Published online March 8, 2006

Arsenate and Arsenite Sorption on Carbonate Hosted Precious Metals Ore
David L. Decker,* Charalambos Papelis, Scott W. Tyler, Mark J. Logsdon, and Jirka Šimůnek
Price, 2000). Northeastern Nevada and small areas ofwestern Utah contain the richest gold-producing region
yet discovered in North America. This region is known
as the Carlin Trend, and it contains several goldproducing ore types that are often combined under themoniker “Carlin-type.” One of the characteristics of theCarlin Trend deposits is the co-occurrence of As mineralogy with gold (Tooker, 1985). Typically, gold is extracted from Carlin-type ores by large-scale, heap
leaching of crushed ore using alkaline cyanide solutions.
The prevalence of As mineralogy combined with thelarge-scale mining operations has led to concerns regarding the potential for release of As into surface water andgroundwater resources not only during heap-leaching
operations but also after mining due to long-term draindown of residual fluids or leaching by meteoric water infiltration.
The objective of this work was to determine the pH
and oxidation state–dependent sorption behavior of
As(III) and As(V) on two representative samples ofCarlin Trend ores. In turn, the sorption behavior will be
integrated into a variably saturated reactive transportmodel to simulate As release and transport from heapleach facilities. This model will be used to facilitate thedesign and evaluation of long-term As mitigation strategies for heap-leach facilities. The need for experimental
sorption data and a mathematical formulation to describe pH and oxidation state–dependent sorption behavior is driven by the mineralogical characteristics ofthe ore deposits, the techniques of gold mining in theCarlin Trend, and the regulatory and industry requirements for mine closure.
The Carlin Trend is generally composed of formationsof silicified sedimentary rocks containing a large percentage of carbonate, siltstone, chert, and shale. The
ore-zone mineralogy of Carlin Trend deposits is characterized by disseminated gold with quartz, dolomite,calcite, sericite, pyrite, marcasite, orpiment, realgar, kaolinite, illite/smectite and barite, limonite, goethite, andhematite. The altered zones are primarily products ofweathered, reduced ores and are characterized by theoxidation products of the primary ore minerals andgenerally contain less than one percent sulfides (Shevenell et al., 1999; Tempel et al., 2000).
All of the gold ore types found in Nevada and Utahthat are generally classified in the Carlin Trend arecharacterized by As mineralogy (Dickson et al., 1979;
Singer, 1996; Shevenell et al., 1999). Arsenic is so prevalent in these ore bodies that it is a recommended
indicator for new gold deposit exploration efforts(Rytuba, 1985). Arsenic occurs in Carlin Trend ore bodies

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

ABSTRACTThe societal impacts of As in water resources in the arid western
USA are potentially acute as a consequence of the combined effects oflimited water supplies and the pervasive occurrence of naturally occurring As in subsurface geologic formations, including the carbonatehosted, disseminated gold-bearing formations of the Carlin Trend. The
prevalence of As in secondary minerals in gold-bearing carbonatehosted ores is of interest because of the potential for As release asa result of ore development. A key component to gold mining is theengineering and construction of large-scale heap-leach and waste-rockcontainment structures that are characterized by variably saturatedhydrology. Estimating As release behavior from these structures with avariably saturated reactive flow and transport numerical model requires the quantification of the significant differences in the sorption
behavior for the stable redox states for As. Therefore, the objective of
this study was to quantify this sorption behavior and to represent theobserved behavior with an isotherm formulation. The pH-dependent
sorption behavior of arsenite, As(III), and arsenate, As(V), onto two
carbonate-hosted gold ores is presented. The experimentally determined pH-dependent sorption behavior for both As(III) and As(V) is
consistent with sorption on metal oxides as reported in studies on rockand soils with similar bulk mineralogical properties. The experimental
sorption data are represented with two modified isotherm formulations. Modified formulations of the Langmuir isotherm and of the Sips
isotherm are presented that include the pH of the sorbate solution
as an additional model parameter. These formulations are applied toboth As(III) and As(V) sorption data to generate an isotherm surface.
The pH-dependent isotherm methodology can be incorporated readily
into numerical models for the purposes of estimating As transport behavior in field-scale, variably saturated environments.

A

RSENIC is present in the environment in a variety ofmineral and aqueous forms and is found throughout the USA (Welch et al., 2000). Arsenic can be toxic atmoderate levels of 1026 to 1025 M in natural and drinking water, thereby requiring treatment of this water toreduce the concentration of As to the drinking water
standard of 1026.87 M (10 mg L21). The state of Nevada is
the largest producer of gold in the contiguous USA,producing more than 226 800 kg (8 million ounces) per
year from 179 ore deposits (Driesner and Coyner, 2000;
D.L. Decker, Desert Research Institute, 2215 Raggio Parkway,
Reno, NV 89512 USA; C. Papelis, Desert Research Institute, 755 E.
Flamingo Road, Las Vegas, NV 89119 and Water Resources Management Graduate Program, Univ. of Nevada, 4505 Maryland Pkwy., Las
Vegas, NV 89154-4029 USA; S.W. Tyler, Dep. of Natural Resources
and Environmental Sciences and Dep. of Geological Sciences andEngineering, MS 175, Univ. of Nevada, Reno NV 89557-0180 USA;
M.J. Logsdon, Geochimica, Inc., 9045 Soquel Dr., Suite 2, Aptos, CA
95003 USA; J. Šimůnek, Dep. of Environmental Sciences, A135
Bourns Hall, Univ. of California, 900 University Ave., Riverside CA
92521 USA. Received 14 Mar. 2005. *Corresponding author (dave.
decker@dri.edu).
Published in Vadose Zone Journal 5:419–429 (2006).
Original Researchdoi:10.2136/vzj2005.0038ª Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: DDW, deionized-distilled water; LOI, loss on ignition;
NBMG, Nevada Bureau of Mines and Geology; XRD, X-ray diffraction; XRF, X-ray fluorescence.

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420

VADOSE ZONE J., VOL. 5, FEBRUARY 2006

primarily as complexes in sulfide and oxide minerals.
The most common of the sulfide mineral hosts is arsenopyrite (FeAsS), and As also commonly occurs as a contaminant in pyrite (FeAsxS22x). Arsenic also occurs in othersulfides such as orpiment (As2S3), realgar (b–As2S2), andperhaps as a coprecipitate with jarosite in heap-leachpads [(K,As,Fe)(SO4)2(OH)6]. Arsenic might also occur
in oxidized zones as arsenolite/claudetite (As2O3).
The industrial process of mining a gold-ore heap, coupled with the presence of sulfide mineralogy, results inthe potential for large swings in pH and redox potential
within the heap. The aqueous chemistry of As is characterized by redox processes over a wide range in pH anda narrow redox potential range (Nordstrom and Archer,2003). As a result, oxidation changes can readily occur ina heap either during mining operations or during postclosure natural hydrologic cycling of water through theheap. The redox sensitivity of As is important because
the sorption characteristics, and therefore the transport
behavior, of As depend on the oxidation state (e.g.,
Goldberg, 2002). Therefore, any efforts to estimate As
flux from gold-ore heaps must include the redox chemistry and the pH-dependent sorption characteristics ofboth As(V) and As(III).
While cyanidation is intended to target the mineralized gold in the heap-leach pad ore, other metals andmetalloids (e.g., As) are released during the leachingprocess by either dissolution or by complexation withcyanide (Flynn and McGill-Haslem, 1995). These otheraqueous metal and metalloid species are not removed
from the pregnant liquor in the mill circuit and are returned to the top of the heap with the recirculated, reclaimed cyanide leaching fluid. Thus, with time andsuccessive cycles through the mill circuit, the concentration of these constituents can rise from trace to milligram per liter concentration levels in the heap leachate
during the course of the mine life of a heap-leach pad.
These byproducts of cyanide leaching of gold are theprimary concern in effective environmental closure of aheap-leach facility.
Typically, concentration requirements for chemical
constituents at closure are met by rinsing the heap withfresh water until the required concentrations of the constituents of concern are obtained in the heap discharge
fluid. Such rinsing requires very large volumes of fresh
water, an extremely valuable commodity in arid Nevada. Furthermore, the rinsing process can alter thechemistry and mineralogy of the heap. For example,
rinsing with fresh water can remove readily available
alkalinity from the heap, leaving the residual rock pile ina geochemical condition that is conducive to sulfide
oxidation and acidification. Ongoing engineering efforts
to design and implement effective closure strategies forthese facilities are driven by the concerns over water
quality and quantity associated with heap-facility draindown and uncertainty associated with the long-term meteorologically mediated chemical flux through the heap.
The development of the capability to estimate the longterm As mass flux from heap-leach pads would be usefulin prioritizing mitigation resources and evaluating thelong-term performance of closure design alternatives. A

coupled, variably saturated flow and geochemical model
has been developed to simulate As transport (Decker
et al., 2006). A key component of this model is the pHdependent sorption of As compounds reported here.
There are numerous studies of As pH-dependent sorption behavior onto geomedia that are potentially applicable to a complex metal ore. Studies that have examined
As sorption onto agricultural soils observed that the pH
dependence observed for both As(III) and As(V) was
similar to behavior reported for sorption onto pure ferricoxide (Manning and Goldberg, 1997a) and was comparable to sorption behavior onto pure-phase clay minerals
identified in the bulk soil medium (Goldberg and Glaubig,1988). Industrially contaminated soils with high levels of
As have been described through a process of identifying
the dominant sorbent minerals and applying pure-phasemineral surface complexation model parameters to estimate As mobility (Lumsdon et al., 2001). There have been
numerous studies conducted on a wide range of purephase minerals, including clays, metal oxides, calcite, andiron sulfides. A few of these studies are applicable tothe present effort through the identification of specificprimary and secondary mineral phases. Studies of As
sorption onto clay minerals (Manning and Goldberg,
1996a, 1997b; Cornu et al., 2003) and onto metal oxides(Manning and Goldberg, 1996b; Manning et al., 1998;
Goldberg and Johnston, 2001) are potentially applicable
to a metals-bearing heap-leach facility and provide a basis
for comparison to sorption behavior onto bulk soil orrock. Additional work to examine competitive sorption
on pure-phase minerals is also of interest as a consequenceof complex interstitial fluid geochemistry (e.g., Goldberg,
2002). There have been a large number of studies using
the iron oxide and oxyhydroxide mineral family that describe pH-dependent sorption behavior (e.g., Pierce andMoore, 1982; Dzombak and Morel, 1990; Bowell, 1994;
Bowell et al., 1994; Jain et al., 1999). These studies describe
significant pH-dependent sorption behavior between As(III) and As(V). Pyrite, arsenopyrite, and arsenian pyrite
are all As-bearing iron-sulfide minerals that have been
observed in Carlin Trend heap-leach facilities. Arsenite
sorption onto pyrite and troilite (FeS) occurs in a manner
analogous to sorption onto ferric oxide, but with a muchsharper sorption edge at near-neutral pH (Bostick andFendorf, 2003). Arsenic has been observed to coprecipitate with carbonate minerals in hydrothermal springs,
resulting in the sequestration of As in these systems, a
process that is not describable through surface complexation modeling or isotherm relationships (Le Guernet al., 2003), while arsenate sorption onto soil carbonates
has been inferred from studies on an agricultural soil
(Goldberg and Glaubig, 1988).
Applying these studies to the Carlin Trend heap-leachrock is problematic. The rock is not monomineralic andincludes a specific and unusual mineral assemblage. To
apply the literature directly would require identifying individual sorbent mineral species, quantifying their relative
abundances, and selecting internally consistent thermodynamic and sorption parameters from the literature forthe aqueous species of interest. Furthermore, for thosemineral species that are present in the Carlin Trend rocks,

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www.vadosezonejournal.org

but that are not reported in the As sorption literature,
spectroscopic studies of these minerals with As would beneeded to quantify sorption behavior for these specificminerals. One would also have to consider the possible
range of mineral compositional heterogeneity at several
spatial scales to establish a comprehensive set of possible models for field-scale heap-leach facilities. As analternative, a series of whole-rock As sorption experiments encompassing the range in pH expected in a goldore heap-leach pad for both As(III) and As(V) was
undertaken. This experimental dataset was then used
to provide a basis for the development of a mathematicaldescription of As sorption on the bulk geomedia.
MATERIALS AND METHODS
The rock samples obtained for this study were provided
by two separate mining operations on the Carlin Trend, one
within the Humboldt hydrographic basin in northeastern
Nevada and a second from a mine in western Utah. Samplesof heap material were obtained with an excavator by the mine
operators from the near-surface of heaps that had been leachedand subsequently rinsed. The specific locations as well as theleach and rinse history of these samples were not provided due
to proprietary restrictions. These samples were assigned identification numbers CT-1 and CT-2.
In an effort to obtain a representative sample from thesample drums from each location, a 9-kg sample was removed
and oven dried at 1058C for 48 h. Using a jaw-crusher, both
samples were then reduced to grains smaller than 1 cm. Eachsample was then split with a soils sample splitter and one ofthese splits was pulverized with a tungsten-carbide ball mill,and dry-sieved through a 100-mm screen. Rock smaller than
100-mm was used for all sorption experiments and whole-rockanalyses. The process of rock drying, grinding, and sieving has
been used in previous metal sorption research (Um andPapelis, 2003, 2004). A significant assumption inherent in thisapproach is that the mineralogical composition of the study
samples did not change in the course of sample collection,
preparation, and experimentation and that the gross sorption
behavior should be unaffected by these activities.
The physical and geochemical properties of ground samples
CT-1 and CT-2 were determined with a variety of analytical techniques. The Nevada Bureau of Mines and Geology(NBMG) analytical geochemistry laboratory analyzed heap
samples CT-1 and CT-2 for bulk mineralogy using X-ray diffraction (XRD). Both CT-1 and CT-2 contain calcite andquartz, with trace amounts of kaolinite and muscovite, andminor amounts of alunite. The bulk mineralogy is consistent
with the mineralogical character of the Carlin Trend deposits(Rytuba, 1985; Tooker, 1985; Shevenell et al., 1999). The presence of the minor and trace minerals is significant because theseclay–mica minerals could serve as sorption surfaces (Manning
and Goldberg, 1996a). In addition, the Al and Fe in these minerals could form Al- or Fe-oxides through the dissolution andsubsequent hydrolysis reactions of these minerals (Charlet andManceau, 1994). The XRD technique is sensitive to crystalline
minerals in part-per-hundred relative mass quantities. As a result, many of the ore mineral phases that exist at low concentration, or in amorphous or poorly crystalline phases, are not
identifiable with XRD.
Heap samples CT-1 and CT-2 were analyzed for the major
elements using X-ray fluorescence (XRF) by the NBMG. The
results are shown in Table 1 as the corresponding metal oxides.
Silicon is the largest elemental component in both samples.

The large amount of calcium is consistent with the high percentage of carbonates expected in a Carlin Trend ore-bodyrock. Additionally, Al and Fe are a significant mass percentage
of both rock samples. The presence of relatively large masspercentages of Al and Fe are important because oxides ofthese elements are known to be effective sorbent minerals foroxyanions (Arai and Sparks, 2002; Goldberg, 2002). Since bothrock samples were taken near the surface of each spent heapleach pad, both samples should be significantly oxidized. This
supposition is further supported by the relatively low masspercentage of total S, reported as SO422, given in Table 1,suggesting a low percentage of pyrite or other sulfide-bearing
mineral species. Both of the samples were obtained fromcarbonate-hosted, sulfide-bearing, hard-rock ore bodies. As aconsequence, the total mass fraction of organic C in the reported loss on ignition (LOI) mass percentages for both samples is likely to be below the detection level. The reported LOI
for CT-1 is 18.3%, and the reported CaO is 19.5%, while the
LOI for CT-2 is 4.67%, and the CaO is 2.5% The XRD results
indicate a significant fraction of calcite in CT-1, and the LOI
mass fraction is likely to be nearly entirely composed of CO2
originating from the thermal decomposition of calcite during
the XRF ignition process. Similarly, although CT-2 is not dominated by carbonate mineralogy, it is likely that the comparatively smaller LOI mass percentage is also primarily due toinorganic C loss.
The results of the complete whole-rock digestion, usinghydrofluoric acid (HF) and inductively coupled plasma mass
spectrometry analysis, are given in Table 2. The relatively large
concentration of As in both samples is consistent with theprevalence of As mineralogy in Carlin Trend ore deposits.
Both rock samples were obtained from heap-leach pads that
had been extensively leached. Evidence of extensive cyanide
leaching is suggested by the low gold concentrations remaining
in both samples (Table 2).
Sorption experiments were conducted with similar techniques used by Um and Papelis (2003, 2004), using batch reactors consisting of 15-mL polystyrene centrifuge tubes. The
total fluid volume of each batch reaction experiment was
10 mL. This resulted in an approximately 5-mL head space toassist in mixing. Mixing was accomplished with Labquake
Table 1. Major elemental composition from X-ray fluorescenceand SO22
4 analysis.†Amount of major elements,
as metal oxides
Component

CT-1

CT-2
% total mass

SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2 O
P2O5
SO22
4
LOI‡
Total

49.0
0.38
7.64
2.58
0.05
0.74
19.5
,0.01
1.34
0.16
0.30
18.3
99.69

80.9
0.30
6.01
2.50
0.03
0.67
2.50
,0.01
1.07
0.31
0.21
4.67
98.96

† Sulfate analysis conducted by the Nevada Bureau of Mines and Geology
with the method given in Furman (1938, p. 906–911).
‡ LOI, loss on ignition, representing loss of volatile substances such as H2O,
CO2, organic C, Hg, Se, graphite (Baedecker, 1987). However, the geologic origin and the significant amount of calcite determined from X-ray
diffraction analysis for both samples preclude the possibility of a significant mass fraction of organic C in either sample. The total calculation
includes SO22
4 as a component of LOI, and not as a separate component.

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VADOSE ZONE J., VOL. 5, FEBRUARY 2006

Table 2. Minor elemental composition as determined from wholerock digestion and inductively coupled plasma mass spectrometry analysis.
Mass of element
Component

CT-1

CT-2

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

21

Ag
As
As inferred from desorption experiments
Au
Ba
Be
Bi
Cd
Co
Cr
Cu
Mo
Ni
Pb
Sb
Se
Sr
Th
U
V
Zn
Total mass, %

mg kg
0.294
0.778
865
336
55.57
49.82
0.239
0.208
1200
1740
2.82
1.33
,0.001
0.002
0.38
4.64
8.14
7.07
59.7
66.6
4.18
41.5
4.61
15.3
34.1
48.7
43.1
50.7
59.7
214
1.13
8.74
584
219
2.91
7.71
3.87
9.34
34.5
579
45.9
316
0.30
0.37

(Barnstead/Thermolyne, Dubuque, IA) rotating mixers using
end-over-end rotation. Deionized-distilled water (DDW) wasused for all solutions. The solution volume was assembled using
the necessary solutions to obtain the desired As concentration,
ionic strength, and pH. Arsenic solutions were made using crystalline NaAsO2 for As(III) and crystalline Na2HAsO47H2O for
As(V). Ionic strength was adjusted to 0.01 M using 1 M NaClsolution. The final pH of the reactor solution was adjusted using
either 0.1 M NaOH or 0.1 M HCl. The solid concentration was
1.0 g L21.
The solution pH was determined as a function of acid orbase addition for both heap materials by performing 24-h leachexperiments with a range of concentrations of acid and base.
These results were used to determine the required volumes ofacid/base to obtain a desired pH for both heap materials.
Additional precursor experiments were conducted to determine that a 24-h sorption reaction time was needed to reach an
approximate equilibrium.
Arsenate was found to be initially present in both rock samples, and this led to a significant effort to remove the arsenate
mass before conducting As(III) batch sorption experiments.
Several attempts were made at reducing the As(V) to As(III)in situ with little to no success. A problem with this approach isthat while the reduction of As(V) is desired, additional reactions with either the As or the rock must not occur. Ultimately,
the method chosen was to sequentially extract the As(V) by
desorption at high pH. This was accomplished by weighing
solid into the reactor tubes in the usual manner, except that thesolution was adjusted to pH 10 to promote As(V) desorption.
These reactors were equilibrated for 8 h, centrifuged for 30 min
at 1580 relative centrifugal force, and the supernatant extracted
via a vacuum pipetter. Fresh pH 10 solution was then added
to the reactors, and the entire process repeated. The entire
extraction process was repeated a total of four times. While themultistep extraction process described does not completely
remove the preexisting As(V), the relatively low As(V) concentrations compared with the As(III) concentrations used in
the isotherm development experiments were assumed to notaffect the isotherm experiment results significantly.
The As(V) extraction methodology is justified on the basisthat the geochemical conditions of the reactors are consistent

with operational conditions of the heap-leach facility. Duringheap-leach mining, the mine operator must maintain the
NaCN leachate solution between pH 10.5 and 12 to preventthe following reaction from occurring (Bartlett, 1992):

NaCN(aq) 1 H2 O ! HCN(g) 1 Na1 1 OH2[1]Preventing this reaction from occurring is important to ensure the safety and economic viability of the operation by
reducing the losses of cyanide to the atmosphere. Therefore,
during the mining phase for a heap-leach facility, leachate fluid
is applied in excess of pH 10.5 for an extended period of time,
typically several years. Therefore, the rock on a heap-leachfacility is exposed to a significantly more aggressive treatment
than what was done in the laboratory in preparation for the
As(III) sorption experiments. In conclusion, the pretreatment
methodology to remove As(V) from the rock is consistent withthe prior geochemical history of the rock samples, and it is very
unlikely that any additional Fe or Al mineral dissolution or
precipitation reactions occurred in the laboratory that had
not already occurred during the several years that the rockwas mined.
A hydride-generation atomic absorption spectrometry method
was used to speciate and analyze the sorption experiment eluent
samples (Manning and Martens, 1997). The carrier eluent used
was 25 mM NaOH and 1% (v/v) MeOH in de-aired DDW. The
reductant solution used in the hydride generation process was
3 g of NaBH4 dissolved into 500 mL of de-aired DDW with 2.5 gof NaOH. The acid solution used in the hydride generator was
6 M HCl using ACS plus reagent-grade acid. Carrier eluent andhydride generator solutions were made fresh before each analysis. All de-aired DDW was prepared by stirring under vacuumfor 24 h.
Calibration standards were made before each analysis withthe same solid-phase chemicals used to make solutions. Check
standards were made using chemicals from a second chemical
supplier. The calibration standards were made by diluting with
DDW water at the same ionic strength as the experiment.
The method reported by Manning and Martens (1997)
was adapted for use with a computer controlled Varian 8050
(Varian, Palo Alto, CA) spectrophotometer with an electric
furnace and continuous hydride generation. The adaptations
to the method were limited to the manipulation of the softwarethat controls the operation of the spectrophotometer detector
to enable continuous data collection during sample speciation.

RESULTS AND DISCUSSION
Both As(V) and As(III) on CT-1 and CT-2 exhibitedstrong pH-dependent sorption behavior. The As(V) concentrations used for both CT-1 and CT-2 were 100 mg
L21 (1025.87 M), 200 mg L21 (1025.57 M), 500 mg L21(1025.18 M), and 1000 mg L21 (1024.87 M) as As(V). An
additional concentration of 2000 mg L21 (1024.57 M) wasused with CT-2. The results for the As(V) experiments
are shown in Fig. 1 and 2.
The As(III) concentrations used for both CT-1 andCT-2 were 100, 500, and 1000 mg L21, with an additional
concentration of 3500 mg L21 (1024.33 M) for CT-1 (Fig. 3
and 4). Because both CT-1 and CT-2 had an initialsorbed mass of As(V) and because of the possibility ofAs(III) oxidation during the experiment, As(V) concentrations were determined for these experiments inaddition to As(III). Detectable amounts of As(V) wereobserved in the As(III) sorption experiments for bothCT-1 and CT-2 in the 100 mg L21 and 500 mg L21 As(III)Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

www.vadosezonejournal.org

423

Fig. 1. Arsenate sorption data for heap material CT-1. Error bars are inclusive of analytical and experimental error.

experiments, and in the 3500 mg L21 As(III) CT-1 experiment. The average As(V) concentrations in these experiments were 20, 50, and 100 mg L21, respectively.
The As(V) results for both CT-1 and CT-2 were generally consistent with As(V) sorption onto iron oxyhydroxide and agricultural soils as reported in previous
studies (Dzombak and Morel, 1990; Manning and Goldberg, 1996a, 1997a). In particular, the decrease in apparent
sorption concentration with increasing pH is consistent
with results reported in the literature. The shape of thesorption envelopes for As(V) are similar to that reported
for amorphous aluminum and iron oxide (Fig. 1 and 2)(Dzombak and Morel, 1990; Goldberg, 2002).
The As(III) results are also generally consistent withthe published literature dealing with sorption onto soils
and minerals common to soils (Manning and Goldberg,

1996a, 1997a; Manning et al., 1998; Goldberg, 2002).
The smaller degree of pH dependence and the opposite
pH-dependent behavior as compared with the As(V)data are also comparable to the observations reported inthese previous studies.
A critical assumption built into sorption isotherms
is that the oxidation state of the aqueous species is thesame as that of the adsorbed species. If the sorption
mechanism is an inner-sphere complexation reaction thatproceeds relatively rapidly, then this assumption is reasonable. Alternatively, it is possible that a heterogeneous
mixture of sorbed As(III) and As(V) anionic complexes
could be represented on the mineral surface as a resultof redox reactions at the mineral surface. Previous studies with anions have suggested a relationship between
acid–base pair chemistry and oxyanion complex binding

Fig. 2. Arsenate sorption data for heap material CT-2. Error bars are inclusive of analytical and experimental error.

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

424

VADOSE ZONE J., VOL. 5, FEBRUARY 2006

Fig. 3. Arsenite sorption data for heap material CT-1. Error bars are inclusive of analytical and experimental error.

strength (Hingston et al., 1967, 1972; Stumm et al., 1980;
Stumm, 1992). Differences in acid–base chemistry of the
two As oxidation states, therefore, would be expected toresult in differences in the sorption affinity of arsenate
and arsenite oxyanions. There is some evidence that anonhomogeneous distribution of As oxyanions may bepresent on the mineral surfaces of both rock samples.
The relatively larger mass of As released from the hydrofluoric acid digestions as compared with the adsorbed As
mass calculated from the As desorption experiments(Table 2) suggests that a significant amount of the As
mass initially bound to the rock is not released by theexperimental techniques used to conduct the sorption
work. The possibility of As-bearing mineral phases contributing to the digested As mass cannot be ignored, but
is discounted by the lack of XRD evidence of theseminerals and the extensive cyanide leaching of both ofthese samples. It is also possible that although both rocksorbent samples were ground smaller than 100 mm, thewhole-rock digest analyses released sorbed As that remained unavailable during the surface-controlled desorption experiments. Additional micromineralogy andsurface spectroscopy would provide the data necessary fora conclusive analysis.
The possibility of a heterogeneous distribution of sorbate–sorbent bond energies necessarily implies the possibility of a distribution of As(III) and As(V) oxyanion
species on the sorbent surface, even while attempts weremade to ensure a homogeneous sorbate solution. Therefore, the sorption data presented may well reflect contributions from both oxidation states. However, there
are no clear trends in the sorption data that would

Fig. 4. Arsenite sorption data for heap material CT-2. Error bars are inclusive of analytical and experimental error.

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Table 3. Keren-type pH-dependent isotherm model parameters. Units for the calculated isotherm surface plot are: Ce, mg L21; qe, mg g21;
pH 5 2log[H1].

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

CT-1

CT-2

Parameter

As(V)

As(III)

As(V)

As(III)A
B
C
D
21
qm, mg grockr2
RMSE
F statistic
F critical, a0.0007962
46.99
548 600
0.1250
1200
0.82
62
1.17
1.13, 0.35

0.0002831
6297
3526 000
1.003
377
0.91
26
0.98
0.55, 0.05

0.0003601
22.66
379 600
1.297
1188
0.80
74
1.16
1.14, 0.35

0.0006856
984.8
169 500
32.49
201
0.81
24
1.22
1.18, 0.30

reflect an isotherm superposition of As(III) and As(V)behavior. The significant differences in pH-dependent
sorption behavior between As(III) and As(V) for bothsorbate rock samples, coupled with similar results reported in the soils literature, generally support the conclusion that the data do not reflect a significant competitive
sorption behavior.

Modeling Arsenic Oxyanion Sorption
There are several models that are commonly used todescribe sorption processes at the mineral–water interface. Empirical isotherm models are the most commonly
applied approach because of their ease of use and wide
range of applicability in mass transport and geochemical
simulation codes. Isotherm models require a qualitativeknowledge of mineral composition and are dependent
on the observed relationships between aqueous andadsorbed phases of the adsorbing species (Koretsky,
2000). There are limitations in the isotherm approach innumerical transport models (Bethke and Brady, 2000);
however, properly applied isotherm relationships are avalid representation of water–rock sorption reactions
and are the most commonly applied means of incorporating water–rock reactions into transport simulations.
Surface-complexation models are widely used in mineral–water interface studies, but they are not routinely
incorporated into reactive flow and transport models
due to the difficulties in obtaining the necessary model
parameters and unique parameter sets when inversion
methodologies are applied to batch reaction sorption
data. Surface-complexation models require a thorough,quantitative understanding of sorbent micromineralogy
because these models depend on parameters that aremineral and mineral–ion complex specific. Therefore, touse a surface-complexation model to simulate sorption
in a highly heterogeneous rock matrix such as the rocksamples that are the subject of this paper would requirethat model parameters for individual mineral species
and ions be obtained or estimated through a numericalinversion process. Further, a means of representing subsurface mineral heterogeneity must be developed toadequately simulate the geochemical complexities of thereactive rock matrix at scales that are meaningful tofield-scale transport problems. Combining the uncertainty involved in surface complexation model parameter estimation from numerical inversion of experimental
data with the difficulty in calculating a unique numericalsolution with multiple minerals and mineral-specific estimated parameters, in a complex, variably saturated
flow regime, challenges the usefulness of the surfacecomplexation approach in all but the most refined andcharacterized mineralogical systems. Many of these
aspects of the surface-complexation approach are thesubjects of ongoing research to provide a thermodynamically based representation of sorption chemistry at
multiple scales of chemical and physical heterogeneity(Davis et al., 1998; Al et al., 2000; Appelo et al., 2002).
An advantage of an isotherm approach is that theisotherm is developed for a “whole-rock” sample that isinclusive of at least some significant subset of the mineral heterogeneity present in a representative volumeof rock at the field scale. However, one of the tenets of
the isotherm approach is that isotherms are valid only
for a constant pH or, alternatively, that sorption is independent of pH. Because As oxyanion sorption exhibits
strong pH dependence, the applicability of the Freundlich or Langmuir isotherm formulations is problematic.
One possible approach to using a standard isotherm torepresent As oxyanion sorption is to develop a set ofisotherms, each identified with a unique and constant
pH. However, the application of this approach to a flow
and transport model is impractical.
A pH-dependent formulation that was developed todescribe B sorption on several mineral species and soils
has the generalized form of the Langmuir equation(Keren et al., 1981a, 1981b). This formulation describes
2
the sorption of B(OH)3, B(OH)2
with the4 , and OH
assumption that these complexes are all competing forthe same sorption sites. The Keren formulation is, inpart, dependent on the B hydrolysis reaction
B(OH)3 1 2H2 O  B(OH)24 1 H3 O1[2]

The Keren pH-dependent B sorption formulation is
described byqe 5 qm

kHB cHB 1 kB cB
1 1 kHB cHB 1 kB cB 1 kOH cOH[3]

where qe is the metal ion (in this case B) concentration
sorbed on the solid phase at equilibrium (mol g21); qm is
the Langmuir monolayer saturation capacity (mol g21);
cHB, cB, and cOH are the solution activities (mol L21) of the2
species B(OH)3, B(OH)2
4 , and OH ; and kHB, kB, andkOH are sorption coefficients related to the binding

426

VADOSE ZONE J., VOL. 5, FEBRUARY 2006

energies of these species (Communar et al., 2004). The
relationship in Eq. [3] is reduced through the use of
relationships describing total B concentration to concentrations of each of the two B species to obtain

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

KL Ce
qe 5 qm
1 þ KL Ce
KL 5(kHB 1 AkB )(1 1 A)(1 1 kOH cOH )[4]where A » Kh(104cOH) is the coefficient dependent on
the solution OH2 concentration, Kh is the B hydrolysis
constant associated with the reaction in Eq. [1] (Kh 5
5.9 3 10210 at 2988K), and Ce 5 cHB 1 cB (Communar
et al., 2004).
Keren’s model is successful in describing B sorption
on pyrophyllite (Keren et al. 1994), montmorillonite(Keren et al. 1981a, 1981b; Keren and Gast 1983), andseveral soils (Mezuman and Keren, 1981; Communar
et al., 2004). However, in spite of the apparent successof this approach to incorporate pH dependence, Kerenet al. (1981a) concluded that the sorption coefficients of
the individual sorbing species have no physical significance, and they are used as model-fitting parameters.
Therefore, Keren’s formulation, for the purposes of thispaper, can be simplified to the following:

KL 5

KL Ce
qe 5 qm
1 1 KL Ce[5]

A 1 BcOH(1 1 CcOH )(1 1 DcOH )[6]where A, B, C, and D are constants. The relationship for
KL in Eq. [6] can be recast in terms of pH ascOH 5 10( pH 2 14)
KL 5

[7]

A þ B 3 10( pH 2 14)
1 1 C 3 10( pH 2 14) 1 D 3 [10( pH 2 14) ]2[8]

Keren’s pH-dependent formulation can be applied todescribing As oxyanion sorption. The aqueous hydrolysis reactions for arsenite are described by Nordstrom
and Archer (2003)
H3 AsO3  H2 AsO23 1 H1

pK 5 9:17

1
H2 AsO23  HAsO22
3 1 H

pK 5 14:1

32
1
HAsO223  AsO3 1 H

pK 5 15[9]where only the first deprotonation step is applicable to aheap-leach environment because of the pH range. The
aqueous complexation reactions for arsenate are similarly described by Nordstrom and Archer (2003)
H3 AsO4  H2 AsO24 1 H1

pK 5 2:3

1
H2 AsO24  HAsO22
4 1 H

pK 5 6:99

32
1
HAsO224  AsO4 1 H

pK 5 11:8[10]

where only the first two reactions are applicable to aheap-leach environment. These reactions can be reduced
to a single reaction (Appelo and Postma, 1993)1
pK 5 9:1
H3 AsO4 HAsO22
4 1 2H[11]

Since both arsenate and arsenite hydrolysis reactions
can be described by a single reaction involving two components for each oxidation state, Keren’s pH-dependent
Langmuir isotherm formulation can be analogously applied to each oxidation state, assuming that the As species compete for the same sorption sites.
As an alternative to the Keren approach, a choice wasmade to incorporate a pH-dependent function into theSips (1950), or Langmuir–Freundlich, isotherm. The Sips
isotherm is written as(aS Ce )n1 1 (aS Ce )nS

qe 5 qm[12]

S

where aS is the Sips isotherm constant, nS is the Sips
isotherm exponent, and the remaining terms are as
defined above (Ho et al., 2002). In the special casewhere nS 5 1, this formulation reduces to the familiar
Langmuir isotherm form.
To incorporate pH-dependence into Eq. [12], one
possible form is(aS Ce )n1 1 (aS Ce )nS

qe 5 qm k(pH)

S

[13]

where k(pH) is a function of pH that matches theobserved pH dependence. This form of the Sips isotherm empirically incorporates pH as an additional
independent variable to a standard isotherm. The function k(pH) can take any form that is suitable. A form
that is suitable for the As(V) and As(III) datasets forboth CT-1 and CT-2 isk(pH) 51
1 1 de2epH[14]

where d and e are constants.
This expression encompasses two analytically measured parameters, Ce and pH, and one calculated parameter, qe. The remaining parameters are used to fit
the pH-dependent isotherm in Eq. [13] and [14] to theobserved data.
The pH-dependent Keren formulation in Eq. [5] and[8] and the Sips isotherm formulation given in Eq. [13]and [14] were used to fit isotherms to the experimental
sorption data for both As(V) and As(III) onto heap
samples CT-1 and CT-2. The inversion was accomplished using an iterative, nonlinear, numerical routine
in a computer spreadsheet. Several different optimization functions were examined, including the modified
Marquardt approach described in Ho et al. (2002).
However, the sum of the squared error produced thesmallest RMSE while providing the best overall fit to theexperimental data. The regression results for the Kerenisotherm are given in Table 3, and that for the Sips
isotherm are given in Table 4.
The pH-dependent sorption surfaces using the Kerenformulation for As(V) and As(III) onto CT-1 are shownwww.vadosezonejournal.org

427

Table 4. Sips-type pH-dependent isotherm model parameters.
Units for the calculated isotherm surface plot are: Ce, mg L21;
qe, mg g21; pH 5 2log[H1].

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

CT-1

CT-2

Parameter

As(V)

As(III)

As(V)

As(III)de
21qm, mg grock
aS
nS
r2
RMSE
F statistic
F critical, a0.000004891
21.404960
0.001151
1
0.81
63
1.22
1.18, 0.30

4.925
0.4211337
0.002011
1
0.84
35
1.38
1.34, 0.20

0.00007911
21.0631400
0.0003007
1
0.82
71
1.16
1.14, 0.35

14.62
0.2795401
0.004123
1
0.77
27
1.35
1.30, 0.20

in Fig. 5 and 6. The experimental data are shown withthe fitted isotherm surfaces superimposed onto the plotsof sorption data. The general shapes of the Keren formulation isotherm surfaces for CT-2 are very similar
to that of CT-1 (Table 3). The Pearson’s correlation
coefficients (r 2 values) for all four Keren formulation
isotherms are at least 0.80 (Table 3). However, the regressions using the Keren isotherm formulation are significant at 65% for the As(V) data, 70% for the CT-2
As(III) data, and 95% for the CT-1 As(III) data. Though
the regression significance is generally quite low withthe exception of the CT-1 As(III) result, the Keren isotherm does capture the general behavior and the overall quantitative characteristics of the experimental data.
The results for As(V) are consistent with the pH dependence illustrated for soils and hydrous ferric oxides, andthe “s”-shaped surfaces in the As(III) isotherms are consistent with sorption onto metal oxides and clays as reported in several previous studies (e.g., Goldberg, 2002).
The pH-dependent Sips isotherm formulation was
also applied to the CT-1 and CT-2 As(V) and As(III)batch sorption data. The Sips isotherm parameters are
given in Table 4, and the isotherm surfaces for CT-1 areshown in Fig. 7 and 8. The results for CT-2 are similar tothat of CT-1 (Table 4). The r2 values for the regressions
using the Sips isotherm are not quite as large as thoseusing the Keren isotherm. However, the RMSE values
and the regression significance statistics are quite similar(Tables 3 and 4). Therefore, the Sips isotherm does notprovide an improved representation of the data. The
Keren isotherm appears to be capable of producing a
sharper sorption edge than the Sips isotherm. However,this difference may only be significant for some applications. Literature results for As(III) sorption onto montmorillonite and illite, for example, are characterized by adiffuse sorption envelope (e.g., Goldberg, 2002).
The differences in the model parameters for both theKeren-type (Table 3) and the Sips-type (Table 4) isotherms are not as large as might be expected given the
differences in gross mineralogy and in total SiO2
(Table 1). However, the similarities in isotherm parameter sets for As(V) and As(III) between CT-1 and CT-2
are matched by nearly identical total Fe and Al masspercentages in both samples. This observation, coupledwith qualitative comparisons to monomineralic studies,Fig. 5. CT-1 As(V) experimental sorption data with the Keren formulation isotherm surface.

Fig. 7. CT-1 As(V) experimental sorption data with the Sips formulation isotherm surface.

Fig. 6. CT-1 As(III) experimental sorption data with the Keren formulation isotherm surface.

Reproduced from Vadose Zone Journal. Published by Soil Science Society of America. All copyrights reserved.

428

VADOSE ZONE J., VOL. 5, FEBRUARY 2006

Fig. 8. CT-1 As(III) experimental sorption data with the Sips formulation isotherm surface.

rated into a variably saturated reactive transport solver
to estimate As transport behavior in heap-leach facilities(Decker et al., 2006).
The advantage of the sorption isotherm approach,compared with a surface complexation approach, is that
an isotherm approach describes laboratory batch reaction data that are inclusive of the whole-rock mineralogy
without explicitly quantifying either the mineralogicalcomposition or the relative abundance of these minerals.
For application to full-scale heap-leach systems for which
detailed mineralogical data do not exist and would be
difficult, if not impossible, to obtain due to proprietaryconcerns, the isotherm approach used in this researchallows for an empirically based analysis of whole-rockbatch reactor sorption data that reflect whole-rock mineralogy and mineral surface chemistry while providing ameans to rapidly assess sorption behavior from easily
conducted batch reaction experiments.
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sulfide mineral surfaces is also probable in view of themineralogic setting (Bostick and Fendorf, 2003; Goldberg and Glaubig, 1988).

CONCLUSIONSThe As(V) and As(III) pH-dependent sorption behavior observed on the two carbonaceous gold ore samples
studied is consistent with the literature for mineralogically heterogeneous soils. The differences between CT-1
and CT-2 sorption behavior are noticeable, but are notas significant as initially expected given that these samples originated from two different mine sites that are
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