cadmium and copper release kinetics in relation to afforestation of cultivated soil

PII S0016-7037(00)00602-5

Cadmium and copper release kinetics in relation to afforestation of cultivated soil

BJARNE W. STROBEL,1,* HANS CHRISTIAN BRUUN HANSEN,1 OLE K. BORGGAARD,1 MARTIN K. ANDERSEN,2 andKARSTEN RAULUND-RASMUSSEN

2

1Chemistry Department, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK–1871 Frederiksberg C, Denmark2Danish Forest and Landscape Research Institute, Hørsholm Kongevej 11, DK–2970 Hørsholm, Denmark

(Received November8, 1999;accepted in revised form November1, 2000)

Abstract—Afforestation of cultivated soils causes soil acidification and elevated concentrations of dissolvedorganic matter (DOC) in the soil solution, and hence, aggravate the risk of heavy metal leaching. The kineticsof cadmium and copper release from an unpolluted arable soil applied with forest floor soil solution wasinvestigated in the laboratory, and the release rates correlated to pH and DOC in solution through log–logequations. The soil solution was isolated from Norway spruce (Picea abies(L.) Karst.) by centrifugation, andthe solution passed a cation-exchange column to remove metal cations and to protonate the DOC. Soil samplesfrom an arable Ap horizon were placed in completely mixed flow cells, and influent solutions with 0 to 5 mMDOC were applied. The solution pH was adjusted to achieve effluent pH values in the range 3.6 to 6.9 in theflow cells at steady-state conditions. Cadmium release rates were very low at pH. 5 and increasedexponentially as pH decreased to,5. The release rate was correlated to solution pH in a simple model:log(cadmium release rate)5 20.21 pH2 15.28 (R2 5 0.48), and no significant effect of DOC was observed.The kinetics of copper release from the soil was more complicated with effects of both pH and DOC. Inexperiments without DOC, the release rate of copper was slightly lower at high pH than at low pH. Inexperiments above pH 5, the presence of 5 mM DOC in the solution increased the release rate of copper.However, the copper release was retarded by DOC in the range pH 3.8 to 5.0, which coincided with amaximum retention of DOC in the flow cells. The release rate of copper was correlated to solution pH andconcentration of DOC, including an interaction of pH and DOC: log(copper release rate)5 0.86 pH2 1.26logDOC 1 0.24 pH z logDOC 2 19.26 (R2 5 0.60). If the changes in soil chemical conditions afterafforestation influence the cadmium and copper release rates in a similar way as observed in the flow cellexperiments, then the release rate of cadmium will increase exponentially at soil solution pH, 4.5. Theinhibition of copper release by DOC observed at pH 3.8 to 5.0 indicates that copper is retained in the soil byinteractions with adsorbed organic matter.Copyright © 2000 Elsevier Science Ltd

1. INTRODUCTION

Formation of good-quality groundwater and protection ofaquifers against contamination with heavy metals is paid spe-cial attention worldwide. In agricultural production and inarable soils, cadmium and copper are among the most toxicheavy metals with potential effects to organisms. The solubilityof cadmium in soils is mainly influenced by pH (Christensen,1989; McBride et al., 1997; Filius et al., 1998; Ro¨mkens andSalomons, 1998), whereas copper forms strong complexes withorganic ligands in soils and soil solutions (Davis, 1984; Berg-gren, 1989; 1992a; Temminghoff et al., 1994; McBride et al.,1997). Intensive cultivation of arable soils includes applicationof inorganic and organic fertilizer and sludge containing ad-mixed contaminants such as cadmium and copper. The regularaddition of lime keeps soil pH circumneutral where heavymetal cations have limited solubility (Davis, 1984; Christensen,1989; Temminghoff et al., 1994; Salam and Helmke, 1998;Schwarz et al., 1999).

Afforestation of intensively cultivated arable soils leads tomajor changes in the chemical conditions in the soil as theregular liming and fertilization cease. Increased air pollution inforest, compared with open land, root respiration and the pro-tons exuded as charge compensation from the tree root growthaccelerates soil acidification together with the organic acid

production in the litter layer. An increase in soil organic matterat the forest floor is a source of dissolved organic carbon(DOC), which leaks into the underlying mineral soil layers(Cronan and Aiken, 1985).

Part of soil solution DOC is sorbed onto soil minerals with amaximum sorption often observed at pH 4 to 5 (Davis, 1982;Jardine et al., 1989; Moore et al., 1992; Drever, 1994). More-over, DOC complexed with di- and trivalent metal cations atmineral surfaces might promote detaching of metal complexesand, hence, increase the total dissolved concentration of thesecations (Tyler, 1981; Hue et al., 1986; Pohlman and McColl,1988; Berggren, 1992a). The Cu–DOC complexation is rela-tively strong, and the high concentrations of Cu–DOC com-plexes in the soil solution may increase copper leaching fromthe A horizon to deeper soil layers, drains, or aquifers. Cad-mium forms weaker complexes with DOC than copper and isgenerally more soluble at low pH (Berggren, 1992a;b; Tipping,1994; McBride et al., 1997; Temminghoff et al., 1997).

DOC has been found to enhance mineral weathering more atslightly acidic to neutral pH than at acidic to strongly acidic pH(Furrer and Stumm, 1986; Bennett et al., 1988; Raulund–Rasmussen et al., 1998). Furthermore, DOC seems to enhancethe dissolution of minerals in ultramafic rocks at low pH, butmight under the same pH conditions decrease or even inhibitdissolution of minerals in granitic rocks (Davis, 1982; Lund-strom and Ohman, 1990; Welch and Ullman, 1992; Ochs et al.,1993). In addition to the release of the major cation constitu-*Author to whom correspondence should be addressed ([email protected]).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 65, No. 8, pp. 1233–1242, 2001Copyright © 2000 Elsevier Science LtdPrinted in the USA. All rights reserved

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1233

ents, the weathering and dissolution reactions include release oftrace elements from the minerals, e.g., cadmium and copper.These different and sometimes opposite effects of DOC suggestthat for soil being composed of minerals from various rocks, itmight be expected that the percolating DOC sometimes en-hances mineral dissolution and desorption reactions in themineral soil layers, but inhibition of these processes may alsobe seen (Temminghoff et al., 1997; Raulund–Rasmussen et al.,1998).

The processes related to afforestation of arable soils, includ-ing soil acidification, DOC–mineral interactions, mineralweathering, and release of heavy metals may be simulated in aflow cell system. Continuous influx of solution into a flow cellproduces steady-state conditions for reactions with the soilsample and allows frequent sampling. The initial release ratescan be observed in flow cell experiments, even when reactionmechanisms change with time, or the reaction mechanisminclude inhibition (Gfeller et al., 1997). The continuous influxof solution is a major advantage, because the chemical condi-tions in the flow cell change gradually to steady-state condi-tions. This avoids the abrupt changes in solution concentrationsand pH often seen in batch experiments when the solution isreplaced. The abrupt shifts in solution pH in batch experimentsmay cause alternating desorption and resorption reactions toefface the differences in the release rates. Consequently, theflow cell setup may better simulate the gradual changes takingplace during afforestation, when DOC and the flux of protonsfrom the litter layer are entering the nutrient-rich and circum-neutral arable soils far from equilibrium. The apparent steady-state release rates of cadmium and copper observed in the flowcells simulate the natural release during the period with chang-ing soil conditions after afforestation of arable land. The chang-ing soil chemical conditions include acidification, precipitationor flocculation of DOC, and leaching of cations such as alumi-num, iron, and heavy metals from the cation-exchange complexand other readily available pools.

The objective of this study was to determine effects of pHand DOC concentration in soil solution on release rates ofcadmium and copper from an unpolluted cultivated soil, i.e., anagricultural soil without elevated contents of cadmium andcopper due to contamination. The kinetic data obtained fromflow cell experiments are correlated with pH and DOC in thesolution by use of simple log–log models.

2. MATERIALS AND METHODS

2.1. Soil Solution

In December 1997'500 L of litter was collected in black plasticbags from a 2-cm-deep O horizon in a Norway spruce (Picea abies(L.)Karst.) stand in Tisted Nørskov in Jutland, Denmark. The bags werekept cold until soil solution was isolated from the naturally moist litterby centrifugation at 40003 g for 30 min. The solution was passedthrough a filter paper (S&S 2893) and a Durapore membrane filter type0.45-mm HVLP (Raulund–Rasmussen and Vejre, 1995). Cadmium,copper, and other metal cations were removed from the soil solution,and the DOC was proton saturated by passing successively throughthree cation-exchange columns; 2.53 50 cm of Dowex 50W–X8cation-exchange resin (mesh 16–40) at a flow rate of 13 mLz min21

and stored at 4°C. Selected properties of the DOC solution are shownin Table 1.

2.2. Soil Solution Analysis

Aliquots of the DOC solution were analyzed for aluminum, calcium,iron, potassium, magnesium, and sodium by ICP–AES (Perkin ElmerOptima 3000XL, AS90) and the concentration of cadmium and copperin solution samples was determined by graphite furnace atomic absorp-tion spectroscopy (GFAAS) (Perkin Elmer 5100, Zeeman 5100). Inor-ganic anions were analyzed by capillary zone electrophoresis (West-ergaard et al., 1998). The concentration of DOC was quantified with atotal organic carbon analyzer (Shimadzu TOC–500). The absorbance at465 and 665 nm of a 20 mM DOC solution with pH adjusted to 4.0 wasrecorded with a Shimadzu UV–1601 spectrophotometer, and the E465/E665 ratio was calculated. All measurements were made in duplicate.

The content of carboxylic acid and phenolic groups in DOC wasdetermined by titration of 25 mL DOC solution with 0.05 mol/L NaOHin presence of a 10 mM NaCl background electrolyte in two replicates.The DOC solution aliquots were bubbled with argon before and duringthe titrations to prevent interferences from carbon dioxide. The titra-tions were performed with a Metrohm 665 dosimat with 0.02 mL of0.05 mol/L NaOH added every 30 s and pH recorded potentiometricallyuntil pH 11 was reached. Titratable acidity of the protonated DOC wascalculated by using Eqn. (1).

A2 5CNaOH 2 @OH2# 2 Canions

CDOC(1)

Where A2 denotes the titratable acidity of DOC (molz mol21 C),CNaOH (mol z L21) is the total concentration of NaOH added, [OH2](mol z L21) is hydroxide ion concentration,Canions (mol z L21) iscorrection for the concentration of inorganic anions in the DOC solu-tion, i.e., chloride, nitrate, sulfate andortho-phosphate, andCDOC (molC z L21) is the concentration of DOC in the 25-mL solution. The totalnumber of carboxylic acid groups was calculated as total titratableacidity of DOC up to pH 7.0. The content of phenolic groups wascalculated as total titratable acidity between pH 7.0 and 11.0 (Davidand Vance, 1991).

2.3. Soil Material

The A horizon from an intensively cultivated Danish sandy loamfrom Christianssæde in Lolland (southeastern Denmark) was used astest soil after air drying. The soil is classified as Fine-loamy, mixed,mesic Typic Hapludalf (Soil Survey Staff, 1997) and developed on

Table 1. Selected properties of the DOC isolated from Norwayspruce forest floor in Tisted Nørskov before and after treatment withcation exchange resin (Dowex 50W–8X).

Parameter Unit Beforea Aftera

pH 5.206 0.01 2.706 0.01Dissolved organic carbon mM 716 1 706 1Carboxylic acid groups mmolz mol21 C —b 70.36 0.2Phenolic groups mmolz mol21 C — 47.76 0.2E465/E665 ratio — 13.36 1.8

Cadmium nM 8.36 0.4 0.166 0.03Copper nM 3556 3 526 2

Aluminium mM 90 6 2 306 2Calcium mM 16876 37 106 1Iron mM 32 6 2 146 1Magnesium mM 3726 1 86 1Potassium mM 5246 1 0 –Sodium mM 7146 8 366 6

Chloride mM — 0.506 0.06Nitrate mM — 0.296 0.01Phosphate mM — 0.186 0.08Sulphate mM — 0.066 0.02

a Average value6 SD.b Not determined.

1234 B. W. Strobel et al.

calcareous till from the Weichsel Glaciation; the calcium carbonate hasbeen completely dissolved and leached from the upper 70 cm of the soilprofile (Table 2).

The total content of cadmium, copper, and phosphorus in the soilwas determined by aqua regia extraction and analyzed by ICP–AES(Niiskavaara, 1995). The total content of carbon and nitrogen in the soilwas determined by dry combustion with a LECO CNS–2000. Theexchangeable cations were determined by extraction with 1 mol/LNH4NO3 (Stuanes et al., 1984). The oxalate extractable aluminum andiron were determined according to Schwertmann (1964). All measure-ments were made in duplicate.

2.4. pH-Stat Experiments

Release kinetics of cadmium and copper from soil was initially tested inpH-stat experiments with constant pH at 3.9 and 5.2 in the solution. In apolypropylene beaker, 10.0 g of soil was suspended in 10 mL of deionizedwater by intense magnetic stirring to wet all surfaces. Stirring was reducedafter 30 min, and 90 mL water was added. A portion of ultrapure HNO3

was added to adjust pH. Solution pH was maintained with 0.1 mol/Lultrapure HNO3 for 24 to 32 h by a Metrohm 665 dosimat and Metrohm691 pH meter. Aliquots of the solution were filtered through a resistantRC–membrane, 0.2mm, Ø13 mm (Sartorius, Go¨ttingen) and kept invials with 0.07 mL 2.5 mol/L ultrapure HNO3, and the concentration ofcadmium and copper was determined with GFAAS.

2.5. Flow Cell Experiments

Release kinetics of cadmium and copper from the soil was investi-gated in magnetic stirred flow cells each placed in a waterbath ther-mostated at 25°C (Fig. 1). The flow cells hold 94 mL and are made ofpolycarbonate (Plexiglas) with Teflon packing and at the outlet aresistant RC–membrane, 0.45mm, Ø25 mm (Sartorius, Goettingen).

Influent solution was continuously pumped through the flow cellswith a flow rate of 1.7 mLz h21 applied with a peristaltic pump(Alitea–XV) and Aliprene tubings (Alitea, Stockholm). The outlet wasdirected through Teflon tubings to a fraction collector with 10-mLpolypropylene vials that was initially added 0.2 mL of 2.5 mol/Lultrapure HNO3 to prevent adsorption of metal cations to the vials andto preserve the samples until analysis.

Influent solutions were made by dilution of the proton-saturatedDOC solution to contain 1.0, 3.0, and 5.0 mM DOC, plus a referencesolution without DOC, and pH was adjusted to five levels in the rangepH 3.0 to 4.0 with ultrapure HNO3 or NaOH. Flow cell experimentswere run for 7 days each, and effluent fractions of'2.8 mL were

collected every 100 min, giving a total of 100 fractions collected fromeach experiment. To obtain sufficient volume of solution for determi-nation of metal cations, the content of two succeeding vials weremixed, giving a composite sample with twice the solution volume('5.6 mL). The total concentration of cadmium and copper wasanalyzed in the composite vials number 112, 617, 11112, etc. Thetotal concentration of aluminum, calcium, iron, magnesium, potassium,and sodium cations was analyzed in the composite vials number 314,819, 13114, etc. Concentration of DOC and pH was analyzed in vialsnumber 5, 10, 15, etc. In total, each flow cell experiment comprisedanalysis of 10 analyte concentrations in 20 fractions, giving a total of200 determinations in each of the 21 experiments.

The concentrations of cations and DOC as well as pH in the effluentsolutions were used for chemical speciation with the WHAM model(Tipping, 1994). DOC was specified as fulvic acid in the calculations.The WHAM Water10 database (Tipping, 1994) was used for the calcula-tions, except that the content of carboxylic acid groups in the fulvic acidwas replaced by the actual content determined by titration (Table 1).

Initial tracer tests where a pulse of chloride was pumped into theflow cell with 5 g of quartz as solid phase, and the chloride concen-tration was measured in the bottom, center, and top of the flow cell after0, 5, 10, and 30 min. These tests showed that the solution in the flowcell was ideally mixed after 10 min. Therefore, release of metal cationsin the flow cell was calculated as from a “batch experiment withcontinuous sampling,” i.e., effluent concentrations were determined foreach fraction and the concentration in the flow cell solution was thesame as in the effluent at any time. The accumulated release,Racc ofcadmium and copper was calculated in molz g21 by using Eqn. (2) thatincludes two terms: one for the effluent fraction and another for thechange in concentration in the flow cell solution.

Racc5 On51,6,11

961

5.00FOi50

4

Vn2i z~Cn;n11 1 Cn25;n24

2

1 ~Cn;n11 2 Cn25;n24! z 92G (2)

In all flow cell experiments, 5.00 g of soil was used. In this expressionVn–i is the actual volume of each fraction, i.e., 2.8 mL610%,Cn,n11

(M) is the cation concentration in the combined vials number n andn11. In the bracket, the first term is the effluent content, and the secondterm is the change in the amount of metal cation of the 92-mL solutionin the flow cell.

Modeling of the cadmium and copper release kinetics was performedby multiple linear regressions with SAS proc GLM (SAS, 1996).Linear log–log equations with log(release rate) of cadmium and coppercorrelated to pH and logDOC were tested.

3. RESULTS

3.1. pH-Stat Experiments

Cadmium concentrations in pH-stat experiments increasedthroughout the 24 to 32-h experiments at pH 3.9 and 5.2, and

Table 2. Selected soil properties of the Ap horizon of the intensivelycultivated arable soil at Christianssæde.

Parameter Unit Ap horizona

Clay % 206 2Silt % 146 2Fine sand % 406 3Coarse sand % 266 3pHH2O 6.56 0.1Carbon gz kg21 12.06 0.2Ntotal g z kg21 0.986 0.05Ptotal

b mg z kg21 4936 6Cdtotal

b mmol z kg21 4.96 0.1Cutotal

b mmol z kg21 1186 2Alox mmol z kg21 1206 6Feox mmol z kg21 206 1Aluminiumc cmolc z kg21 0.846 0.04Calciumc cmolc z kg21 11.66 0.6Magnesiumc cmolc z kg21 0.556 0.03Potassiumc cmolc z kg21 0.156 0.02Sodiumc cmolc z kg21 0.076 0.01

a Average value6 SD.b Aqua regia extraction.c Exchangeable cation.

Fig. 1. Schematic diagram of the flow cell equipment.

1235Cadmium and copper release kinetics

no equilibrium was attained within this period (Fig. 2a). Aftera fast initial release, the concentration of cadmium increasedfrom 20 nmolz L21 after 2 h to 34nmol z L21 after 24 h at pH3.9. The initial release was negligible at pH 5.2, but the con-centration increased at a constant rate to 20 nmolz L21 after32 h. Copper concentrations in the pH-stat experiments werealmost constant at'150 nmolz L21 from 6 to 32 h (Fig. 2b).

3.2. Flow Cell Performance

For all experiments, effluent pH was higher than influent pH,whereas the concentration of DOC in the influent was higherthan in the effluent (Fig. 3a). Average pH and concentration ofDOC was calculated for the period after 2 to 3 days where onlyminor changes were observed.

The percentage of DOC retained in the flow cell was calcu-lated for each experiment with 5 mM DOC in the influent byusing Eqn. (3):

DOCretained5 SDOCinfluent 2 DOCeffluent

DOCinfluentD z 100 (3)

where DOCinfluent is the influent DOC concentration (0, 1, 3, or5 mM) and DOCeffluent is the effluent DOC concentration. Theamount of DOC retained in the flow cell was found to be pHdependent with a maximum retention of'40% of influentDOC near pH 4.5.

The concentration of cadmium and copper in the effluent wasinitially high but decreased within,1 day (Fig. 3b). Theconcentration of cadmium tended to reach a steady state inexperiments with low pH, whereas at higher pH the concen-tration decreased throughout the experiment. The effluentconcentrations of copper decreased throughout all experi-ments, but the decreases were very small after 1 day ofreaction.

3.3. Release of Cadmium and Copper

The release rates of cadmium and copper were calculated bylinear regression of the accumulated release based on thesteady-state period of 24 to 150 h.

Fig. 2. Concentration of cadmium (a) and copper (b) in solutionversus time at constant pH at 3.9 (●) and 5.2 (E) obtained in pH-statexperiments with soil from Christianssæde Ap horizon.

Fig. 3. The concentration of DOC (Œ) and pH (‚) (a), and totalconcentrations of cadmium (●) and copper (E) (b) in effluents duringflow cell experiments C53x and C53z. Solid lines show average valuesfor pH and DOC.


The accumulated release of cadmium from the soil was 4times higher at pH 4 than at pH 6 (Fig. 4a,b). In experimentswith 5 mM DOC in the influent the accumulated release wasalmost the same as without DOC in the influent at both pHvalues. Consequently, the release rates of cadmium calculatedas the slope of these regression lines were similar for experi-ments with and without DOC at both pH values, which showsthat DOC does not influence cadmium release rates from thesoil (Table 3).

The accumulated release of copper in experiments withoutDOC was 1.5 times higher at pH 4 than at pH 6 after 1 weekof reaction (Fig. 4c,d), whereas the corresponding release rateswere almost the same (Table 3). This apparent contradiction

arises from the very fast initial release of copper followed by aslower steady-state release rate.

DOC increased the initial release of copper at pH 4, but thefollowing steady-state release rate was remarkably slower thanexperiments without DOC added (Fig. 4c). After 1 week, theaccumulated release of copper was higher in experiments with-out DOC at pH 4 than experiments with 5 mM DOC. At pH 6,both the accumulated release and the release rate of copperincreased with higher DOC.

The release of copper was enhanced by DOC at pH 6, but atpH near 4 the release was reduced compared with experimentswithout DOC added. A comparison of cadmium and copper,therefore, shows that the release of copper was much less

Fig. 4. Accumulated release of cadmium (a, b) and copper (c, d) from the Ap horizon of the Christianssæde soil inexperiments with and without DOC at pH 4 (a, c) and pH 6 to 7 (b, d). Release rates are calculated as the slope of the linearregression lines.


influenced by a decrease in pH than cadmium, whereas DOCstrongly affected the release of copper but not the release ofcadmium (Fig. 4).

3.4. Modeling of Release Kinetics

Modeling of cadmium release kinetics by multiple linearregressions shows a strong correlation with pH (p , 0.01), butno correlation with DOC in the effluent (Fig. 5). Consequently,the cadmium release rate from soils appears to be governed by

pH in the solution (Table 4). The copper release kinetics seemsmore complicated. Modeling of the copper release kinetics asdepending on DOC and pH was conducted for experimentswith steady state pH$ 3.7. This analysis reveals that thecopper release rate depends on DOC and pH, including aninteraction term of DOC and pH (R2 5 0.60) (Table 4). Toshow the suitability of the model to describe copper releaserates at different pH and DOC concentrations, calculated cop-per release rates are plotted against measured copper releaserates in Figure 6. Except for one outlier, the model provides asatisfactory description of the data.

4. DISCUSSION

4.1. Release of Cadmium and Copper

4.1.1. Initial tests of release kinetics of cadmium and copper

The continuous increase in the solution concentration ofcadmium observed in pH-stat experiments at constant pH 3.9and 5.2 shows that release of cadmium from soil is kineticallycontrolled (Fig. 2). Cadmium concentrations in the pH-statexperiments are'20 times higher than concentrations ob-served in flow cell experiments, indicating that concentrationsare even farther away from equilibrium in the flow cells.Copper was apparently in a sort of sorption equilibrium in thepH-stat experiments with almost constant concentrations after6 h (Fig. 2). The concentrations are'5 times higher thancopper concentrations observed in flow cell experiments. How-ever, a slow release rate might cause small changes in copper

Table 3. Experimental conditions and results for the kinetic experiments.

Series pHina pHeff

bDOCin

a

(mM)DOCeff

b

(mM)

Cd ratec Cu ratec

[Cd]e

(nM)[Cu]e

(nM)[Al] e

(mM)[Fe]e

(mM)[Ca]e

(mM)amol z

g21 z s21 R2dfmol z

g21 z s21 R2d

C03x 3.0 3.696 0.09 0.0 0.226 0.07 — — 1.5 0.96 — 216 5 7.16 0.9 1.46 0.4 0.296 0.02C032x 3.2 3.806 0.09 0.0 0.156 0.04 78 0.94 2.1 0.94 1.016 0.20 266 7 8.26 0.6 0.96 0.2 0.166 0.02C032y 3.2 3.676 0.06 0.0 0.176 0.07 112 0.96 1.4 0.98 1.166 0.21 186 4 6.66 1.0 0.96 0.1 0.166 0.02C035x 3.5 4.956 0.13 0.0 1.076 0.35 14 0.42 — — 0.236 0.14 — 5.76 1.4 1.26 0.6 0.136 0.02C0375x 3.75 6.146 0.25 0.0 0.256 0.07 21 0.54 — — 0.476 0.24 — 4.86 0.9 — 0.096 0.01C04x 4.0 6.956 0.17 0.0 0.196 0.02 51 0.58 1.5 0.93 0.206 0.08 216 6 1.56 0.7 — 0.086 0.01C04y 4.0 6.916 0.19 0.0 0.176 0.03 — — 0.8 0.51 — 156 8 1.36 0.6 — 0.076 0.01C132x 3.2 4.076 0.03 1.0 1.26 0.1 81 0.92 0.9 0.65 1.456 0.11 186 2 7.36 0.3 1.66 0.1 0.166 0.01C14x 4.0 6.526 0.32 1.0 0.96 0.1 20 0.86 1.3 0.96 0.306 0.04 226 3 2.16 0.1 0.96 0.1 0.096 0.01C34y 4.0 6.176 0.14 3.0 2.56 0.3 30 0.95 2.1 0.98 0.386 0.08 286 4 3.06 0.3 1.56 0.1 0.066 0.01C53x 3.0 3.676 0.11 5.0 3.36 0.3 — — 3.0 0.98 — 416 7 256 7 8.86 2.5 0.326 0.02C53z 3.0 3.676 0.06 5.0 3.06 0.3 202 0.99 2.0 0.92 2.026 0.15 336 9 9.96 1.0 3.26 0.5 0.256 0.03C532x 3.2 3.926 0.04 5.0 3.26 0.7 86 0.96 0.7 0.56 1.456 0.10 206 4 9.66 0.8 2.76 0.3 0.126 0.01C532y 3.2 3.886 0.01 5.0 3.86 0.3 93 0.90 1.1 0.51 1.426 0.28 306 4 9.56 0.3 2.86 0.1 0.146 0.01C532z 3.2 3.946 0.19 5.0 3.46 0.7 104 0.94 0.6 0.49 0.996 0.18 256 4 7.56 1.0 2.26 0.3 0.136 0.01C532v 3.2 4.156 0.14 5.0 3.06 0.9 99 0.99 0.5 0.39 1.176 0.10 196 6 6.06 1.1 1.86 0.4 0.166 0.01C535y 3.5 4.726 0.07 5.0 2.66 0.7 21 0.60 1.1 0.74 0.546 0.09 356 7 5.06 1.3 1.96 0.5 0.106 0.01C5375x 3.75 5.116 0.17 5.0 3.96 0.3 42 0.50 1.5 0.27 0.876 0.32 476 15 6.56 1.1 3.46 0.9 0.106 0.01C5375y 3.75 5.116 0.20 5.0 3.76 1.0 32 0.61 0.8 0.42 0.566 0.21 356 11 5.66 0.6 2.46 0.7 0.106 0.01C5375z 3.75 5.026 0.15 5.0 3.26 0.4 25 0.90 1.4 0.94 0.336 0.07 246 6 2.96 0.6 1.26 0.5 0.076 0.01C54z 4.0 6.036 0.42 5.0 3.76 0.4 37 0.75 3.5 0.97 0.636 0.12 436 3 4.26 0.6 2.16 0.3 0.066 0.01

a pHin and DOCin denotes pH and DOC concentration in the influent solution. standard deviation is 0.02 for pH and 5% for DOC.b pHeff and DOCeff denotes pH and DOC concentration6 standard deviation in the effluent solution at steady-state conditions in the flow cell.c Cd rate and Cu rate is cadmium and copper release rate caclulated as the slope of the accumulated release curve.d R2 denotes correlation coefficient.e [Xx] denotes total concentration6 standard deviation of the element in the effluent solution at steady-state conditions in the flow cell.

Fig. 5. Release rates of cadmium vs. pH at steady state for Aphorizon of Christianssæde arable soil.● 5 mM DOC,‚ 3 mM DOC,3 1 mM DOC, andE no DOC. The solid line represents model fit todata: log(Cd release rate)5 215.21 pH20.28.


concentrations that it would be impossible to distinguish inpH-stat experiments.

4.1.2. Release rates of cadmium and copper

In the 1-week flow cell experiments up to 5% of the totalcadmium and up to 2% of the total copper content are released(Table 2 and Fig. 4). The available fraction of cadmium andcopper in the soil was determined in long-term batch experi-ments running for 88 weeks with and without DOC (Strobel etal., 2001). Here it was found that'45% of the aqua regiaextractable cadmium in the soil from Christianssæde was re-leased at almost constant rates throughout the experiment.Moreover, these batch experiments showed release of'20% ofthe aqua regia extractable copper in the soil when the solutioncontained DOC, which was 3 times more than in experiments

without DOC. On the assumption that 45% of aqua regiaextractable cadmium and 20% of copper, respectively, in thesoil is available for release and assuming that the release ratesdetermined in the flow cells can be extrapolated to longerreaction time, the extractable copper would be released after'200 days at pH 5. The extractable cadmium would be re-leased from the soil after'900 days at pH 5 and 275 days atpH 3.6, i.e.,'3 times faster at the lower pH. However, therelease period under field soil conditions would be much longerbecause kinetic weathering and dissolution rates have beenconsidered 10 to 1000 times faster in laboratory experimentscompared with rates in field soils (Velbel, 1993; Swoboda–Colberg and Drever, 1993). Transfer of laboratory release ratesto field soil is impeded by soil physical conditions such asincomplete contact between solution and solid phase in fieldsoils, which cause a reduced mineral surface area exposed tothe solution. Retention of soil solution in micropores and lim-ited exchange of solution between micropores and bulk soilsolution could cause elevated pH at the mineral surfaces (Swo-boda–Colberg and Drever, 1993; Clow and Drever, 1996).

In the flow cell experiments, the release rates of cadmium arestrongly correlated to pH, whereas DOC has no effect (Fig. 5).The release rate of cadmium can be modeled by a simple linearcorrelation between log(cadmium release rate) and pH in thesolution withR2 5 0.48 (Table 4). The modeling of the cad-mium release rate does not improve by including DOC in thecorrelation analysis. The addition of DOC might have affectedthe release kinetics slightly by increasing the ionic strength ofthe solution, because the ionic strength in the influent solutionswas kept low to simulate forest soil solution conditions.

4.1.3. Cadmium reactions

The concentration of cadmium in the effluent solutions atsteady-state conditions decreases at higher pH, and DOC seemsnot to influence the total cadmium concentration in solution(Table 3). Accordingly, DOC has limited effect on speciationof cadmium in solution as seen in Figure 7. This result is ingood agreement with several other investigations reportingonly minor interactions between cadmium and organic ligands(Davis, 1984; Christensen, 1989; Berggren, 1992b; Ro¨mkensand Salomons, 1998).

Christensen (1989) and Ro¨mkens and Salomons (1998)

Table 4. Multiple regression equations for modeling the log(release rate) for release of cadmium and copper from soil.

Element Regression equation R2a Eqn. no.

log(Cd release rate)b 5 20.21 (60.03) pHc 215.28 (60.13)d 0.48**e (4)20.21 (60.03) pH20.06 (60.05) logDOC215.42 (60.18) 0.49N.S. (5)20.28 (60.05) pH20.02 (60.01) pHz logDOC 215.22 (60.13) 0.50N.S. (6)

log(Cu release rate)b 5 0.05 (60.02) pH215.19 (60.11) 0.09N.S. (7)0.07 (60.05) logDOC215.13 (60.13) 0.04N.S. (8)0.05 (60.02) pH20.06 (60.04) logDOC215.35 (60.16) 0.11N.S. (9)0.08 (60.08) pH21.26 (60.13) logDOC10.24 (60.03) pHz logDOC 219.26 (60.42)f 0.60** (10)

a R2 denotes correlation coefficient of the regression equation.b Release rate (molz g21 z s21) of cadmium and copper calculated as the slope of accumulated release.c pH and DOC concentration (M) in the effluent at steady-state conditions in the flow cell.d Figure in parenthesis is 95% confidence interval.e *, **, ***, and N.S. denote significance level at 0.05, 0.01, 0.001, and nonsignificant.f Eqn. (10) applies to release rates at pH. 3.6 in Table 3.

Fig. 6. Calculated copper release rates plotted against measuredrelease rates. Calculated release rates were obtained by the model:log(Cu release rate)5 0.86 pH2 1.26 logDOC1 0.24 pHz logDOC219.26 (Eqn. 10, Table 4). The solid line is the 1:1 line.


found a linear correlation between the distribution coefficientKd and pH, i.e., logKd 5 a z pH 1 b including no effect ofDOC. It may be argued that if cadmium sorption and desorptionreactions are fast, the solution cadmium concentrations mea-sured in the flow cell experiments will be at equilibrium andthus defined by the distribution coefficient. This in turn willresult in a log(cadmium release rate) showing a linear decreasewith increasing pH that is the relationship actually observed(Table 4, Fig. 5). However, it is also well known that thelogarithmic rates of proton induced mineral-weathering reac-tions decreases linearly with pH in the acid range (Bennett etal., 1988; Blum and Lasaga, 1988). Therefore, equilibriumdistribution of cadmium between solid and solution cannot beinferred from the linear log(rate)–pH relationship observed inthe flow cell experiments.

4.2. Cu–DOC Interactions

The overall minimum in the release rate of copper wasobserved in experiments with the highest concentration of DOC

in the influent and steady-state pH near 4.2 (Fig. 8). In regionII (Fig. 8), i.e., the pH range 3.8 to 5.0, the release rate ofcopper seems inhibited by DOC, because the release of copperin experiments with DOC added is slower than without DOC.In column experiments at pH 3.9, Temminghoff et al. (1997)leached more copper without DOC compared with solutionscontaining 8.3 mM DOC. In mineral-weathering studies, Lund-strom and Ohman (1990) and Ochs et al. (1993) found thatmineral dissolution at pH near 4 to be inhibited by DOC. Theapparent DOC inhibition of the copper release rates coincideswith a maximum DOC retention in the flow cell (Fig. 8). Theretention might be due to flocculation and precipitation of DOCin the flow cell and/or sorption to the soil solids. The maximumsorption of DOC to mineral particles often found at pH 4 to 5may increase the number of sorption sites at the mineral–DOCsurfaces, which may increase the interactions between adsorbedDOC and cations with high affinity for DOC, e.g., aluminum,copper, and iron and thereby cause the observed inhibition ofrelease rates (Jardine et al., 1989; Moore et al., 1992; Ro¨mkenset al., 1996).

Hydrophobic humic substances have higher molecularweights and higher affinity for sorption to the soil solid phasesthan hydrophilic humic substances (Kaiser and Zech, 1996;1998; Gu et al., 1995). In soils and flow cell systems withcontinuous influx of DOC comprising hydrophilic and hydro-phobic substances, this means that sorption of the larger hy-drophobic molecules to the soil mineral surfaces may displacethe hydrophilic substances (Jardine et al., 1989; Kaiser andZech, 1996). Preferential sorption of larger hydrophobic sub-stances may block surface sites toward dissolution by DOCadsorbed as di- or polynuclear surface complexes decreasingthe release of copper from the mineral surfaces (Davis, 1984;Chin and Mills, 1991; Ochs et al., 1993).

At pH , 3.8 (region I in Fig. 8), the increasing protonationof functional groups on DOC at lower pH reduces formation ofcoordinative bonds between DOC and soil mineral surfaces.This implies the formation of fewer and weaker surface com-plexes, such as monodentate mononuclear surface complexes,

Fig. 7. The estimated absolute (a) and relative (b) amounts of DOCcomplexed cations in the effluent for experiments with 5 mM DOCinfluent. The points are calculated with the speciation programWHAM, and lines are fitted to the points.‚ Al–DOC, h Fe–DOC,●Cu–DOC,E Cd–DOC, and3 Ca–DOC.

Fig. 8. The percentage of DOC retained (E) in the flow cell and thecopper release rate (●) vs. pH at steady state in experiments with 5 mMDOC in the influent. The dashed line is the copper release rate withoutDOC in the influent (modeled by Eqn. 10 in Table 4); solid lines aredrawn as a guide to the eye.


with little effect on release rates (Furrer and Stumm, 1986;Ochs et al., 1993).

At pH . 5 (region III), the sorption of DOC decreases withincreasing pH that reduces the copper adsorption to surfacebound DOC. Because less DOC is adsorbed, more DOC isavailable for in-solution copper complexation, which may ex-plain the enhanced copper release rate observed at pH. 5(Davis, 1984; Berggren, 1989; Berggren, 1992a; Ro¨mkens andSalomons, 1996; McBride et al., 1997; Temminghoff et al.,1997).

4.3. Speciation of Effluent Solutions

In addition to cadmium and copper, other cations such asaluminum, calcium, and iron are released from the soil samplesin flow cell experiments. Because these cations also formcomplexes with DOC, the interactions between DOC and cop-per are reduced. The extent of this complexation is indicated bychemical speciation of the effluent solutions as calculated byWHAM (Tipping, 1994). According to this calculation, Al–DOC, Ca–DOC, and Fe–DOC complexes dominate at the low-est pH, but the Fe–DOC concentration decreases almost twoorders of magnitude as pH increases from 4 to 6 (Fig. 7a). Thetotal amount of dissolved iron was complexed by DOC in allexperiments, whereas uncomplexed aluminum occurs at pHbelow 4 (Fig. 7b). The high concentrations and high affinity forbinding to DOC implies that iron and aluminum displace di-valent cations for binding to DOC, e.g., copper.

Although the amounts of Ca–DOC and Cd–DOC increasewith increasing pH, a very small fraction of calcium and limitedamounts (0–50%) of cadmium are bound to DOC. The relativeamount of Cu–DOC is also strongly influenced by pH with'25% of total copper being complexed at pH 3.6 and almost100% at pH. 5 (Fig. 7b). This dependence reflects reducedcompetition from aluminum and iron due to practically reducedsolubility at increasing pH, leaving increasing concentrations ofDOC for complexation of copper at higher pH (Davis, 1984;Temminghoff et al., 1997; Salam and Helmke, 1998).

These calculations showing strong complexation of copper atpH . 5 indicate that in-solution complexation by DOC maycontrol the release of copper at higher pH (Fig. 8). In contrast,small amounts of copper are complexed by DOC at pH, 4(Fig. 7b), limiting the importance of in-solution complexation.Accordingly, the release of copper at low pH may be due tosurface complexation rather than in-solution complexation.

4.4. Implications for Afforestation

Afforestation of arable soils decreases soil pH two unitswithin 10 to 20 yr after afforestation (Jug et al., 1999; Vejre etal., 1999). If these changes in soil chemical conditions causedby afforestation influence the release of cadmium and copperfrom the upper soil layer in a similar way as observed in theflow cell experiments, such a decrease in pH will increase thecadmium release rate significantly (Fig. 5). Consequently, af-forestation of former cultivated soils will lead to leaching ofcadmium with percolating water from the Ap horizon to deepersoil layers. In cultivated soils with cadmium-enriched topsoilbut low cadmium sorption capacity in the subsoil, afforestationmay induce cadmium pollution of shallow groundwater. The

copper complexation by DOC and soil organic matter in theupper forest soil layers complicates estimation of the risk ofcopper leaching. The inhibition of copper release by DOCobserved in the flow cell experiments at pH near 4 indicatesthat afforestation might not increase copper leaching as pHdecreases; if anything, copper is retained in the soil by inter-actions with adsorbed organic matter.

5. CONCLUSIONS

The release rate of cadmium from soil in flow cell experi-ments is controlled by the solution pH with no effect of DOC.The release rate of cadmium is very low at circumneutral pH,whereas it increases exponentially at pH, 5. The copperrelease rate depends on solution pH and DOC, including aDOC–pH interaction effect. The release rate of copper withoutDOC added decreases slightly as pH increases from 3.6 to 6.9.The presence of DOC in the solution enhances the release rateof copper significantly in the pH. 5. In the pH range 3.8 to 5,DOC inhibits the copper release rate. This range of pH coin-cides with a maximum retention of DOC in the flow cellexperiments. Apparently, the kinetics of copper release fromsoil involves both in-solution complexations with DOC andstrong interactions between copper and sorbed organic sub-stances, besides the proton effect within the range of pH foundin forest soils.

Acknowledgments—We thank F. Frederiksen and R. Holtze for makingthe flow cells and H. Nancke–Krogh for skillful technical assistance inthe laboratory. The Commission of the European Communities sup-ported the project, FAIR3-CT96-1983.

Associate editor:G. Sposito

REFERENCES

Bennett P. C., Melcer M. E., Siegel D. I., and Hassett J. P. (1988) Thedissolution of quartz in dilute aqueous solutions of organic acids at25°C.Geochim. Cosmochim. Acta52, 1521–1530.

Berggren D. (1989) Speciation of aluminum, cadmium, copper, andlead in humic soil solutions—A comparison of the ion exchangecolumn procedure and equilibrium dialysis.Intern. J. Environ. Anal.Chem.35, 1–15.

Berggren D. (1992a) Speciation of copper in soil solutions fromPodzols and Cambisols of S. Sweden.Water Air Soil Pollut.62,111–124.

Berggren D. (1992b) Speciation and mobilization of aluminium andcadmium in Podzols and Cambisols or S. Sweden.Water Air SoilPollut. 62, 125–156.

Blum A. and Lasaga A. (1988) Role of surface speciation in thelow-temperature dissolution of minerals.Nature331,431–433.

Chin P.–K. F. and Mills G. L. (1991) Kinetics and mechanisms ofkaolinite dissolution: Effects of organic ligands.Chem. Geol.90,307–317.

Christensen T. H. (1989) Cadmium soil sorption at low concentrations.VIII. Correlation with soil parameters.Water Air Soil Pollut.44,71–82.

Clow D. W. and Drever J. I. (1996) Weathering rates as a function offlow through an alpine soil.Chem. Geol.132,131–141.

Cronan C. S. and Aiken G. R. (1985) Chemistry and transport ofsoluble humic substances in forested watersheds of the AdirondackPark, New York.Geochim. Cosmochim. Acta49, 1697–1705.

David M. B. and Vance G. F. (1991) Chemical character and origin oforganic acids in streams and seepage lakes of Central Maine.Bio-geochemistry12, 17–41.

Davis J. A. (1982) Adsorption of natural dissolved organic matter at theoxide/water interface.Geochim. Cosmochim. Acta46, 2381–2393.


Davis J. A. (1984) Complexation of trace metals by adsorbed naturalorganic matter.Geochim. Cosmochim. Acta48, 679–691.

Drever J. I. (1994) The effect of land plants on weathering rates ofsilicate minerals.Geochim. Cosmochim. Acta58, 2325–2332.

Filius A., Streck T., and Richter J. (1998) Cadmium sorption anddesorption in limed topsoils as influenced by pH: Isotherms andsimulated leaching.J. Environ. Qual.27, 12–18.

Furrer G. and Stumm W. (1986) The coordination chemistry of weath-ering. I. Dissolution kinetics ofd–Al2O3 and BeO.Geochim. Cos-mochim. Acta50, 1847–1860.

Gfeller M., Furrer G., and Schulin R. (1997) Use of completely mixedflow-through systems: Hydrolysis of phenyl picolinate.Environ. Sci.Technol.31, 3692–3701.

Gu B., Schmitt J., Chen Z., Liang L., and McCarthy J. F. (1995)Adsorption and desorption of different organic matter fractions oniron oxides.Geochim. Cosmochim. Acta59, 219–229.

Hue N. V., Craddock G. R., and Adams F. (1986) Effect of organicacids on aluminium toxicity in subsoils.Soil Sci. Soc. Am. J.50,28–34.

Jardine P. M., Weber N. L., and McCarthy J. F. (1989) Mechanisms ofdissolved organic carbon adsorption to soil.Soil Sci. Soc. Am. J.53,1378–1385.

Jug A., Makeschin F., Rehfuess K. E., and Hofmann-Schielle C. (1999)Short-rotation plantations of balsam poplars, aspen and willows onformer arable land in the Federal Republic of Germany. III. Soilecological effects.For. Ecol. Manage.121,85–99.

Kaiser K. and Zech W. (1996) Sorption of DOM and DOM fractions toforest soils.Geoderma74, 281–303.

Kaiser K. and Zech W. (1998) Rates of dissolved organic matter releaseand sorption in forest soils.Soil Sci.163,714–725.

Lundstrom U. and Ohman L.–O. (1990) Dissolution of feldspars in thepresence of natural, organic solutes.J. Soil Sci.41, 359–369.

McBride M., Sauve´ S., and Hendershot W. (1997) Solubility control ofCu, Zn, Cd, and Pb in contaminated soils.Eur. J. Soil Sci.48,337–346.

Moore T. R., de Souza W., and Koprivnjak J.-F. (1992) Controls on thesorption of dissolved organic carbon by soils.Soil Sci.154, 120–129.

Niiskavaara H. (1995) A comprehensive scheme of analysis for soils,sediments, humus and plant samples using inductively coupledplasma atomic emission spectrometry (ICP–AES). Geological Sur-vey of Finland, Special paper.20, 167–175.

Ochs M., Brunner I., Stumm W., and Cosovic B. (1993) Effects of rootexudates and humic substances on weathering kinetics.Water AirSoil Pollut.68, 213–229.

Pohlman A. A. and McColl J. G. (1988) Soluble organics from forestlitter and their role in metal dissolution.Soil Sci. Soc. Am. J.52,265–271.

Raulund–Rasmussen K. and Vejre H. (1995) Effect of tree species andsoil properties on nutrient immobilisation in the forest floor.PlantSoil 168/169,345–352.

Raulund-Rasmussen K., Borggaard O. K., Hansen H. C. B., and OlssonM. (1998) Effect of natural organic soil solutes on weathering ratesof soil minerals.Eur. J. Soil Sci.49, 397–406.

Romkens P. F., Bril J., and Salomons W. (1996) Interaction between

Ca21 and dissolved organic carbon: Implications for metal mobili-zation.Appl. Geochem.11, 109–115.

Romkens P. F. A. M. and Salomons W. (1998) Cd, Cu and Znsolubility in arable and forest soils: Consequences of land usechanges for metal mobility and risk assessment.Soil Sci. 163,859–871.

Salam A. K. and Helmke P. A. (1998) The pH dependence of free ionicactivities and total dissolved concentrations of copper and cadmiumin soil solution.Geoderma83, 281–291.

SAS. (1996) SAS (r) Proprietary Software Release 6.12 TS050, SASInstitute, Cary.

Schwarz A., Wilcke W., Sty´k J., and Zech W. (1999) Heavy metalrelease from soils in batch pHstat experiments.Soil Sci. Soc. Am. J.63, 290–296.

Schwertmann U. (1964) Differenzierung der Eisenoxide des Bodensdurch Extraction mit Ammoniumoxalat-Lo¨sung.Z. Pflanzenerna¨hr.Dung. Bodenk.105,194–202.

Soil Survey Staff. (1997) Keys to soil taxonomy, 7th Edition. Poca-hontas Press, Blackburg.

Strobel B. W., Hansen H. C. B., Borggaard O. K., Andersen M. K., andRaulund-Rasmussen K. (2001). Release kinetics of cadmium andcopper from soil studied in batch experiments. (Submitted).

Stuanes A. O., Ogner G., and Opem M. (1984) Ammonium nitrate asextractant for soil exchangeable cations, exchangeable acidity andaluminium.Commun. Soil Sci. Plant Anal.15, 773–778.

Swoboda–Colberg N. G., and Drever J. I. (1993) Mineral dissolutionrates in plot-scale field and laboratory experiments.Chem. Geol.105,51–69.

Temminghoff E. J. M., van der Zee S. E. A. T. M., and Keizer M. G.(1994) The influence of pH on the desorption and speciation ofcopper in a sandy soil.Soil Sci.158,398–408.

Temminghoff E. J. M., van der Zee S. E. A. T. M., and de HaanF. A. M. (1997) Copper mobility in a copper-contaminated sandysoil as affected by pH and solid and dissolved organic matter.Environ. Sci. Technol.31, 1109–1115.

Tipping E. (1994) WHAM—A chemical equilibrium model and com-puter code for waters, sediments, and soils incorporating a discretesite/electrostatic model of ion-binding by humic substances.Com-puters Geosci.20, 973–1023.

Tyler G. (1981) Leaching of metals from the A-horizon of a spruceforest soil.Water Air Soil Pollut.15, 353–369.

Vejre H., Madsen M., Marott T., Olsen T., and Zlateva T. (1999)Evaluation of soil acidification vulnerability in afforestation areas. InLandscape Ecology, The Science and the Action, 5th World Con-gress IALE,155–156 (abstr.).

Velbel M. A. (1993) Constancy of silicate-mineral weathering-rateratios between natural and experimental weathering: Implications forhydrologic control of differences in absolute rates.Chem. Geol.105,89–99.

Welch S. A. and Ullman W. J. (1993) The effect of organic acids onplagioclase dissolution rates and stoichiometry.Geochim. Cosmo-chim. Acta57, 2725–2736.

Westergaard B., Hansen H. C. B., and Borggaard O. K. (1998) Deter-mination of anions in soil solutions by capillary zone electrophoresis.Analyst123,721–724.


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