Characterization of Arsenic-Containing Water Treatment Residuals

Chapter 23

Characterization of Arsenic-Containing Water Treatment Residuals

Downloaded by UNIV LAVAL on October 29, 2015 | http://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch023

Hun-Young Wee and Timothy A . Kramer Department of Civil Engineering, Texas A & M University, 3136 T A M U , College Station, T X 77843-3136

The characterization of water treatment residuals containing arsenic was examined and arsenic desorption and leachability from the residuals were focused. Solution pH is a critical variable in controlling arsenic stability in the residuals. The release of arsenic from the residuals was elevated at low and high pH due to the increase dissolution of the adsorbents such as Fe and Al hydroxides. The negatively charged surfaces of the adsorbents at high pH reduced arsenic anion adsorption. Competition with phosphate can play a significant role for the arsenic leaching from residuals because phosphate tends to compete with As(V) on the sorption sites. The cooperative effects of calcium can help to bind arsenic on metal hydroxides. Arsenic leaching from water treatment residuals was found to be underestimated by the toxicity characteristics leaching test (TCLP) due to the pH of the leachates being favorable for As(V) adsorption.

© 2005 American Chemical Society

In Advances in Arsenic Research; O'Day, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Arsenic is commonly recognized as a toxic and carcinogenic metal compound (1). The adverse health effects when humans are exposed to arsenic compounds are well documented (2). Arsenic contamination of groundwater and/or drinking water has been reported as a critical water quality issue in Vietnam, Bangladesh, and West Bengal, India (3, 4, 5). In particular, 70 million people in Bangladesh have been poisoned due to naturally occurring arsenic in wells used for drinking water sources (6), and highly elevated concentrations of arsenic (greater than 1 mg/L) have been commonly detected. The U.S. Environmental Protection Agency (USEPA) has lowered the maximum contaminant level ( M C L ) of arsenic from 50 to 10 μg/L, effective in 2006 (2). Numerous treatment technologies have been developed and applied to arsenic removal from various contaminated waters. Theses technologies may be grouped to four major categories: precipitative processes, membrane processes, adsorptive processes, and ion exchange (7, 8). Most arsenic compounds are physically separated from waters and condensed. In addition, appreciable volumes of arsenic-contaminated residuals are expected to be produced to match the revised M C L . The overall objective of this paper is to discuss the arsenic desorption characteristics depending on the pH and the presence of solutes such as phosphate, sulfate, and calcium. Arsenic leaching by the toxicity characteristic leaching procedure (TCLP) is also studied.

Arsenic in Water Treatment Residuals Limited work has been conducted on the characterization of arsenic containing residuals. Amy et al. (8) investigated arsenic leachability in various water treatment plant (WTP) residuals using the T C L P . The results showed that the arsenic concentration in the residuals generated from WTPs was vastly different and soluble arsenic concentrations in the leachates also varied depending on residuals. However, the concentrations of arsenic in the leachates were not over the limit of 100 times of M C L for arsenic (5 mg/L). Thus, the landfill disposal alternative was recommended for arsenic containing W T P residuals. Soluble arsenic concentrations in residuals generated from iron-based precipitation treatment were studied depending on the redox conditions (9). They divided three redox zones: adsorption, mobilization, and reductive fixation. In the mobilization zone, soluble arsenic concentration was great due to the reductive dissolution of iron oxides and oxidative dissolution of arsenic containing minerals such as arsenian pyrite (Fe(S,As) ) and arsenopyrite (FeAsS). To minimize arsenic mobility, the maintenance of high redox and 2


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neutral p H was suggested when arsenic leaching behaviors in municipal sewage sludge were investigated depending on the redox conditions and pH (10).

Arsenic Desorption Studies


Effects of pH on Arsenic Desorption Solution pH is a critical variable in controlling arsenic stability in solid matrices due to the ionization characteristics of As(III) and As(V) being significantly different. The p K values of inorganic arsenic species are As(III) (pK, = 9.2 and p K = 12.1) and As(V) (pK, = 2.2, p K = 6.96, and p K = 11.5) (11). A t normal natural pH environments (pH 4-9), H A s 0 * and H As0 ~ are the dominant species for As(V) and H A s O for As(III) (12). The six different residuals were collected from different water treatment plants. These residuals represent granular ferric oxy-hydroxide (Residual 1 and 3), granular activated alumina (Residual 2), ferric chloride addition (Residual 5), alum addition (Residual 6), and lime softening (Residual 4). The concentrations of major metals obtained from the residuals are shown in Table I. a

2

2

3

2

4

2

4

0

3

3

Table I. Concentrations of major metals in residuals. Residual 1 2 3 4 5 6

Total As (mg/kg) 2700 33 2300 15 150 400

Total Fe (mg/kg) 520000 21000 500000 24000 50000 20000

Total Al (mg/kg) 210 300000 990 18000 30000 46000

Total Ca (nig/kg) 1500 5600 5700 36000 18000 1900

Total Mn (mg/kz) 740 10 850 430 710 1400

Figure 1 shows the soluble As(III) and As(V) concentrations in the residuals as pH changes. Higher arsenic concentrations were observed at low and high pH. As(III) concentrations were highest at pH 4, and soluble As(V) concentrations were highest at pH 10. The maximum adsorption for As(V) and As(III) on iron and aluminum hydroxides usually occurs around pH 5 and pH 9, respectively (13, 14). Results of soluble Fe and A l concentrations were similar to those of



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Figure 1. Effect ofpH on arsenic leaching: (A) As(III), and (B) As(V). NOTE: Unit of soluble As is based on the dry solids.; Weight ratio of dry solids to extract solution (Dl water titrated to a desired pH) was 1:20.



325 arsenic (data not shown). Interestingly, higher leached Fe and As(V) concentrations in residuals samples 1 and 3, both a granular ferric hydroxide, were observed simultaneously at pH 10 due to the dissolution of the Fe hydroxides and, subsequent, As(V) release. The As(III) and As(V) desorption trends can be explained by the dissolution of Fe and A l hydroxides (i.e., increase of solubility) and the charge of the arsenic species. The solubility diagrams of the Fe hydroxides and A l hydroxides as a function of pH are shown in Figure 2 and Figure 3, respectively (15). Activity coefficients for the species were ignored, so activities and concentrations were assumed to be equal. By the amphoteric behavior, the solids, such as Fe and A l hydroxides, are dissolved to form cationic species at low pH and to form anionic species at high pH (15). During the dissolution of Fe and A l hydroxides, surface arsenic species can also be dissolved. At pH 4, the predominant As(III) species is H A s 0 ° , which has a neutral charge. The surfaces of Fe and A l hydroxides are positively charged at low pH. Therefore, the dissolution of metal hydroxides and a positively charged surface of the metal hydroxides and a neutral charge of dominant As(III) species facilitate the release of bound arsenic, and readsorption of leached As(III) cannot readily occur. At pH 4, the prevalent species for As(V) is H A s 0 " , which can be easily adsorbed onto the metal hydroxides. Thus, soluble As(V) concentration in the leachate was lower than that of As(Iil) at low pH. The surfaces of Fe and A l hydroxides are negatively charged at high pH. Therefore, the increasingly negative surface potential with increasing pH makes for unfavorable conditions for the adsorption of anionic arsenic species such as H A s 0 " for As(V) and H A s 0 " for As(III) at pH 10. In brief, the increasing negative surface charge of the metal hydroxides with increasing pH promote the desorption of As(V). The neutrally and negatively charged species of As(III) are predominant at pH 10 and thus, As(III) should be released due to repulsion from the negatively charged surface of metal hydroxides. It is hypothesized that, at pH 10, the reason why leached As(III) concentrations were less than As(V) might be the lower concentrations of As(III) contained in the residuals and the presence of neutrally charged species of As(III). At neutral pH (pH 6 and 8), the released arsenic, soluble Fe, and A l concentrations were very low due to the insolubility of the Fe and A l hydroxides and the predominant arsenic species. When the disposal of arsenic tainted residuals is planned without any post-treatment, the neutral pH condition of the system should be maintained to minimize arsenic solubility and mobilization. 3

2

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3


3


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327 Effects of Phosphate on As Desorption The mobilization of bound arsenic on adsorbents is strongly influenced by the presence of iigands that can compete with arsenic for surface sorption sites. In particular, phosphate tends to compete with As(V) for sorption sites on the surface of the metal hydroxides (equation 1) and thus, tends to extract the As(V) compounds (16, 17, 18). The adsorption behavior of phosphate and As(III) are vastly different and a significantly higher concentration of bound As(III) compared to As(V) was observed on iron minerals in the presence of 0.1 M sodium phosphate (19, 20). 2

2

Fe-oxide-As0 H + H P 0 " -> Fe-oxide-P0 H + H A s 0 " Downloaded by UNIV LAVAL on October 29, 2015 | http://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch023

4

4

4

4

(1)

As(III) and As(V) concentrations released by the competition with phosphate are shown in Figure 4. The results indicated that As(V) was the dominant arsenic species extracted by phosphate. The granular ferric hydroxide residuals, 1 and 3, contained high concentrations of total arsenic (data not shown) and thus, the concentration of As(V) released in the two residuals was much higher than that in the other residuals samples.

400 -,

Residuals Figure 4. Competitive desorption of arsenic with phosphate. Note: Unit of soluble As is based on the dry solids.; Weight ratio of dry solids to extract solution (0.1 M phosphate, pH 7) was 1:20.


328 Effects of Calcium on As Desorption Calcium can play an important role to stabilize the bound arsenic compounds on metal hydroxides at high pH (21). Figure 5 shows that arsenic leaching from the residuals which were prepared with the addition of calcium hydroxide as a neutralizing agent is less than release from the residuals with the addition of sodium hydroxide. The negatively charged surface of iron hydroxides changes to positive by sorption of calcium (equation 3 and 4) so that favorable conditions for negatively charged arsenic sorption exists (22).


w

=Fe OH° + C a =Fe OH° + C a s

2+

2+

w

+

= =Fe OCa + H = =Fe OHCa s

+

(3) (4)

2+

The cooperative effect of calcium was found in the phosphate adsorption on goethite in seawater, i.e. the enhancement of phosphate adsorption was observed in the presence of calcium at high pH (23). The addition of calcium shows that a less teachable arsenic containing residual can be generated by desorption studies in the presence of phosphate (Figure 6). The formation of solid phase calciumphosphate and calcium-arsenate compounds may also be responsible for the reduced arsenic desorption in the presence of calcium.

10

12

pH Figure 5. Desorption ofAs(V) at different pHs in the two residuals. NOTE: Unit of soluble As is based on the dry solids.; Weight ratio of dry solids to extract solution (DI water titrated to a desired pH) was 1:20.


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250 50

200 -

ε 150

lubie

Characterization of Arsenic-Containing Water Treatment Residuals

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