Pamela A. Waller

Sorption of antimony species by humic acid

The retention of antimony (a potential toxin) in polluted soils or waterway sediments can involve interaction with several component phases. One of these, humic acid, has been found to adsorb antimony (III) from solutions of Sb(OH)3 or potassium antimonyl tartrate (C8H4K2Sb2O12) in accordance with Langmuir type isotherms. Using Sb(OH)3 solutions (initial Sb levels < 10 μM) the bonding constant value (at pH 4) was 6 × 105, with a calculated saturation capacity of 23 μmol g−1. In the antimonyl tartrate systems (initial Sb levels 0.5 to 75 μM) the bonding constant value for the sorbed species was 1.6 × 105 and the saturation capacity 53 μmol g−1. Addition of small amounts of HCl or NaOH (to vary the pH between 3.1 and 5.4) had little effect on the amount sorbed from KSbT solutions but with Sb(OH)3 solutions uptake was reduced (by about 15%). In the presence of NaCl (0.5 or 0.05M) Sb uptake increased (by about 15%). Antimony (V) (introduced as KSb (OH)6) was not sorbed from solutions < 10 μM in this salt. Using more concentrated solutions, uptake gradually increased, reaching a plateau value of around 8 μmol g−1 with solutions initially 50 or 75 μM.

Determination of antimony (III) and (V) by differential pulse anodic stripping voltammetry

Re-evaluation of DPASV procedures for determining low levels of Sb (III) and Sb (V) in solution identified several problem areas, e.g. anomalous ASV behaviour, possible formation of an intermediate valency state during the analytical cycle, and chemical interactions in acidified test solutions containing both valency states. Specific determination of Sb (III) can be achieved using base solutions composed of 0.2M HCl (detection limit 10 nM) or acetic acid/acetate buffer (detection limit 600 nM). For the determination of Sb (V), analysis in 2M HCl is recommended [with response in 0.2M HCl being used to correct for any Sb (III) present].

The lability of copper ions sorbed on humic acid

AbstractInteraction between copper (II) ions and humic acids can yield at least three different types of complex. The soluble forms promote migration of the metal ion in environmental systems and the labile content can be more ‘biologically active’. In our study, the distribution of copper between ‘fixed’, ‘non-labile’ and ‘labile’ complex forms at different pH values has been evaluated by equilibrating Cu-loaded humic acid with ion exchange resins of different types (and counter ion forms). Metal loadings of 35 to 225 μmol g−1 were obtained by equilibrating 10 or 50 mg of purified acid with Cu (II) solutions (10−4 M, pH 2 to 4.5). After removal of the aqueous phase, the Cu-loaded particles were re-suspended in water and a porous cage containing excess resin exchanger was added to each sample vial. After overnight mixing, the cage was retrieved and phases separated. Analysis of the aqueous phase determined the non-labile soluble copper released at the equilibrium pH; the ‘labile’ fraction value was found ...

The effect of pH on the lability of lead and cadmium sorbed on humic acid particles

AbstractSorption of lead(II) or cadmium(II) ions on humic acid particles (at pH 3.5) yields surface complexes which are sparingly soluble in the pH 3.55 region. Interaction of these species with acetic acid or dilute nitric acid released ASV labile metal species (mainly hydrated metal ion). When the pH was adjusted between 4 and 7, increasing amounts of the metal humate species (and humic acid substrate) dissolved and low levels of ASV labile species were detected. Overnight interaction with an excess of acetate ions (0.01 M) led to higher lability values (attributed to acetate/humate ligand exchange). The percentage of labile metal ion detected in acetate solutions varied with the amount of metal ion initially sorbed (range 100 to 500 mmol g−1). With the lower loadings, the ASV labile levels peaked in the pH 6–7 region (at around 12% of total sorbed cadmium, and 4% of total lead). In alkaline solutions, the percentage of labile Cd fell to about half of the peak value, but with Pb, lability values increas...

The effect of ion exchange on the solubility of fluoride compounds

The influence of ion exchange processes on the apparent solubility of fluoride compounds was examined by observing changes in free F− and total F levels when suspensions of sparingly soluble fluoride species (e.g., CaF2, AlF3) and aluminium smelter wastes were equilibrated with a range of materials having different cation exchange capacities. The exchanger materials used included synthetic resins, clay minerals, a humic acid and Mn(IV) oxide. The amount of fluoride ion released from the fluoride salts and F− rich wastes was found to increase in the presence of solids capable of exchanging cations, and the magnitude of the effect tended to be determined by the number of exchange sites available and the affinity of the fluoride compound cation for the exchange material. In some instances (e.g., with illite and alumina wastes) the released fluoride ion attacked the substrate and formed soluble complex ions.

Effect of time and pH on the lability of copper and zinc sorbed on humic acid particles

AbstractThe relative lability of copper and zinc present as soluble humate complexes or sorbed on humic acid particles has been evaluated by measuring the degree of metal transfer to cation exchangers, after mixing for time periods ranging from 2 to 114 h, with system pH values ranging from 2.7 to 8. Three major forms of metal humate species were postulated, namely, metal chemisorbed on solid; a soluble non-labile complex (possibly MA2); and a soluble labile complex (MA). The amount present in each form varied with time, pH, temperature, amount of exchanger resin present, and amount of metal ion initially loaded onto humic acid particles (40 to 480 μmol g−1). The percentage of total metal content which remained ‘fixed’ to the humic acid particles peaked in the pH 4 to 5.5 region. Below pH 4 an increasing percentage of the metal was mobilised, presumably as a result of protons competing for the functional groups involved in metal bonding while at pH 5.5 the solid particles dissolved and most metal ion was ...

The lability of zinc humate species

AbstractAt low pH, interaction between zinc(II) ions and humic acids yields sparingly soluble surface complexes; in alkaline conditions there are soluble complexes which differ in stability, stoichiometry and lability. The distribution of zinc between ‘fixed’, ‘non-labile’ and ‘labile’ complex forms at different pH values has been evaluated by equilibrating Zn-loaded humic acid (20 to 300 nmol μmol−1) with ion exchange resins of different types (and counter ion forms). Analysis of the supernatant aqueous phase determined the non-labile soluble zinc content (at the equilibrium pH); the ‘labile’ fraction value was found by back-extracting the washed resins into 0.05 M EDTA (pH 7). When the system pH was <6, about 5% was released as a soluble non-labile complex and the majority of the Zn remained firmly fixed to the solid phase. Above pH 6, the substrate dissolved and the percentage present as non-labile species increased from 5 to 70% as the pH changed from 5 to 8.5. Around pH 7, the labile content peaked a...

The evaluation of inorganic phosphate species in salt water lake sediments

AbstractSurface sediments drawn from 10 shallow bays have been subjected to selective extraction in order to sub-divide the total P content into sub-categories such as water soluble P, Ca-P, Al-P and Fe-P. The reagents selected were similar to systems used in soil analysis, but evaluation of the procedures showed that the species values varied with time of extraction, weight of sediment taken, volume of extradant and chemical nature of the sediment. In water extractions, the P levels appeared to be determined by saturation with a sparingly soluble salt, while in acidic media P extract levels peaked (using different experimental conditions) due to loss of extracted P as a new phase (e.g. CaHPO4) or through re-adsorption on other components.The optimum conditions for P speciation in sediments must be determined from a series of preliminary studies because each of the five sediments studied in detail displayed individual characteristic behaviour.

Determination of ‘labile’ phosphate in lake sediments using anion exchange resins: a critical evaluation

AbstractTransfer of phosphate ions from sediment particles to an anion exchanger (Amberlite IR-4B or Duolit A378) has been used to determine the ‘labile’ (or ‘available reactive’) phosphorus content of several salt-water lake sediments. An evaluation of the procedure identified several sources of error, e.g. (i) incomplete transfer of P to the exchanger [due to competition for sites by other anions present in the dried sediments]; (ii) slow intermediate processes such as dissolution of inorganic phosphate compounds; (iii) sorbed P not fully retrieved from the exchanger resins [due to the limited efficiency (∼90%) of the displacement process]; and (iv) absorbance readings in the colorimetric procedure used to determine phosphate can be enhanced by light scattered from fine particles. Accordingly, for the determination of the amount of ‘labile’ phosphate present in sediments it is recommended that one (a) stirs the exchanger/suspension system for at least 24 hours; (b) analyses both the aqueous phase and th...

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...