Perchlorate removal by acidified zero-valent aluminum and aluminum hydroxide
Hsing-Lung Lien
*, Chia Ching Yu, Ya-Ching Lee
Department of Civil and Environmental Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan, ROC
a r t i c l e
i n f o
Article history:
Received 25 February 2010 Received in revised form 8 May 2010 Accepted 10 May 2010
Available online 2 June 2010 Keywords: Perchlorate Aluminum Aluminum hydroxide Adsorption Corrosion
a b s t r a c t
Removal of perchlorate using either acid-washed zero-valent aluminum or aluminum hydroxide was studied in batch reactors under ambient temperature and pressure. Approximately 90–95% of perchlorate was removed within 24 h in the presence of 35 g L1aluminum at acidic pH (4.5 ± 0.2). Although
alumi-num is a strong reductant, this study indicated no explicit evidence to support perchlorate reduction while it was found that an adsorption process is involved in the perchlorate removal. The adsorbed per-chlorate ions were desorbed effectively using a 1.0 N MgSO4solution. The effective composition for the
perchlorate adsorption is confirmed as aluminum hydroxide (bayerite), which is a product of the alumi-num corrosion. Rapid adsorption of perchlorate was observed in the presence of alumialumi-num hydroxide. The perchlorate adsorption by aluminum hydroxide is dependent on the solution pH. The removal mech-anism can be attributed to the ion-pair formation at the aluminum hydroxide surface.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Perchlorate salts such as ammonium perchlorate have been used as oxidants in solid propellants for rockets, missiles, and fire-works (Urbansky, 2002). Widespread perchlorate contamination of groundwater and drinking water supplies in 25 US states and Puer-to Rico has been reported (US EPA, 2004). Perchlorate is a health concern because it inhibits iodine uptake in the thyroid gland caus-ing the reduced production of thyroid hormones (Urbansky, 2002). The US Environmental Protection Agency has established an official perchlorate reference dose of 0.0007 mg kg1d1, which
corre-sponds to a drinking water equivalent level of 24.5 ppb (US EPA, 2005).
As a strong oxidant with the high oxidation state of the chlorine, +7, perchlorate reduction is a thermodynamically favorable pro-cess under ambient conditions (Urbansky, 2002).
ClO4þ 8H þ
þ 8e! Cl
þ 4H2O Eo¼ 1:287 V ð1Þ
However, studies have indicated a large kinetic barrier for per-chlorate reduction (Gu et al., 2003; Cao et al., 2005). Because of the kinetic barrier, researchers have devoted great efforts to develop-ing remedial technologies for perchlorate reduction usdevelop-ing various reactive media (e.g., iron) (Gu et al., 2003; La´ng and Hora´nyi, 2003; Moore et al., 2003; Cao et al., 2005; Oh et al., 2006; Son et al., 2006; Yu et al., 2006). However, under ambient conditions, incomplete removal of perchlorate (66%) was found in 2 wk when a high iron dose (1250 g L1) was applied at neutral pH (Moore
et al., 2003). To effectively reduce perchlorate, reaction systems usually require incorporating with microbial reduction processes (Son et al., 2006; Yu et al., 2006) or operating under particular con-ditions such as an elevated temperature (Gu et al., 2003; Cao et al., 2005; Oh et al., 2006; Xiong et al., 2007), high metal doses (Moore et al., 2003), or extremely acidic conditions (Gu et al., 2003; La´ng and Hora´nyi, 2003). More recently, Wang et al. (2008)reported the use of pressurized hydrogen gas for perchlorate removal in the presence of metallic catalysts. It was found that the Ti–TiO2
catalyst can remove more than 90% of initial perchlorate concen-trations (1 ppm) in 3 d. They also reported an indirect electro-chemical reduction process for effective removal of perchlorate using a titanium electrode (Wang et al., 2009).
While the chemical and microbial reduction can transform per-chlorate to chloride, different approaches for perper-chlorate removal such as adsorption and ion exchange have also been developed (Yoon et al., 2009). It has been reported that surfactant-tailored activated carbon is capable of adsorbing perchlorate at ppb level and both perchlorate and surfactant can be destroyed during the thermal regeneration process of activated carbon (Parette and Cannon, 2005). Lehman et al. (2008) reported a combination of the exchange process with biological brine treatment to remove perchlorate from the groundwater.
Aluminum metal is a strong reducing agent (Eo= 1.662 V) that
has a standard reduction potential more negative than zero-valent iron (Eo= 0.43 V). Our previous work has demonstrated that
alu-minum can serve as a reactive reagent for reductive degradation of chlorinated organic compounds through aluminum corrosion (Lien and Zhang, 2002; Lien, 2005; Chen et al., 2008). Electrochemical studies found the reduction of perchlorate occurs in the presence 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2010.05.013
*Corresponding author. Tel.: +886 7591 9221; fax: +886 7591 9376. E-mail address:[email protected](H.-L. Lien).
Contents lists available atScienceDirect
Chemosphere
of aluminum (Ujva´ri et al., 2002a; La´ng and Hora´nyi, 2003; Li et al., 2004; La´ng et al., 2005), but the conversion ratio was very low (1.4%). On the other hand, the aluminum corrosion products such as aluminum hydroxide/oxyhydroxides are known as good adsor-bents (Sposito, 1996). The use of aluminum-based drinking water treatment residual for effective adsorption of perchlorate has been reported (Makris et al., 2006).
In this study, acid-washed zero-valent aluminum was tested for perchlorate removal under ambient conditions. The use of acidification to treat aluminum is to activate its reducing power by dissolving the passive oxide layer and to create a favorable con-dition for perchlorate adsorption. The objective is armed at inves-tigating removal mechanisms of perchlorate in reaction with aluminum. Both chemical reductions and surface adsorption pro-cesses are taken into consideration in this study. Characteristics of aluminum and its reactions in Al0–H2O systems were
systemi-cally examined to understand the influence of aluminum corrosion on the perchlorate removal. In this study, it was found that the adsorption of perchlorate is the major removal process. In order to determine the effective composition, aluminum hydroxide, a corrosion product of the zero-valent aluminum, was therefore syn-thesized and tested for its capability of perchlorate adsorption un-der various pH conditions.
2. Materials and methods 2.1. Chemicals
Aluminum powder (325 mesh (44
lm), 99.5%) was purchased
from Alfa Aesar. Sodium perchlorate (NaClO4, ACS reagent, 99+%)and sodium chloride (NaCl, 99.5+%) were purchased from Sigma Aldrich. Concentrated hydrochloric acid (37.5%) with trace metal grade and magnesium sulfate (MgSO47H2O, ACS reagent) obtained
from J.T. Baker. Deionized water (>18 MXcm, ELGA LabWater) was used in all experiments.
2.2. Preparation of acid-washed aluminum and aluminum hydroxide Concentrated HCl (13 mL) was slowly added to a 1000 mL glass beaker containing 5.0 g aluminum powder in 10 mL aqueous solu-tion and the suspension was mixed with a magnetic stirrer in a fume hood under ambient conditions. Immediate fume evolution was observed. A small amount of deionized water was quickly added to dissipate heat and the suspension was stirred for 60 s. The acid-washed aluminum samples were then filtered and rinsed with 500 mL deionized water. After acid washing, about 3.5 ± 0.4 g (n = 5) aluminum particles were recovered in each batch prepara-tion. Synthesized aluminum hydroxide was prepared by oxidizing acid-washed aluminum at 40 °C for 24 h. X-ray diffraction (XRD) spectra indicated it was in the form of bayerite.
2.3. Batch experiments
Batch experiments were conducted in 200 mL serum bottles (Wheaton, actual volume was approximately 220 mL) sealed with polytetrafluoroethylene-lined, butyl rubber septa and aluminum crimp caps. In a typical experiment, a desired amount of acid-washed aluminum (0.5–3.5 g) was placed in 100 mL perchlorate-contaminated water. The initial concentration of perchlorate was 10 mg L1. Because of a significant hydrogen evolution in the
reac-tion systems containing acid-washed aluminum, the sealed reactor was connected to a 70 mL plastic syringe using Teflon tubing to re-lease gas and monitor the production of hydrogen gas. The initial pH of the solution was about 5.2 and was adjusted with 1.0 N HCl to pH 4.5 ± 0.2 prior to perchlorate addition unless indicated
otherwise. This procedure produced an average concentration of soluble chloride at approximately 115 ± 13 mg L1 (n = 4), which
was considered as a background level of soluble chloride. The solu-tion pH was measured at the beginning and end of the experiment. The serum bottles were placed on a platform shaker in horizontal position (180 rpm) at room temperature (24 ± 1 °C). Solution pH and temperature were measured with a Sension 3 laboratory pH meter (HACH Co.) equipped with a combination pH electrode and a thermocouple probe, respectively. For pH effects on the perchlo-rate adsorption by synthesized aluminum hydroxide, 50 g L1
alu-minum hydroxide was placed in a 200-mL serum bottle containing 10 mg L1perchlorate in 100 mL aqueous solution. The solution pH
was adjusted by 1.0 N HCl or NaOH at the beginning of the reaction and monitored periodically throughout the experiment.
For repetitive cycles of adsorption–desorption of perchlorate by synthesized aluminum hydroxide, in each cycle, 10 mg L1
per-chlorate was added in a batch reactor containing 35 g L1
alumi-num hydroxide in a 100 mL aqueous solution. After perchlorate desorption, the solution was discarded and the aluminum hydrox-ide was carefully washed by 300 mL deionized water to remove residual perchlorate and was then resealed for the next cycle. 2.4. Analytic methods
At selected time intervals, 1 mL aqueous aliquot withdrawn by a 5 mL gastight syringe was diluted with 4 mL deionized water for ion chromatograph (IC) analysis. The total sampling volume did not exceed 10% of the total solution volume. The solution was fil-tered through a 0.2
lm cellulose membrane filter (Millipore, MA)
to remove particulates prior to IC analysis. Perchlorate and chloride were analyzed on a Metrohm 861 Advanced Compact IC equipped with a Metrosep A Supp 5–100/4.0 column. Eluent contained 9 mM Na2CO3/2.8 mM NaHCO3was used.Hydrogen gas was determined by HP 4890 GC–TCD equipped with a 60/80 Carboxen-1000 column (Supelco, 457 cm 0.32 cm). Dissolved aluminum concentration was measured by an induc-tively coupled plasma-optical emission spectrometry (PerkinElmer Optima 2000DV, PerkinElmer Inc.). The wavelength of aluminum ion was set at 396.153 nm.
XRD measurements were performed using a X-ray diffractome-ter (Siemens D5000) with a copper target tube radiation (Cu Ka1) producing X-rays with a wavelength of 1.54056 Å. Samples were placed on a quartz plate and were scanned from 20 to 80° (2h) at a rate of 2° min1. The specific surface area of solids was measured
by Brunauer–Emmett–Teller (BET) N2method using a COULTER SA
3100 surface area analyzer (Coulter Co.). Zeta potential of fresh acid-washed aluminum and synthesized aluminum hydroxide in aqueous solutions was measured by a zeta potential analyzer (ZetaPlus, Brookhaven Instruments Corporation). The solid loading was 0.2 g L1in 102M NaCl aqueous solution. Prior to analysis,
the solution pH was adjusted to desired values by adding 1.0 M HNO3or KOH.
3. Results and discussion
3.1. Characteristics of acid-washed aluminum and its reactions in the Al0–H
2O system
Fig. 1 depicts XRD spectra for fresh acid-washed aluminum (Fig. 1a) and aged aluminum particles taken after reacting with 10 mg L1perchlorate for 24 h. (Fig. 1b). The characteristic peaks
of aluminum appear at 38.6°, 44.9°, 65.2°, and 78.5° where the main diffraction peak is near at the diffraction angle (2h) of 38.6°. The diffraction angle of peaks shown inFig. 1a indicated that the fresh acid-washed aluminum is mainly consisted of zero-valent
aluminum. Compared to the XRD pattern of fresh acid-washed alu-minum, aged aluminum particles showed a significant change of their composition at the surface. The XRD pattern of aged alumi-num particles revealed the diffraction peaks at diffraction peaks appeared at 20.4°, 27.9°, 36.6°, and 40.7° indicating aluminum hydroxide (bayerite, Al(OH)3) was formed.
Surface area analyses showed that specific surface areas of the acid-washed aluminum were dramatically increased during the reaction in the aqueous solution. The fresh particles have the spe-cific surface area about 12 m2g1while it increased to 70 m2g1
after 16 h and 130 m2g1after 24 h. As indicated by XRD analysis,
zero-valent aluminum was converted to aluminum hydroxide after 24 h. Aluminum hydroxide has been known as porous media hav-ing specific surface areas greater than 100 m2g1(Sivaraj et al.,
1986; Phambu, 2003). The high specific surface area of particles after reactions can therefore be attributed to the aluminum hydroxide formation.
The zero point of charge (pHzpc) for fresh acid-washed
alumi-num and synthesized alumialumi-num hydroxide particles was deter-mined to be at pH 7.2 and 8.0, respectively. At pH below pHzpc,
the surface is positively charged because it adsorbs more protons; conversely, at pH above pHzpc, the surface is negatively charged
(Kasprzyk-Hordern, 2004). This indicated that the surface of both types of particles is positively charged under acidic conditions that may be beneficial to perchlorate removal through the electrostatic attraction since perchlorate is an anion.
In the Al0–H2O system, the characteristic reaction of aluminum
corrosion (Eq.(2)) results in hydrogen evolution, aluminum disso-lution, and increasing pH and temperature under acidic conditions (Abiola et al., 2004; Studart et al., 2005; Ishii et al., 2007). Al0þ 3H2O ! Al3þþ 1:5H2þ 3OH ð2Þ
As shown inFig. 2a, no hydrogen gas was produced within the first 6 h in the reaction system containing 10 mg L1perchlorate
and 17.5 g L1acid-washed aluminum. However, the rate of
hydro-gen evolution increased to about 40 mL h1within 8–13 h. During
the experimental period of 16–20 h, a vigorous evolution of hydro-gen was monitored at a significantly higher rate (233 mL h1). At
the end of the experiment (24 h), the rate declined to near zero. The hydrogen evolution process during the aluminum corrosion involving an incubation period, a rapid increasing stage and a pla-teau period under both acidic and alkaline conditions has been re-ported (Abiola et al., 2004; Ishii et al., 2007). Similar to the hydrogen evolution, no significant change of pH was observed within the first 4 h while the solution pH quickly increased to 7 at 8 h and then remained relatively constant at 8.1 ± 0.5 through-out the experiment.
The aluminum dissolution was confirmed by the measurement of the dissolved aluminum ion concentration. Dissolved aluminum ions were found in the aqueous solution during the perchlorate re-moval. The aluminum concentration peaked at about 30 mg L1at
8 h and then gradually declined to below detection limit at 16 h. The high dissolved aluminum concentration can be attributed to initial acidic conditions of the reaction system. Nevertheless, the dissolved aluminum was removed from the aqueous solution when the solution pH increased to about 8.1 during the hydrogen evolu-tion (Eq.(2)). Under such alkaline conditions, dissolved aluminum is readily precipitated as an insoluble form of polymeric aluminum hydroxide (Deltombe and Pourbaix, 1958). This is consistent with the XRD analysis indicating the formation of aluminum hydroxide. Aluminum corrosion is an exothermic reaction that releases heat and causes an increase in temperature. Although batch reac-tors were placed under ambient temperature (24 ± 1 °C), a signifi-cant increase of the solution temperature was observed (Fig. 2a). As depicted inFig. 2a, the solution temperature rose from 24 to 40 °C at 10 h and then gradually declined to 24 °C.
3.2. Perchlorate removal using acid-washed aluminum
Studies have shown that the perchlorate reduction in the pres-ence of various metals takes place in acidic media (Ujva´ri et al., 2002a,b; Li et al., 2004; La´ng et al., 2005).Fig. 2b shows the per-chlorate removal with acid-washed aluminum under ambient con-ditions at initial pH of 4.5. The metal dose used in the experiments ranged from 5 to 35 g L1 in 100 mL perchlorate solutions. A lag
phase was found during the perchlorate removal. The lag phase is defined as the time required removing at least 20% of the initial perchlorate concentration. For example, the lag time was 16 and 8 h at the metal dose of 5 and 17.5 g L1, respectively. The
perchlo-rate concentration decreased rapidly after the lag phase even at the low metal dose. The overall perchlorate removal efficiency reached to about 85–95% in all three different conditions of the metal dose. As discussed in detail later, the existence of the lag phase is in fact a consequence of the reaction time requirement for the formation of aluminum hydroxide from the aluminum oxidation.
Chloride has been widely documented as a final product in the perchlorate reduction by zero-valent iron under elevated tempera-ture conditions (Ujvári et al., 2002b; Moore et al., 2003; Gu et al., 2003; Cao et al., 2005). As discussed above, the aluminum corro-sion leads to increase temperature and produce hydrogen gas. The increase in temperature tends to enhance reaction rates and the vigorous hydrogen evolution suggests an increasing rate of electron transfer. Therefore, one could expect that perchlorate undergoes a chemical reduction to chloride (Eq.(1)) through the aluminum oxidation.
To investigate this possibility, experiments were conducted to analyze the chloride formation in this study without using HCl to adjust pH. In the batch reactor containing 10 mg L1perchlorate
and 17.5 g L1acid-washed aluminum in 100 mL aqueous solution,
the initial pH was about 5.2 and the background chloride concen-tration was about 3 mg L1because the acid-wash step during the
aluminum preparation introduced residual chloride into the reac-tion system. Surprisingly, instead of observing an increase in the chloride concentration, it was found that the total chloride concen-tration decreased along with perchlorate removal after the lag phase. The same behavior was also observed in all cases under acidic conditions. As illustrated inFig. 2c, simultaneous disappear-ance of both perchlorate and chloride occurred right after the lag phase in the presence of 17.5 g L1acid-washed aluminum at pH
4.5. With pH adjustment, the initial concentration of chloride was measured about 118 mg L1 while it rapidly reduced to
6 mg L1at 16 h. Thus, attempts to verify the possibility of
perchlo-20 30 40 50 60 70 80
2
θ
R
ela
ti
v
e I
n
te
n
sity
(
a.
u
.)
(a) Fresh Acid-wahsed Aluminum
(b) Aged Aluminum
rate reduction to chloride in zero-valent aluminum systems were unsuccessful.
The adsorption at the solid surface is another possible process to explain the perchlorate removal as well as the simultaneous dis-appearance of both perchlorate and chloride. It has been reported that chloride accelerates the pitting corrosion of aluminum by pe-netrating the aluminum oxide film and then is immobilized at the
surface through the incorporation with oxide to form oxide–chlo-ride complexes (Kolicset al., 1998; Branzoi et al., 2003; McCafferty, 2003). To verify whether the disappearance of chloride and per-chlorate is a consequence of the surface adsorption, desorption experiments were performed. Studies have indicated that the adsorption strength of sulfate is higher than that of perchlorate and chloride at the aluminum and aluminum oxide surface
Temperature ( oC) 0 10 20 30 40 50
pH
2 4 6 8 10 12 14 H2 (mL) 0 500 1000 1500 2000 2500 3000 3500 Temperature pH H2 Perchlorate concentration (mg L -1) 0 2 4 6 8 10 12 35 g L-1 17.5 g L-1 5 g L-1Time (h)
0 10 20 30 40 (C/C o ) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 ClO4 -Cl -0.5 mL, 1 N MgSO4a
b
c
Fig. 2. (a) Hydrogen evolution, pH and temperature change during the perchlorate removal with acid-washed aluminum; (b) perchlorate removal with acid-washed aluminum at various metal doses; (c) adsorption and desorption of chloride and perchlorate ions in the perchlorate removal with acid-washed aluminum. The initial chloride concentration was measured about 118 mg L1
(Hora´nyi and Joó, 2000; Kolics et al., 1998). Therefore, the desorp-tion experiment was conducted by adding sulfate ions into batch reactors. The addition of 0.5 mL MgSO4(1.0 N) corresponding to
240 mg L1SO2
4 was carried out when the significant
disappear-ance of both perchlorate and chloride was achieved at 24 h. It was found that more than 99% of sulfate ions quickly disappeared with a corresponding increase in both perchlorate and chloride concentrations in the aqueous solution after 12 h of the sulfate addition (Fig. 2c). The desorbed perchlorate accounted for 96% of the total amount of perchlorate. The observation of perchlorate desorption can be attributed to the high adsorption affinity of sul-fate capable of replacing the adsorbed perchlorate at the surface. Similarly, chloride was desorbed after sulfate was added. The recovery of desorbed chloride was about 98%.
Accordingly, this result demonstrates that the disappearance of perchlorate in the presence of acid-washed aluminum is mainly re-sulted from the surface adsorption process. It should be noted that the disappearance of sulfate is not a consequence of the sulfate reduction because sulfate was added while aluminum was oxidized.
3.3. Effective composition of aluminum for perchlorate removal Because the adsorption is the major process for perchlorate re-moval in the presence of acid-washed aluminum, experiments were conducted to investigate the reactive composition of solids for perchlorate adsorption. Fig. 3 illustrates repetitive cycles of adsorption–desorption for perchlorate removal using synthesized aluminum hydroxide at initial pH 4.5. Aluminum hydroxide, con-firmed by the XRD analysis, was prepared by oxidation of acid-washed aluminum at 40 °C for 24 h. For each cycle of perchlorate adsorption and desorption, 10 mg L1perchlorate was added in a
batch reactor containing 35 g L1synthesized aluminum hydroxide
in a 100 mL aqueous solution. As shown inFig. 3, rapid adsorption of perchlorate was found in the first cycle where about 90% of per-chlorate was removed within 15 min. Perper-chlorate was readily des-orbed after the addition of 0.5 mL MgSO4 (1 N). The desorbed
perchlorate accounted for 80% of total amount of perchlorate added. Compared to the acid-washed aluminum (Fig. 2c), synthe-sized aluminum hydroxide shows a similar pattern for perchlorate adsorption and desorption. As depicted inFig. 2c, acid-washed alu-minum in the form of zero-valence at the first few hours showed only minor effect for perchlorate removal. The effective removal of perchlorate did not take place until the lag phase where alumi-num hydroxide was formed. As a result, it is clear that alumialumi-num
hydroxide is a key composition for serving as an effective adsor-bent for perchlorate removal.
In this study, we conclude that perchlorate is primarily removed by aluminum hydroxide that is a product from the alumi-num corrosion in the Al0–H
2O system. The lag time of perchlorate
removal shown inFig. 2b actually reflects the length of reaction time for the oxidation of aluminum to aluminum hydroxide. It was found that increasing metal dose tends to increase the solution temperature that facilitates the aluminum oxidation. Once alumi-num hydroxide is formed, perchlorate adsorption occurs. Conse-quently, the lag time can be reduced as increasing metal doses.
Adsorption of anions onto the aluminum hydroxide surface can undergo an ion-pair formation with positively charged surface sites (Eq.(3)) or ligand exchange with surface hydroxyls (Eq.(4)) (Sposito, 1996). AlOHþ2þ A ! AlOHþ2A ð3Þ AlOH þ A! AlA þ OH ð4Þ where AlOHþ
2and AlOH represent the surface hydroxyl
(alumi-nol) groups and Arepresents an anion, which is ClO
4in this study.
The iron-pair formation, caused by the electrostatic attraction be-tween anions and positively charged hydroxide surface, is depen-dent on pHzpcwhile the ligand-exchange reactions usually lead to
increase pH.
Fig. 4shows the effect of pH on perchlorate adsorption by syn-thesized aluminum hydroxide. Perchlorate adsorption on the solids increased with decreasing pH. At pH 2.0, more than 98% of perchlo-rate was adsorbed whereas only 20–30% of perchloperchlo-rate was re-moved at pH 8–10. The aluminum hydroxide that has a high affinity for perchlorate at lower pH can be attributed to the ion-pair formation at the positively charged surface of the solids as measured by zeta potentials. The electrostatic attraction between perchlorate and the positively charged surface occurred at lower pH whereas the electrostatic repulsion between perchlorate and the negatively charged surface took place as increased pH.
Measurements of pH change before and after the perchlorate adsorption with synthesized aluminum hydroxide were con-ducted. Same measurements were also carried out for a control test in which the batch reactor contained only synthesized aluminum hydroxide without perchlorate. It was found that the solution pH increased from 4.5 to about 6.0 during the perchlorate adsorption with aluminum hydroxide; however, same behavior was also ob-served in the control test. This indicated that the increase in pH was not caused by ligand-exchange reactions, which replace
Time (h)
0 5 10 15 25 30 35Perchlorate (mg L
-1)
0 2 4 6 8 10 12 MgSO4 MgSO4 ClO4-, HCl ClO4 -, HCl ClO4 -MgSO4Fig. 3. Repetitive cycles of adsorption–desorption for perchlorate removal by synthesized aluminum hydroxide. The perchlorate concentration and metal loading was 10 mg L1and 35 g L1, respectively.
Time (h)
0.0 0.5 1.0 1.5 2.0 2.5 3.0Perchlorate (mg L
-1)
0 2 4 6 8 10 12 pH 2 pH 4 pH 6 pH 8 pH 10Fig. 4. Effects of pH on the perchlorate adsorption by synthesized aluminum hydroxide. The perchlorate concentration and metal loading was 10 mg L1and 50 g L1, respectively.
hydroxyls on the surface by perchlorate. Instead, the pH increase might be due to the amphoteric nature of hydrated alumina sur-face (Sposito, 1996). Accordingly, this study suggested that per-chlorate does not undergo ligand-exchange reactions at the surface of aluminum hydroxide.
4. Conclusions
In this study, we examined the use of acid-washed zero-valent aluminum for perchlorate removal. Perchlorate can be effectively removed after a lag phase. There is no evidence to support that the perchlorate removal is caused by the chemical reduction. A desorption experiment indicated that perchlorate undergoes pri-marily an adsorption process. The addition of 0.5 mL MgSO4
(1.0 N) led to a quick desorption of perchlorate. The desorbed per-chlorate accounted for 96% of the total amount of perper-chlorate. Alu-minum hydroxide, a product of the aluAlu-minum corrosion, has been confirmed as the effective composition for the perchlorate adsorp-tion. Synthesized aluminum hydroxide is capable of adsorbing per-chlorate rapidly at slightly acidic pH and desorbing perper-chlorate by sulfate addition. The adsorption mechanisms can be attributed to the ion-pair formation at the aluminum hydroxide surface. Acknowledgment
The authors would like to thank National Science Council (NSC), Taiwan ROC for the financial support through NSC Grants (NSC 95-2221-E-390-013-MY3).
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