Murray B. McBride

Health risk from heavy metals via consumption of food crops in the vicinity of Dabaoshan mine, South China.

Heavy metal contamination of soils resulting from mining and smelting is causing major concern due to the potential risk involved. This study was designed to investigate the heavy metal (Cu, Zn, Pb and Cd) concentrations in soils and food crops and estimate the potential health risks of metals to humans via consumption of polluted food crops grown at four villages around the Dabaoshan mine, South China. The heavy metal concentrations in paddy and garden soils exceeded the maximum allowable concentrations for Chinese agricultural soil. The paddy soil at Fandong village was heavily contaminated with Cu (703 mg kg(-1)), Zn (1100 mg kg(-1)), Pb (386 mg kg(-1)) and Cd (5.5 mg kg(-1)). Rice tended to accumulated higher Cd and Pb concentration in grain parts. The concentrations of Cd, Pb and Zn in vegetables exceeded the maximum permissible concentration in China. Taro grown at the four sampled villages accumulated high concentrations of Zn, Pb and Cd. Bio-accumulation factors for heavy metals in different vegetables showed a trend in the order: Cd>Zn>Cu>Pb. Bio-accumulation factors of heavy metals were significantly higher for leafy than for non-leafy vegetable. The target hazard quotient (THQ) of rice at four sites varied from 0.66-0.89 for Cu, 0.48-0.60 for Zn, 1.43-1.99 for Pb, and 2.61-6.25 for Cd. Estimated daily intake (EDI) and THQs for Cd and Pb of rice and vegetables exceeded the FAO/WHO permissible limit. Heavy metal contamination of food crops grown around the mine posed a great health risk to the local population through consumption of rice and vegetables.

Adsorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution

Biochars produced by pyrolysis of hardwood at 450 °C (HW450) and corn straw at 600 °C (CS600) were characterized and investigated as adsorbents for the removal of Cu(II) and Zn(II) from aqueous solution. The adsorption data were well described by a Langmuir isotherm, with maximum Cu(II) and Zn(II) adsorption capacities of 12.52 and 11.0 mg/g for CS600, 6.79 and 4.54 mg/g for HW450, respectively. Thermodynamic analysis suggested that the adsorption was an endothermic process and did not occur spontaneously. Although Cu(II) adsorption was only marginally affected by Zn(II), Cu(II) competed with Zn(II) for binding sites at Cu(II) and Zn(II) concentrations ≥ 1.0mM. Results from this study indicated that plant-residue or agricultural waste derived biochar can act as effective surface sorbent, but their ability to treat mixed waste streams needs to be carefully evaluated on an individual basis.

MOBILITY AND SOLUBILITY OF TOXIC METALS AND NUTRIENTS IN SOIL FIFTEEN YEARS AFTER SLUDGE APPLICATION

The increased use of sewage sludge as a fertilizer, combined with reports that large fractions of sludge-borne heavy metals cannot be accounted for several years after land application, indicates that more detailed study of potential mobility of these metals in soils is needed. A field site that

Toxic metals in sewage sludge-amended soils: has promotion of beneficial use discounted the risks?

Abstract Land application of contaminated waste products has been defended as beneficial use by some scientists and regulators, based on the premise that the behavior of any toxins accumulated in soils from this practice is reasonably well understood and will not have detrimental agronomic or environmental impacts into the foreseeable future. In this review, I use the case of toxic metals in sewage sludges applied to agricultural land to illustrate that metal behavior in soils and plant uptake is difficult to generalize because it is strongly dependent on the nature of the metal, sludge, soil properties and crop. Nevertheless, permitted agricultural loadings of toxic metals from sewage sludges are typically regulated using the sole criterion of total metal loading or concentrations in soils. Several critical generalizing assumptions about the behavior of sludge-borne metals in soil-crop systems, built into the US EPA risk assessment for metals, have tended to underestimate risks and are shown not to be well justified by published research. It is argued that, in the absence of a basic understanding of metal behavior in each specific situation, a more precautionary approach to toxic metal additions to soils is warranted.

Copper solubility and speciation of in situ contaminated soils : Effects of copper level, pH and organic matter

This study attempts to identify the soil properties controlling the fractionation of copper into various soil pools and determine the influence of pH and metal loading on soil-solution free copper activity (pCu2+). The pCu2+ was determined in 0.01 M CaCl2 soil extracts using a copper ion selective electrode. We analyzed a wide variety of soils: urban, agricultural and forest soils from the Province of Québec, New York State and Denmark. The pCu2+ ranged from 12.21 to 6.18. The relationships among pCu2+, total soil copper, total dissolved copper and soil pH are studied for their variability within and between sites as well as for the whole data set. Regression equations are presented for predicting soluble copper as a function of total soil copper and also for predicting pCu2+ as a function of total soil copper and soil pH.

A CRITIQUE OF DIFFUSE DOUBLE LAYER MODELS APPLIED TO COLLOID AND SURFACE CHEMISTRY

The use of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in colloid science has continued to the present day in spite of the inability of this theory to explain, even qualitatively, numerous phenomena exhibited by clays and other colloidal materials. An alternative description of the fundamental forces involved in the formation of dispersions and gels was presented in 1938 by Langmuir, but was never widely accepted among colloid scientists. Recent experimental and theoretical work, particularly in Japan, has revived this alternative view, in which a long-range Coulombic attraction force appears to explain several phenomena, particularly transitions among ordered and disordered phases of colloidal particles in dilute salt solutions.Examples are given from surface chemistry where rather complex models based on diffuse double layer theory are used to explain chemical adsorption behavior that often has a simpler explanation. It is argued that the rule of parsimony (Ockham’s Razor) should be applied to complex models that appear to explain the data at hand before such models are taken to be generally valid. A satisfactory fit of model predictions to experimental data obtained under a very limited range of conditions does not prove the validity of the model. Thus, it is concluded that the DLVO theory and its modifications fail to meet the 2 criteria necessary to the acceptance of a theory: agreement with observations and simplicity.

TRACE ELEMENT CONTENT OF SELECTED FERTILIZERS AND DAIRY MANURES AS DETERMINED BY ICP–MS

The trace element composition of representative fertilizers, liming agents, and dairy manures applied on farms in New York was measured because there has been recent concern about toxic metal contaminants in fertilizers and other soil amendments used in agriculture. Selected commercial fertilizers, lime products, and dairy manures were sampled, digested with hydrofluoric acid mixtures, and analyzed for trace element contaminants by inductively coupled plasma–mass spectrometry (ICP–MS). The trace element and heavy metal concentrations of the commercial fertilizers tested were generally low, although the phosphate component of fertilizer blends contained measurable concentrations of several elements of concern, including cadmium (Cd), uranium (U), arsenic (As), and molybdenum (Mo). At the concentrations in these fertilizers, agronomic rates of application would take decades to significantly increase soil concentrations of these elements above background. The manures were found on average to have low concentrations of all trace elements and heavy metals measured, with the important exceptions of copper (Cu) and zinc (Zn), where feed additives and use of Cu sulfate in treating hoof rot may explain the highest concentrations measured. Annual loadings of about 0.35 kg ha−1 Cu and 0.9 kg ha−1 Zn to dairy farmland are estimated for the median manure composition. Concentrations of lead (Pb), cadmium (Cd), and mercury (Hg) did not exceed 4.3, 0.4, and 0.05 mg kg−1 (dry weight), respectively, in any manure sample. Correlation of manure Pb concentration to aluminum (Al), indicates that soil contamination of the manure accounts for most of the Pb.

Effect of sludge-processing mode, soil texture and soil pH on metal mobility in undisturbed soil columns under accelerated loading.

The effect of sludge processing (digested dewatered, pelletized, alkaline-stabilized, composted, and incinerated), soil type and initial soil pH on trace metal mobility was examined using undisturbed soil columns. Soils tested were Hudson silt loam (Glossaquic Hapludalf) and Arkport fine sandy loam (Lamellic Hapludalf), at initial pH levels of 5 and 7. Sludges were applied during four accelerated cropping cycles (215 tons/ha cumulative application for dewatered sludge; equivalent rates for other sludges), followed by four post-application cycles. Also examined (with no sludge applications) were Hudson soil columns from a field site that received a heavy loading of sludge in 1978. Romaine (Lactuca sativa) and oats (Avena sativa) were planted in alternate cycles, with oats later replaced by red clover (Trifolium pratense). Soil columns were watered with synthetic acid rainwater, and percolates were analyzed for trace metals (ICP spectroscopy), electrical conductivity and pH. Percolate metal concentrations varied with sludge and soil treatments. Composted sludge and ash had the lowest overall metal mobilities. Dewatered and pelletized sludge had notable leaching of Ni, Cd and Zn in Arkport soils, especially at low pH. Alkaline-stabilized sludge had the widest range of percolate metals (relatively insensitive to soils) including Cu, Ni, B and Mo. Old site column percolate concentrations showed good agreement with previous field data. Little leaching of P was observed in all cases. Cumulative percolate metal losses for all treatments were low relative to total applied metals. Leachate and soil pH were substantially depressed in dewatered and pelletized sludge soil columns and increased for alkaline-stabilized and ash treatments.

Reduction of nitrogen loss and Cu and Zn mobility during sludge composting with bamboo charcoal amendment

Background, aim, and scopeComposting is an effective treatment process to realize sludge land application. However, nitrogen loss could result in the reduction of nutrient value of the compost products and the stabilization effect of composting on heavy metal concentration and mobility in sludge has been shown to be very limited.Materials and methodsLaboratory-scale experiments were carried out to investigate the effects of bamboo charcoal (BC) on nitrogen conservation and mobility of Cu and Zn during sludge composting.ResultsThe result indicated that the incorporation of BC into the sludge composting material could significantly reduce nitrogen loss. With 9% BC amendment, total nitrogen loss at the end of composting decreased 64.1% compared with no BC amendment (control treatment). Mobility of Cu and Zn in the sludge may also have been lessened, based on the decline in diethylenetriaminepentaacetic acid-extractable Cu and Zn contents of composted sludge by 44.4% and 19.3%, respectively, compared to metal extractability in the original material.DiscussionAmmonia adsorption capability of BC might be the main reason for the retention of nitrogen in sludge composting materials. Decrease of extractable Cu2+ and Zn2+ in the composting material mainly resulted from the adsorption of both metals by BC.ConclusionsIncorporation of BC into composting material could significantly lessen the total nitrogen loss during sludge composting. Mobility of heavy metals in the sludge composting material could also be reduced by the addition of BC.Recommendations and perspectivesBamboo charcoal could be an effective amendment for nitrogen conservation and heavy metal stabilization in sludge composts. Further research into the effect of BC-amended sludge compost material on soil properties, bioavailability of heavy metals, and nutrient turnover in soil needs to be carried out prior to the application of BC-sludge compost in agriculture.

Mobility and extractability of cadmium, copper, nickel, and zinc in organic and mineral soil columns

The relative mobility of Cd, Cu, Ni, and Zn added to several soils and the simultaneous desorption and leaching of metals (Ca, Mg, K, Al, Na, heavy metals) were determined by eluting soil columns with 0.01 M CaCl2 and analyzing the metals in leachates and soil by plasma emission spectroscopy. Although soil chemical properties generally had a larger effect on the mobility of metals than did the properties of the metals themselves, Cu moved least readily through all soil columns. The least mobility of metals was observed in a mineral soil with a relatively high pH, CEC, and exchangeable base content. The liming of an acid mineral soil reduced metal mobility, but not to the extent expected from the very low mobility observed for a naturally nonacid mineral soil. The order of mobility of the metals in the soils was: Cu ≦ Zn ≤ Ni ≤ Cd. These metals were almost completely extractable by 0.1 N HCl from a limed and unlimed acid soil, but were less extractable from a nonacid mineral soil and an organic soil. This effect is attributed to partially irreversible binding of the metals in organic matter, a phenomenon inhibited by the presence of Al on organic complexation sites.