W.F. Pickering

Influence of clay-solute interactions on aqueous heavy metal ion levels

The extent to which heavy metal ions (Cu, Pb, Zn, Cd) are removed from aqueous solution by clay suspensions (kaolinite, illite or montmorillonite) has been shown to vary with the nature of the clay, solution pH, concentration of competing cations, and the nature (and concentration) of any ligands present.The amount sorbed increases gradually with pH until the threshold value for formation of sparingly soluble hydroxy complexes is reached. The affinity order for divalent cations, and the nature of the sorption process also appear to vary with clay type.

Specific sorption of antimony (III) by the hydrous oxides of Mn, Fe and Al.

The hydrous oxides of Mn, Fe, and Al avidly sorbed Sb from μM Sb(OH), solutions, with uptake levelling off as initial Sb concentration increased. Capacity values decreased along the sequence MnOOH > Al(OH)3 > FeOOH. The amount sorbed by each substrate decreased gradually at pH values > 6. Addition of 0.4M CH3COONa to the aqueous phase (to minimise retention of weakly bound Sb) had little effect on MnOOH uptake capacity (∼160 mmol, kg−1 at pH < 7) but retention dropped rapidly at higher pH. With the other two substrates (pH 6–7) the calculated capacity values for specific Sb sorption were ∼ 45 mmol kg−1 FeOOH and ∼ 33 mmol kg− Al(OH)3; about a third of the total capacity values. On these substrates specific Sb sorption tended to peak in the pH 7 to 8 region. The pH response pattern was modified using Sb tartrate sorbate solutions. Factors influencing Sb sorption included substrate surface charge, chemical form of Sb and surface interactions. Formation of a sparingly soluble metal coating was indicated by the uptake plateaus observed when increasing amounts of solid were added to Sb solutions containing acetate.

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.

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.

Selective extraction of metal ions associated with humic acids

The ability of a range of electrolyte solutions to release metal ions (Cu, Pb, Cd, Zn) presorbed on two samples of humic acid has been investigated. Though treatment with mineral acid or a chelating agent released a high proportion of the retained metal ion, recoveries were never total. Concentrated salt solutions displaced about 80% of the retained Cd or Zn, and about half of any Cu or Pb held by the organic matter, which indicates that most of the adsorbed metal ion is exchangeable, the extraction efficiency being controlled by competing equilibria. The effect of added clay suspensions was also examined. Analytical procedures for fractionating the total metal content of soils into subgroups have been assessed against the observed extraction behaviour.

The effect of pH on the retention of Cu, Pb, Zu and Cd by clay-humic acid mixtures

The amount of metal ion sorbed by the solids increased with increasing pH over the range 3 to 6, and with mixtures of clay-cellulose or illite-humic acid the uptake corresponded to the direct addition of individual substrate adsorption values. When the humic acid samples were admixed with Na+-form kaolinite or montmorillonite, there was some reduction in adsorption, attributable to component interactions, and this effect was most marked in the presence of Cu and Zn ions.In alkaline media there was competition between the ability of the organic material to form soluble metal humates and the tendency of the clays to strongly retain the sparingly soluble metal hydroxy species formed at pH > 6. In most systems studied retention by the solid phase predominated.In the presence of tannic acid there were distinct pH regions in which the four metal ions formed compounds of limited solubility, with the pH for maximum precipitation ranging from 4.5 (Cu) to 7 (Cd). The introduction of clay suspensions increased the amount precipitated/sorbed in these pH regions.The amount of Cu, Pb, Zn or Cd retained by mixed suspensions varied markedly with pH, nature of the clay and the chemical nature of organic components.

The specific sorption of trace amounts of Cu, Pb, and Cd by inorganic particulates

Thin film A.S.V. was used to study the specific sorption of Cd, Pb and Cu by hydrous oxides (Mn, Fe, and Al) or clay mineral suspensions from acetate buffer solutions containing 10 to 100 μg L−1 of each metal ion. The amount sorbed varied with system pH (range 3 to 9), substrate crystal form, the ratio of adsorbent to absorbate present, and the metal ion involved. Uptake by hydrous Mn(IV) oxide was near total over the whole pH range. With other particulates the pH required for onset of sorption varied with solid phase composition, with uptake subsequently increasing steadily with increasing pH. In general, affinity and relative uptake values followed the sequences Pb > Cu > Cd and Mn(IV) oxides > Fe(III) oxides > A1(OH)3 > clays > iron ores. The solid phases loaded with sorbed metal were equilibrated with a range of extractant solutions used in soil/sediment studies, and the results confirmed that chemi-sorption was the main retention process. Significant release was achieved using extractants that attacked the substrate or formed stable complexes with the metal ion.