Mối quan hệ của các dạng hữu cơ hòa tan đến khả năng giải hấp Cd trong môi trường đất
Trang 1Correlation of the partitioning of dissolved organic matter fractions with
the desorption of Cd, Cu, Ni, Pb and Zn from 18 Dutch soils
Christopher A Impellitteria,1, Yuefeng Lua,2, Jennifer K Saxea,3,
Herbert E Allena,*, Willie J.G.M Peijnenburgb
a Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA
b National Institute of Public Health and the Environment, Laboratory for Ecotoxicology, PO Box 1, 3720 BA Bilthoven, The Netherlands
Received 10 February 2002; accepted 12 August 2002
Abstract
Eighteen Dutch soils were extracted in aqueous solutions at varying pH Extracts were analyzed for Cd, Cu, Ni, Pb and Zn by ICP-AES Extract dissolved organic carbon (DOC) was also concentrated onto a macroreticular resin and fractionation into three operationally defined fractions: hydrophilic acids (Hyd), humic acids (HA) and fulvic acids (FA) In this manner, change in absolute solution concentration and relative percentage for each fraction could be calculated as a function of extraction equilibrium pH The soils were also analyzed for solid phase total organic carbon and total recoverable metals (EPA Method 3051) Partitioning coefficients were calculated for the metals and organic carbon (OC) based on solid phase concentrations (less the metal or OC removed by the extraction) divided by solution concentrations Cu and Pb concentrations in solution as a function of extract equilibrium pH are greatest at low and high pH resulting in parabolic desorption/dissolution curves While processes such as proton competition and proton promoted dissolution can account for high solution metal concentrations at low pH, these processes cannot account for higher Cu and Pb concentrations at high pH DOC increases with increasing pH, concurrently with the increase in Cu and Pb solution concentrations While the absolute concentrations of FA and HA generally increase with increasing pH, the relative proportional increase is greatest for HA Variation in HA concentrations spans three orders
of magnitude while FA concentrations vary an order of magnitude over the pH range examined Correlation analysis strongly suggests that
HA plays a major role in increasing the concentration of solution Cu and Pb with increasing pH in the 18 soils studied The percentage of the
OC that was due to FA was nearly constant over a wide pH range although the FA concentration increased with increasing pH and its concentration was greater than that of the HA fraction at lower pH values (pH = 3 – 5) Thus, in more acidic environments, FA may play a larger role than HA in governing organo-metallic interactions For Cd, Ni, and Zn, the desorption/dissolution pattern shows high metal solution concentrations at low pH with slight increases in solution concentrations at extremely high pH values (pH>10) The results presented here suggest that the effects of dissolved organic carbon on the mobilization of Cd, Ni, and Zn may only occur in systems governed by very high pH At high pH, it is difficult to distinguish in this study whether the slightly increased solution-phase concentrations of these cations is due to DOC or hydrolysis reactions These high pH environments would rarely occur in natural settings
D 2002 Elsevier Science Ltd All rights reserved
Keywords: Cadmium; Copper; Nickel; Lead; Zinc; pH, humic acid; Fulvic acid; Soil organic matter
1 Introduction
There is little doubt that organic matter (OM) plays a significant role in metal behavior in the environment
(Schnitzer and Kerndorff, 1981) Many studies have focused
on the sorption of metals by solid phase soil organic matter (SPSOM) (Lion et al., 1982; Sanders, 1980; Sauve et al., 2000; Strawn and Sparks, 2000) Generally, SPSOM in environmental systems is implicated in retention, decreased mobility, and reduced bioavailability of trace metals A significant amount of research has examined the role of
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*
Corresponding author Tel.: +302-831-8449; fax: +302-831-3640.
E-mail address: allen@ce.udel.edu (H.E Allen).
1
Current address: USEPA-National Risk Management Research
Laboratory, 26 W Martin Luther King Drive, Cincinnati, OH 45268, USA.
2
Current address: Connecticut Agricultural Experiment Station,
Department of Soil and Water, New Haven, CT 06511, USA.
3
Current address: Gradient, 238 Main Street, Cambridge,
Massachu-setts 02142, USA.
www.elsevier.com/locate/envint
$
Trang 2dissolved organic matter (DOM) on the behavior of trace
metals Studies on humic acid (HA) metal relationships have
provided information on a wide range of subjects including
sorption characteristics(Spark et al., 1997a), kinetics
(Boni-fazi et al., 1996), stability constants (Pandey et al., 2000),
bonding mechanisms (Frenkel et al., 2000) and modeling
(Liu and Gonzalez, 2000; Robertson and Leckie, 1999)
Studies regarding fulvic acid (FA) – metal systems include
research on binding models (Christensen et al., 1998a;
Leenheer et al., 1998), competitive effects (Mandal et al.,
2000), sorption (Schnitzer and Kerndorff, 1981), binding
strength (Brown et al., 1999; Sekaly et al., 1999), and
stability constants (Schnitzer and Skinner, 1966, 1967)
Research has also been performed on the sorption of organic
molecules onto representative soil solids such as kaolin
(Huang and Yang, 1995) and metal oxides (Spark et al.,
1997a) Many researchers have employed increasingly
com-plex systems to study the reactions between metals, organic
molecules and soil solids including illite – FA – Cu2 + (Du et
al., 1999), kaolin – HA/FA – Cu2 + (Huang and Yang, 1995),
montmorillonite – HA – Cd2 +/Cu2 +/Pb2 +(Liu, 1999), and
kaolin – HA – Co2 +/Cu2 +/Zn2 +(Spark et al., 1997b) Other
researchers have studied these interactions in complex
systems such as soils(Jordan et al., 1997; Temminghoff et
al., 1997, 1998), raw sewage (Kunz and Jardim, 2000),
composts(Hsu and Lo, 2000), lake waters (Xue and Sigg,
1999), biosolids(Han and Thompson, 1999), and estuaries
(Alberts and Filip, 1998) Multiple studies on the effects of
pH on the behavior of SOM consistently show that the
solubility of SOM increases with increasing pH(Andersson
et al., 2000; Erich and Trusty, 1997; Karlik, 1995; You et al.,
1999) Shen (1999) examined the sorption of DOM onto
soil solids and found that DOM sorption reached a
max-imum at pH 4 – 5 with a decrease in DOM sorption with
further increases in pH
Some researchers have also examined the effects of pH
on the nature of DOM Karlik (1995)found an increase in
humic compounds (as defined by separation on a XAD-2
resin) with increasing pH.Temminghoff et al (1994)found
that the humic/fulvic ratio (as defined by size separation in
0.0033 M Ca(NO3)2extracts of a sandy soil) increases from
pH 4.4 to pH 5.7.Erich and Trusty (1997)found changes in
fluorescence emissions and wavelengths in DOM from
forest soil samples with increased lime applications
Ander-sson et al (2000) found an increase in refractory
hydro-phobic acids with increasing lime applications on more
humus
Some researchers have applied the relationships between
increasing DOM with increasing pH to the mobilization and
speciation of metals in environmental systems Much of this
research focuses on Cu Temminghoff et al (1997) found
increased Cu mobility at both low and neutral pH values in a
Cu contaminated sandy soil They found that at pH 3.9, only
30% of Cu in solution was bound by DOC, whereas 99% of
the Cu was bound by DOC at pH 6.6.Strobel et al (2000)
showed that Cu mobilization in a forest soil was related to
both pH and DOC whereas Cd mobilization was related solely to system pH Naidu and Harter (1998) linked Cd mobility to organic ligands Increasing equilibration time and temperature reduced the mobility of Cd caused by organic matter (Almas et al., 1999) Jordan et al (1997)
examined the increased mobility of Pb in the presence of natural organic matter in a sandy soil They found that peat humic acids had a higher binding affinity for Pb than peat fulvic acids They also illustrated the decreased binding of
Pb to the sandy soil when DOM was present in column and batch sorption studies
Research on the relationship between metals and DOM show that organic molecules are, in many instances, respon-sible for the increased mobility of metals in soils This has been shown for Pb(Jordan et al., 1997)and Cu (Temmingh-off et al., 1997) Hsu and Lo (2000) demonstrated an increase in solution Cu with increasing pH and a concurrent increase in DOM This increase of DOM at higher pH values (pH>8) did not result in increased soluble concen-trations of Zn and Mn These results for Cu contradict the general notion that metal sorption increases at higher pH This result is also of critical importance for a myriad of environmental issues involving trace metal contamination including site remediation, modeling, and risk assessment Several studies have also included the effects of Ca on the binding and dissolution behavior of OM in soils and aquatic systems.Mandal et al (2000)examined the effects
of Ca and Mg on Ni binding by FA They concluded that bound Ni tends to be released in the form of Ni2 + ion in the presence of Ca and Mg as these ions out-compete Ni for binding sites Curtin et al (1998) identified soil organic matter as the major source of Ca preferring sites in smectitic Canadian prairie soils Romkens and Dolfing (1998)found that Ca additions precipitated high molecular weight acids and that Cu co-precipitated with these acids Temminghoff
et al (1998)found that Ca as well as pH affects Cu mobility
in a Cu contaminated sandy soil
The research presented here is unique because of the number and variety of soils involved in the desorption/ partitioning experiments The methodology employed for fractionation and characterization of the organic matter allows comparisons to be made between soils both rich and poor in solid phase organic matter We have not made additions to the soils; neither trace metal concentrations nor organic matter contents have been altered This work con-tributes to the mounting evidence that illustrates the impor-tance of the role of organic matter in metal mobility in the environment This research focuses on desorption of organic carbon (DOC) and trace metals from the solid phase to solution in 18 Dutch soils The main objectives of this study are to (1) fractionate the DOC in water extractions of the soils, (2) examine the concentrations of the operationally defined DOC fractions in the extracts as a function of system pH, and (3) correlate Cd, Cu, Ni, Pb and Zn concentrations in the extracts with the DOC fractions, as a function of system pH The experimental findings presented
Trang 3in this work build on existing knowledge by further
eluci-dating more precisely the relations between OM fractions
and metal mobility in a wide variety of unspiked soils
2 Materials and methods
2.1 Definitions
Soluble throughout this work refers to the constituents
passing through a 0.45-Am cellulose fiber membrane filter
(Fisher Scientific, Fairlawn, NJ) DOC fractions are
opera-tionally defined as hydrophilic acids (Hyd), humic acids
(HA), and fulvic acids (FA) DOC fractions were analyzed
for their carbon (C) content by a TOC analyzer (DC 190,
Rosemount Analytical, Dohrmann Division, Santa Clara,
CA) Solid phase organic carbon (SPOC) was calculated
from total carbon measurements using a boat sampler for the
analysis of solids An unamended portion of each soil
sample to be assessed for SPOC was analyzed for total
carbon (TCunacidified) Another portion of the sample was
then thoroughly mixed with 0.5 M HCl (1:1 w/v),
equili-brated for 24 h, and the liquid evaporated under N2 The
dried sample was then re-homogenized and analyzed for
total carbon (TCacidified) The SPOC was then calculated as:
SPOC¼ TCunacidified TCacidified ð1Þ
where all units are mg C/kg soil
2.2 Soil metal extractions
Eighteen soils from the Netherlands were employed in
this study.Table 1provides summary statistics for some key
parameters for all of the soils and information for two of the
soils for which detailed results will be presented Further
information regarding the soil characteristics can be found
elsewhere(Janssen et al., 1997) Each soil was extracted in a
35-ml polypropylene round-bottom centrifuge tube
Mix-tures (3 g:30 ml) of soil and de-ionized (DI), ultrapure (18.3
MV cm) H2O were equilibrated at a minimum of 4 pH
levels (pH here is defined as the pH after the 24-h extraction
period) Trace metal grade HNO3and NaOH (Fisher
Scien-tific) were used to adjust the pH of the mixture The samples were then placed on an orbital shaker (60 rpm) and equilibrated for 24 h The samples were then centrifuged, filtered (0.45 Am), tested for pH (equilibrium pH values ranged from 3 to 9), and analyzed for metals by inductively coupled argon plasma-optical emission spectrometry (ICP, Spectro Analytical Instruments, Kleve, Germany)
We used EPA Method 3051 (USEPA, 1997) for the microwave digestion of soil using HNO3for the determi-nation of total recoverable metals The digestates were analyzed by ICP
2.3 Soil organic carbon extraction and fractionation The 18 soils were then extracted exactly as they were for metal analyses at various pH values in glass centrifuge tubes
at a ratio of 1 g soil/10 ml H2O All soils were extracted at three different pH values Two soils (Budel and Callant-soog) were extracted at five different pH values These particular soils were chosen for more in depth study because
of the relative ease of pH manipulation Budel is a sandy humic soil from land around a Zn factory and Callantsoog is
a sandy, humus poor soil from a shooting range (this sample contains a high concentration of Pb originating from leaded bullets) After filtration and centrifugation, the solution samples were analyzed for DOC and prepared for fractio-nation into operationally defined Hyd, HA, and FA frac-tions The fractionation procedure is modeled after OM fractionation procedures using macroreticular resins(Aiken and Leenheer, 1993; Christensen et al., 1998b; Leenheer and Huffman, 1976; Malcom et al., 1994; Thurman and Mal-colm, 1981) The procedure presented in this work modifies existing procedures by employing the resin as a tool to concentrate soluble organic molecules as well as separate them Known volumes of acidified (pH = 2) soil extracts were passed through a Supelite DAX-8 resin (Sigma-Aldrich, St Louis, MO) contained in a low-pressure liquid chromatography column (Sigma-Aldrich) The resin bed volume (BV) was 8 ml and the loading rate for the extracts was 8 – 10 BV/h Sample passing through the resin con-tained the operationally defined Hyd fraction After loading, the column was back eluted with 0.1 M NaOH at a rate of 2 BV/h The eluate was collected in 10 or 25 ml volumetric
Table 1
Selected soil characteristics for 18 Dutch soils examined in this study
Total Cd (mg/kg)
Total Cu (mg/kg)
Total Ni (mg/kg)
Total Pb (mg/kg)
Total Zn (mg/kg)
OC (mg/kg)
Total metals are from HNO digestions (EPA Method 3051; USEPA, 1997 ) The pH values are from 1:1 (w/v) soil-deionized H O slurries.
Trang 4flasks, sub-sampled and acidified to pH < 1 Samples were
then refrigerated for 24 h with intermittent agitation At the
end of the 24-h period, samples were centrifuged (at 25 jC)
The supernatant was sampled and analyzed for TOC as the
operationally defined FA fraction HA-C was calculated
from the equation:
HA C ¼ DOCtotal ðFA C þ Hyd CÞ ð2Þ
where DOCtotalrefers to the original, unfractionated sample
All units are related to the mass of solid extracted (mg/kg)
Analyses were performed on solutions with defined amounts
of each operationally defined fraction in order to assess
recoveries
2.4 Data analysis Soluble metal (as a function of pH) was modeled by means of parabolic equations This allowed estimates of soluble metals at the exact pH values of the OM extractions Direct metal analyses in each of the operationally defined fractions are of limited value because of the manipulations (especially pH) that are required in the fractionation proce-dure The concentration of organic carbon and the percent-age of the TOC contained in each fraction of the samples were calculated as a function of pH Correlative analyses were then performed between the various fractions and the concentrations of soluble Cd, Cu, Ni, Pb, and Zn For some analyses(Figs 6b,d and 7), data from water extractions with
Fig 1 H O extractable metal as a function of pH for the Budel soil.
Trang 5equilibrium pH between 4 and 9 were exclusively used, as
pH values outside of this range are uncommon in most
environmental settings
3 Results and discussion
3.1 Metals in variable pH extractions
Fig 1 shows the results of metal solubilization as a
function of pH for the Budel soil Similar results are observed
for the remaining 17 soils It is noteworthy that
approx-imately 160 times more Zn than Cu is extracted at the lowest
pH in the Budel soil despite the fact that total Zn and total Cu
differ by no more than a factor of 4.5 Six times more Cd than
Cu is extracted at low pH though total Cd concentration is
substantially less than six times the total Cu concentration
The percentages of each metal extracted in the Budel soil at
low pH are Cd-80%, Cu-2.5%, Ni-22%, Pb-9.2%, and
Zn-87% This suggests that even at lower pH values, stronger
binding of Cu and Pb occurs relative to the binding of Cd, Ni,
and Zn by solid constituents Our results indicate that proton
competition and/or proton promoted dissolution greatly affect the amount of Cd, Ni, and Zn in solution, while the effect on Cu and Pb is less significant Cu and Pb in these soils may be strongly bound to solid forms of OM and, in low
pH environments, have a greater association with the solid phase relative to Cd, Ni, and Zn A significant increase in extractable Cu and Pb occurs as the pH increases from 5 to 9 This increase in water extractable metal as a function of pH is much less significant for Cd, Ni, and Zn Similar results have been published elsewhere(Hsu and Lo, 2000) for Cu, Mn, and Zn in compost extracts and for Cd and Cu in soils(Salam and Helmke, 1998) The increase in solubility of Cu and Pb could be due to the pH-induced solubilization of organic matter This would have important implications for many natural and engineered systems For example, increasing pH
in wastewater treatment to precipitate compounds could actually increase Cu in solution by increasing the solubility
of OM Released organic molecules could transport metals through soils systems to ground and surface waters Depend-ing on the chemistry of the receivDepend-ing waters, a significant portion of the metals could disassociate from the organic molecules and become more biologically active
Fig 2 Dissolved soil organic carbon (DOC) in the fractions (FA, HA and Hyd) as a function of pH (a—Budel, c—Callantsoog) The log scale on the y-axis for (a) and (c) emphasizes the increase in HA as a function of pH (b) (Budel) and (d) (Callantsoog) show percent distribution of the three fractions as a function of the DOC solubilized at the 24-h equilibrium pH.
Trang 63.2 Hyd, HA, and FA in solution
We tested the DOC fractionation procedure with isolated
FA, HA and Hyd from well-characterized solutions of
organic matter isolates using the procedure developed in
this laboratory(Impellitteri, 2000; Lu, 2000) Recoveries of
the isolated FA, HA, and Hyd fractions were 100%, 107%,
and 102%, respectively
Fig 2shows the total concentrations and proportions of
each of the DOC fractions in solution as a function of pH for
two of the soils (Budel and Callantsoog) For seven of the
soils extracted at only three pH values, HA was undetected
at the lowest pH value for the conditions used in this study
Both HA and FA concentrations increase with increasing
solution pH These results have been shown elsewhere for
US coastal plain soils(You et al., 1999) The concentration
of the Hyd fraction remains relatively constant as a function
of pH The percentage of the total DOC that is HA increases
markedly with increasing pH, while the FA fraction
per-centage remains more constant as a function of pH, and the
relative percentage of the Hyd fraction decreases with
increasing pH The y-axes in Fig 2a and care in log form and thus show the striking increase in soluble HA as a function of pH For the 18 soils studied, the increase in soluble HA in the pH range of 3 to 9 was between two and three orders of magnitude The increase in soluble FA typically remained within an order of magnitude Fig 3
shows the partitioning of each fraction as a function of pH
Fig 3 Partitioning of each OC fraction as a function of pH for the (a) Budel
and (b) Callantsoog soils K d-OC is based on SPOC (less the TOC in the
fraction) divided by TOC in each fraction at a particular pH.
Fig 4 Partitioning of each OC fraction as a function of pH for all soils.
K d-OC is SPOC (less the fraction TOC) divided by TOC in each fraction at
a particular pH.
Trang 7The partitioning values, Kd-OC, are calculated based on the
equation:
KdOC¼ ðSPOC TOCall fractionsÞ=TOCfraction ð3Þ
where SPOC (mg/kg) represents the solid phase organic
carbon, TOCall fractions (mg/kg) is the sum of the OC
removed during the extraction and TOCfraction (mg/kg) is
the TOC in each operationally defined fraction The results
show a linear dependency of the log Kd-OCvalues for HA
with equilibrium pH with a strong correlation Strong
correlations also exist for the partitioning of FA; however,
there is little dependency of the partitioning of Hyd on pH
The results for these two soils are representative of the other
16 soils tested For log Kd-OCHyd values vs pH in all 16 soils, the average R2= 0.44 (s = 0.42), for log Kd-OC FA average R2= 0.91 (s = 0.19), and for log Kd-OCHA average
R2= 0.89 (s = 0.20) Thus, within a particular soil, partition-ing of FA and HA correlates well with pH This is in contrast to the data presented inFig 4 Here, all of the data are combined for all of the soils The results in Fig 4
illustrate poor correlation between partitioning of all of the operationally defined fractions and pH This may be explained by the fact that the slope values of the regression lines for partitioning of log Kd-OCvalues vs pH vary among individual soils This suggests that the factors governing the dissolution/desorption of organic carbon from solid to solution vary in these soils This should be expected in this
Fig 5 Partitioning of metal in the Dutch soils vs partitioning of soil organic carbon.
Trang 8sample set of 18 soils, factors such as clay (type and
concentration), metal-oxide content, and presence/absence
of Ca species will all affect the behavior of organic carbon
partitioning It may also be possible that variations in solid
phase carbon source material (e.g soot vs leaf litter) may
play a role in governing the nature of DOC in soils
3.3 DOC partitioning and metal partitioning
Log Kdvalues for all metals (with the exception of Zn)
do not correlate with extraction equilibrium pH when the
data for all soils are combined (Cd-R2= 0.14, Cu-R2= 0.07,
Ni-R2= 0.05, Pb-R2= 0.01, Zn-R2= 0.30).Fig 5 shows the
correlations between log Kdvalues for all the metals and log
Kd-OCwhere Kd-OCis defined as in Eq (3) This data shows
a relationship between the partitioning of Cu and OC in these soils Evidence also exists for a relationship between the partitioning of Pb and OC This relationship has been shown previously for Cu (Temminghoff et al., 1994, 1997, 1998; Yin et al., 2002) Dissolved natural organic matter has also been implicated in preventing the sorption of Pb onto a sandy soil (Jordan et al., 1997)
Further analysis of the data reveal that the presence of Cu and Pb in the water extracts can be more precisely asso-ciated with the operationally defined HA fraction Table 2
shows R2values for log Kdof all metals vs log Kd-OCfor all
of the DOC fractions The data set was further restricted to extracts with equilibrium pH values between 4 and 9 These
pH values are more environmentally realistic, though in some situations with significant anthropogenic disturbances,
pH extremes may occur The R2 values for this restricted range of pH are shown in parentheses in Table 2
Data for Cu and Pb are shown graphically inFig 6.Fig
7 shows the same data as in Fig 6b and dwith the Kd-OC
values normalized by total Ca concentrations in the soils With all else being equal (pH, cation concentrations, etc.), increased Ca concentrations in soils will aid in the floccu-lation of HA from solution causing increased Kd-OCvalues
Table 2
R 2 values for log K d values for all metals vs log K d-OC values for the three
operationally defined DOC fractions across the entire pH range
Log K d-OC Hyd Log K d-OC FA Log K d-OC HA
Log K d Cd 0.0068 (0.00058) 0.020 (0.0027) 0.0022 (0.02)
Log K d Cu 0.028 (0.12) 0.0025 (0.20) 0.21 (0.62)
Log K d Ni 0.0023 (0.037) 0.0017 (0.02) 0.071 (0.14)
Log K d Pb 0.0029 (0.13) 0.014 (0.16) 0.30 (0.46)
Log K d Zn 0.023 (0.017) 0.044 (0.044) 0.021 (0.018)
The data sets for each comparison were also restricted by eliminating
extracts with equilibrium pH values less than 4 or greater than 9 R2values
for the restricted data sets are in parentheses.
Fig 6 Partitioning of Cu and Pb vs partitioning of HA for all water extracts from all soils where HA was quantified in the extracts (b) and (d) show the same data for water extracts where 4 < pH < 9.
Trang 9for HA The partitioning of Cu and Pb in the water
ex-tractions is clearly related to the partitioning of HA in these
soils For Cu, these results build on results generated by
other researchers(Temminghoff et al., 1994, 1997, 1998)
The results for Pb shown inFig 6 are in agreement with
work performed by Jordan et al (1997) They concluded
that HA (peat derived) had a higher affinity than FA for Pb
and prevented the binding of Pb by a sandy soil Here, the
data strongly suggest that the increased
desorption/dissolu-tion of HA is strongly correlated to the increase of the
concentration of Pb in solution As both Cu and Pb form
strong complexes with HA(Tipping, 1994), it is likely that
the correlation of increased soluble Cu and Pb with
increased desorption of HA is a consequence of the
com-plexation of these metals by HA
4 Conclusions
DOC increases with increasing pH The fractionation of
DOC desorbed/dissolved from 18 Dutch soils shows that the
largest relative increase occurs for the operationally defined
HA fraction The Hyd fraction percentage generally de-creases with increasing extraction equilibrium pH The percent FA tends to remain constant over the range of pH values Within soils, the partitioning of FA and HA corre-lates with the system equilibrium pH The partitioning of FA and HA as a function of pH varies widely for the soils studied The Kd-OCvalues for FA and HA tend to decrease (more OC associated with solution phase) with increasing system pH Correlative studies provide indication that HA is capable of transporting Cu and Pb into solution upon desorption/dissolution from the solid phase Ca may antag-onize the solubilization of metals sequestered by HA by flocculation of the HA – metal complex The partitioning of
Cd, Ni, and Zn could be affected by the partitioning of OC
in these soils at very high pH (>10) though in this study, it is impossible to distinguish the effect of OC partitioning and hydrolysis reactions
This study simulates a situation in the environment where soil systems are ‘‘titrated’’ with acid or base in the form of atmospheric precipitation or soil amendment The soil solution is considered to be in a state of pseudo-equilibrium for this study and thus the pH of the soil – solution mixture in nature is of critical importance We presume that the pseudo-equilibrium pH is a master variable governing the desorption/dissolution of organic molecules Metals transported by organic molecules into surface and/or groundwaters will undergo further reactions depending on the chemistry of the receiving waters
Acknowledgements
The International Copper Association, the International Lead – Zinc Research Organization, and the United States Environmental Protection Agency funded this work
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