The reactions of Ni, Zn and Cd with goethite were studied over a range of initial metal concentrations to pH values (4 to 8), reaction times (2h to 42d) and temperatures (5 to 35°C). The adsorption of metals increased with pH, reaction time and temperature. Adsorption of Ni increased relative to Zn and Cd with increasing time and temperature. The initially rapid adsorption of metals within a few hours was followed by a much slower reaction linearly related to time''2, interpreted as diffusion-controlled penetration of goethite. The pH-dependent relative diffusion rates (Ni > Zn > Cd) were

Two Oxisols (Mena and Malanda), a Xeralf and a Xerert from Australia and an Andept (Patua) and a Fragiaqualf (Tokomaru) from New Zealand were used to examine the effect of pH and ionic strength on the surface charge of soil and sorption of cadmium. Adsorption of Cd was measured using water, 0.01 mol dmp3 Ca(NO&, and various concentrations of NaN03 (0.01-1.5 mol dm-3) as background solutions at a range of pH values (3)(4)(5)(6)(7)(8).In all soils, the net surface charge decreased with an increase in pH. The pH at which the net surface charge was zero (point of net zero charge, PZC) differed between the soils. The PZC was higher for soils dominated by variable-charge components (Oxisols and Andept) than soils dominated by permanent charge (Xeralf, Xerert and Fragiaqualf). For all soils, the adsorption of Cd increased with an increase in pH and most of the variation in adsorption with pH was explained by the variation in negative surface charge. The effect of ionic strength on Cd adsorption varied between the soils and with the pH. In Oxisols, which are dominated by variable-charge components, there was a characteristic pH below which increasing ionic strength of NaN03 increased Cd adsorption and above which the reverse occurred. In all the soils in the normal pH range (i.e. pH > PZC), the adsorption of Cd always decreased with an increase in ionic strength irrespective of pH. If increasing ionic strength decreases cation adsorption, then the potential in the plane of adsorption is negative. Also, if increasing ionic strength increases adsorption below the PZC, then the potential in the plane of adsorption must be positive. These observations suggest that, depending upon the pH and PZC, Cd is adsorbed when potential in the plane of adsorption is either positive or negative providing evidence for both specific and non-specific adsorption of Cd. Adsorption of Cd was approximately doubled when Na rather than Ca was used as the index cation.

The risks of contaminants accumulating in soils and crops due to inadvertent addition of impurities in agricultural fertilizers and soil amendments were assessed for Australian conditions. Elements considered of concern were arsenic (As), cadmium (Cd), fluorine (F), lead (Pb) and mercury (Hg). Consideration of background concentrations of these elements in Australian soils, inputs to soil in fertilizers and offtake in harvested crops indicates that Cd and F will accumulate in fertilized soils at a faster rate than As, Pb or Hg. The major factors affecting the accumulation of fertilizer-derived Cd, F, Hg and Pb in soils and their transfer to agricultural crops are reviewed in an Australian context where data are available. Cadmium is the element of most concern as its transfer from soils to the edible portions of agricultural food crops is significantly greater than for other elements. After consideration of the behaviour of F, Hg and Pb in the soil-plant system, we conclude that these elements pose negligible risk of accumulating to toxic concentrations in agricultural food crops. Proposed regulations governing maximum permitted concentrations (MPCs) of F in soils may need review and critical concentrations of F in agricultural soils need definition, given current F loadings to soil from fertilizers. Some agricultural produce currently exceeds Australian MPCs for Cd. However, the levels observed in crops and soils are in a range similar to those found internationally. While Cd concentrations in Australian phosphatic fertilizers have been historically high in comparison with fertilizers used in other countries, lower inputs of fertilizer per unit area and less atmospheric contamination of soils have resulted in similar or lower Cd loadings to agricultural land compared with Europe. In recent years the use of phosphatic fertilizers with lower Cd concentrations and the development of plant cultivars which restrict Cd uptake should assist in control of Cd accumulation by crops. However, acidification and salinization of soils in Australia poses a threat in terms of increasing Cd concentrations in agricultural produce. In comparison with other trace metals, Cd availability to plants appears to decline only slowly with time, if at all. More Cd is currently added to Australian soils than is removed in agricultural produce or by leaching. It is therefore important that the long-term behaviour of Cd in Australian soils be assessed, to determine if Cd concentrations in agricultural produce will slowly increase over time.

Alfisols, Entisols, Inceptisols, Ultisols, Vertisols, and Oxisols are all commonly found in tropical and subtropical regions receiving more than 500 mm mean annual rainfall. Landscapes throughout the tropics and subtropics are, however, dominated by Oxisols and Ultisols occupying extensive areas of potentially highly productive soils. The mineral fractions of these soils consist primarily of low‐activity clays having variable surface charge that differs from high activity clays in the origin of that charge. Low activity clays are dominated by iron (Fe) and aluminium (Al) oxyhydroxides and 1:1 layer silicates (kaolin). Much research has been conducted on the effects of pH, organic matter (OM), and cation composition of the soil solution on the surface charge characteristics of variable charge soils from the tropics. In general, net negative surface charge increases with increasing soil pH and OM content. Adsorption of metal ions by variable charge soils and minerals also increases as their pH, clay, and OM contents increase. Although the precise mechanisms for the change in net negative charge of soil and mineral surfaces with increasing pH are not fully understood, the generation of negative charge either through dissociation of H+ ions from surfaces or consumption of OH− ions by soils is generally accepted. In soils dominated by permanent charge surfaces, heavy metals are not mobile but in variable charge soils, the low surface charge density creates conditions conducive to increased mobility. Consequently, the adsorption of heavy metals, in particular, cadmium (Cd) by strongly weathered soils in relation to the effects of inorganic and organic ligands and the implications for metal transport are reviewed.

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