extraction procedures to investigate the behaviour of lead in the sediment and mechanisms of lead removal in the wetland. The wetland was relatively effective in removing lead and suspended solids, except for heavy flows. The removal of lead in the wetland was attributed to sedimentary processes, adsorption of lead on exchange sites and lead bound to the carbonate and oxidizable phases.

Factors influencing the potential mobility and bioavailability of exchangeable ammonium ion in dried lake sediments

AbstractThe effect of drying temperature and oxidation on the level of exchangeable ammonium ion found in sediments has been examined using samples collected from along a polluted creek and from shallow lake bays. The sediments were dried at temperatures between 20°C and 100°C (either in air or under a nitrogen atmosphere), and the ammonium ion content was extracted into 0.1 M KCl prior to analysis using an ion selective electrode. Exposure to air during the drying stage usually resulted in lower ammonium values, while increasing the drying temperature altered the amount of displaceable (i.e. available) ammonium ion extracted, generally in an upward direction. The amount detected (5–25 μ g−1) varied between sites, and surface sediment values differed from the 10–50 cm core material results. The pH of the extracts varied with the drying temperature used, indicating that the heating process promoted some chemical changes in the test samples. The study has demonstrated that in nutrient level surveys, the ana...