Equilibrium and kinetics studies of arsenate adsorption by soils

GUO Min, WU Wen-zhu, SHI Li-li,CAI Dao-ji

(Nanjing Institute of Environmental Sciences, Ministry of Environmental Protection, Nanjing 210042, China)

Abstracts: The present work is focusing on removal of arsenate from aqueous solution using soils. The adsorption of arsenic(Ⅴ) in six kinds of red soil were investigated under laboratory conditions and the main factors was explored. The results indicated that soils from Yunnan had a good adsorption capacity to arsenic(Ⅴ), which could be better used in the treatment of arsenic(Ⅴ)-contaminated water.The removal of arsenic(Ⅴ)adsorbed by soil particles were found to increase with the increase of initial concentration of arsenic(Ⅴ) solution, oscillation rate and soil quantity, but the margin of increase decreased gradually.The results of L9(34) orthogonal test showed that soil type was predominant than the other factors on the removal of arsenate, and the order of impact degree was soil type, soil quantity, oscillation time and oscillation rate.The mechanism of the adsorption by soils was investigated by FT-IR. FT-IR analysis indicated that the major As-O stretching vibration was located at 793.68cm-1, this means that some poorly crystalline metallic arsenate formed on the surface of red soils after adsorption.The vibration band at 1625~1641 cm-1 is supposed to be the hydroxyl OH stretching band of metallic hydroxide, it means that arsenic(Ⅴ) was adsorbed as a bidentate surface complex in both protonated -FeO2As(O)(OH)- and FeO2As(O)2- forms in the surface of red soils.

Key words: soils; arsenic(Ⅴ); adsorption; L9（34）; mechanism.

Arsenic is a trace element widely present in the crust, which has been listed as a Class A carcinogen by the U.S. Environmental Protection Administration due to its high toxic effects and it has caused great attention [1]. World Health Organization (WHO) has reduced the maximum allowable concentration of Arsenicin water from 0.05mg/L to 0.01mg/L [2];Japan, the United States and the European Union have taken the lead of using this standard;China has also set the forth limit of Arsenic concentration 0.01mg/L in drinking water in the “Drinking Water Sanitary Standard”(GB5749-2006)[3、4].However, China faces groundwater quality problems of enormous proportions from both industrial and natural source. The water contamination caused by arsenic is very severe currently and at least 50 million people are drinking the underground water with Arsenic concentration exceed 0.05mg/L in Asia [5].The impact of arsenic contamination on human health has drawn increasing attention.

YunnanProvince in China has an abundant of mineral resources.With the increasing degree of mineral development in recent years, the issue of heavy metal pollution has been increasingly serious.In 2008, the average value of arsenic concentration in Yangzonghai Lake of Yunnan reached 0.121mg/L. The water quality of YangzonghaiLakewas identified as inferior grade V, which affecting the production and living of the local people seriously. Later, DatunhaiLake in Honghe Prefecture of Yunnan was found to have several times’ pollution of YangzonghaiLake, with the water arsenic content exceeding 96 times of national standards.There are lots of data showing that more than 50% of lakes in HonghePrefecture contaminated by arsenic seriously, contamination control of arsenic admits of no delay consequently.

Adsorption plays an important role in the migration, transformation, bioavailability and destination of Arsenic in environment. Many domestic and foreign scholars have made the study of Arsenic adsorbed in the environment[6、7、8、9]. The results showed that a number of metal oxides (such as ferric hydroxide, aluminum hydroxide, Fe-Al hydroxide, manganese dioxide, etc) can adsorb Arsenicperfectly, and soil had a certain adsorption of arsenic,high soil clay content could also increase arsenic adsorption in water. The red soils in Yunnan havehigh clay content and rich in Fe and Al oxides.Therefore, this paper selected the red soil in Yunnan as adsorbent to study arsenic adsorption technology in dealing with arsenic contaminated water thus exploring fast, efficient, natural and harmless arsenic removal technology, which has important application value and practical significance.

1 Materials and Methods

1.1 Soils for test

6 representative red soils, including 4 kinds of Yunnan red soil, 1 Jiangxi red soil and 1 Guangdong latosol are selected for test, with the basic physical and chemical properties shown in Table 1.

Table 1 Basic physical and chemical properties of six tested soils

Soil / pH / Organic matter（g/kg） / Physical sand（＞0.01mm）% / Physical clay（＜0.01mm）% / Fe2O3/ % / Al2O3/ %
Yunnan red soil 1 / 6.59 / 9.3 / 70.7 / 29.3 / 14.04 / 28.86
Yunnan red soil 2 / 6.47 / 9.2 / 57.3 / 42.7 / 10.05 / 24.48
Yunnan red soil 3 / 5.93 / 10.2 / 60.8 / 39.2 / 12.59 / 26.70
Yunnan red soil 4 / 5.88 / 11.9 / 73.3 / 26.7 / 12.86 / 26.51
Jiangxi red soil / 6.24 / 17.0 / 87.9 / 12.1 / 8.91 / 18.28
Guangdong latosol / 6.55 / 15.1 / 59.2 / 40.8 / 8.29 / 18.56

Note: Yunnan red soil 1 is from Heiqiaomu region of Yunnan, Yunnan red soil 2 is from YangzonghaiLake region of Yunnan,Yunnan red soil 3 is from Gaoliao region of Yunnan, and Yunnan red soil 4 is from Majinpu region of Yunnan.

1.2 Reagents

The reagent purity used in this method is analytical pure or guarantee reagent, and the water for determination is deionized water.

(1) Na2HAsO4·7H2O solid standard: purity 98%, provided by U.S. Alfa Aesar.

(2) 1% potassium borohydride (KBH4) solution: weigh 0.2g of potassium hydroxide to dissolve in 100mL deionized water, and then add 1g of potassium borohydrite solution to mix up.

(3) 2%’s hydrochloric acid solution: take 10mL of concentrated hydrochloric acid to dilute with deionized water to 500mL.

1.3 Instrument

(1) PF6 non-dispersive atomic fluorescence spectrometer, Purkinje General (Beijing).

(2)VECTOR-22 infrared spectrometer, Bruker Optics Inc. (Germany).

(3)E24A-thermostat digital oscillator, NBS (USA).

1.4 Measurement methods

(1) Test methods (including the measurement of arsenic content in water)

Adsorption test: weigh a certain amount of red soil in 200mL glass tank and add 100mL of arsenic solution to shake, and thancentrifuge in the constant temperature digital oscillator to measure the arsenic concentration in the solution left.

Water sample processing: take 1mL water sample into a 10mL colorimetric tube, add 1mL concentrated hydrochloric acid, 0.1g thiourea and ascorbic acid respectively into the tube, and than dissolve all materials with deionized water, finally add deionized water to 10mL to mix up, and measure after 30min.

(2) Measurement of atomic fluorescence spectrometer

Measurement conditions: PMT negative high voltage set 265V, the atomization temperature set 180℃, lamp current set 40mA, carrier gas flow set 300mL/min, shielding gas flow set 600 mL/min.

Standard curve: take 2mL standard stock solutionwith Arsenic concentration 100μg/L into a 10mL colorimetric tube, add 1mL concentrated hydrochloric acid, 0.1g thiourea and ascorbic acid respectively, and than add deionized water to dissolve all meterialswith the finally volume 10mL, measure after 30min by atomic fluorescence spectrometer; the injected sample volumes are 0, 0.1, 0.2, 0.5, 1.0 and 2.0mL, and the standard series concentrations are 0, 1.0, 2.0, 5.0, 10.0 and 20.0μg/L.

(3) Characterization analysis of soil structure changes

Use infrared spectrum to determine the changes of sample spectrum values to characterize the structural changes.

IR analysis conditions: mill and mix the soil/KBr in the proportion of 1/200; take 150mg to press into thin slices under the pressure of 10t cm-2and maintain 1min; use PT-IR spectrometer to measure the sample spectra values within the range of 4000-400cm-1.

2 Results and Discussion

2.1 Impact of different soil types on arsenic adsorption effect

Weigh 0.5g of Yunnan red soil 1, 2, 3, 4, Jiangxi red soil and Guangdong latosol respectively which passing 200 mesh sieve, addthemrespectivelyinto 100mL arsenic water samples with As (V) initial concentration of 10mg/L. And than shake the samples under oscillation rate of 225rmp/min, centrifuge the samples to measure the arsenic concentration left in the solution to calculate the arsenic adsorption rates by different soils. The test results are shown in Figure 1.

Fig.1 The curve of different soils absorbing As(Ⅴ)

It can be seen from Figure 1 that the adsorptions of arsenic in water samples by 6 red soils all achieved balance basically after 10 hours’ oscillation.Three kinds of red soil, namely, Yunnan red soil 1, 3, and 4 have the best effect on As (V) adsorption, with the adsorption quantity of 989.2 μg/g, 895.1μg/g and 879.3μg/g and adsorption rate of 49.5%, 44.8% and 44.0%respectively;whereas Yunnan red soil 2, Jiangxi red soil and Guangdong latosol have weaker effect on adsorb arsenic, with the adsorption rate of 37.2%, 33.6% and 33.5%.

2.2 Impact of different soil quantities on arsenic adsorption effect

Weigh 0.01, 0.02, 0.03, 0.05, 0.1, 0.2, 0.3 and 0.5g of Yunnan red soil 1 and 4 respectively which passing 200 mesh sieve to add into 100mL arsenic water samples with As (Ⅴ) initial concentration of 0.2mg/L, and then shake the samples under oscillation rate of 225rmp/min, centrifuge the samples to measure the arsenic concentration left in the solution to calculate the arsenic removal rates by different soil quantities. The test results are shown in Figure 2.

Fig.2 Effect of two different soil quantities on As(Ⅴ) removal

It can be seen from Figure 2 that with the increase in soil quantity, the arsenic removal rates of Yunnan red soil 1 and red soil 4 increase.When the soil quantities are 0.01, 0.02, 0.03 and 0.05g, the arsenic removal rate of Yunnan red soil 1 is 51.1%, 69.2%, 82.2% and 94.8%, and that of Yunnan red soil 4 is 45.4%, 65.5%, 81.0% and 93.4% respectively. If the soil quantity continues to increase, the increase of arsenic removal effect of Yunnan red soil1 and 4 is no longer evident, indicating that when the soil quantity is 0.05g, the arsenic in the solution can basically be adsorbed.

2.3 Impact of different initial concentrations on arsenic adsorption effect

Weigh 0.03g of Yunnan red soil 1 and 4 respectively which passing 200 mesh sieve to add into 100mL arsenic water samples with As (Ⅴ) initial concentration of 0.05, 0.1, 0.2, 0.5, 1 and 2mg/L, and then shake at 225rmp/min for 0.5h. The adsorption results of arsenic with different initial concentrations are shown in Table 2.

Table 2. Impact of the different initial concentrations on As(Ⅴ) adsorption

Initial concentration(mg/L) / Yunnan red soil 1 / Yunnan red soil 4
Removal rate (%) / RSD
(%) / Adsorption amount (μg·g-1) / Removal rate
(%) / RSD
(%) / Adsorption amount (μg·g-1)
0.05 / 95.1 / 4.0 / 158.5 / 95.5 / 5.7 / 159.1
0.1 / 92.2 / 5.6 / 307.3 / 93.2 / 8.1 / 310.7
0.2 / 79.5 / 5.4 / 529.8 / 82.5 / 5.2 / 550.3
0.5 / 58.8 / 3.6 / 979.2 / 59.3 / 0.7 / 987.7
1.0 / 41.6 / 0.9 / 1387.3 / 42.8 / 1.8 / 1427.7
2.0 / 20.8 / 3.7 / 1389.2 / 24.6 / 0.1 / 1637.1

It can be seen from Table 2 that, with the increase of As (V) initial concentration, the As (V) adsorption of Yunnan red soil 1 and Yunnan red soil 2 gradually increase at a decreasing rate, that is, the As (V) adsorption rate reduces. This is because the soil particles are specific ions, for example, the As (V) adsorption site is certain, with the increase of arsenic amount, the adsorption sites on soil particle surface gradually decreases, resulting in the decrease of arsenic adsorption amount. At the same time, at any As (V) initial concentration, the arsenic adsorption amount of Yunnan red soil 4 is slightly larger than that of Yunnan red soil 1.

2.4 Impact of different oscillation rates on arsenic adsorption effect

Weigh 0.03g of Yunnan red soil 1 and 4 respectively which passing 200 mesh sieve to add into 100mL arsenic samples with As (Ⅴ) initial concentration of 0. 2mg/L, and then shake at the rates of 50, 100, 150, 225 and 300rmp/min for 0.5h. See the arsenic adsorption results at different oscillation rates in Figure 3.

Fig.3 The curve of As(Ⅴ) removal effects at different oscillation rates

It can be seen from Figure 3 that with the increase of oscillation rate, the arsenic removal rate of soil particles is on the increasing trend, but at a decreasing rate. When the oscillation rate reaches 225rmp/min, the effect of arsenic removal is almost stable, indicating that the oscillation rate has a large impact on arsenic adsorption within a certain range.

2.5 Optimization of soil conditions on arsenic adsorption

In order to determine the impact of various factors (soil type, soil quantity, oscillation time and oscillation rate) on the effect of arsenic adsorption, design a L9(34) orthogonal experiment (see Table 3) for the determination of optimum adsorption conditions.

Table 3 The L9(34) orthogonal test design and analysis

No. / A. Soil type / B. Soil quantity (g) / C. Oscillation time (min) / D. Oscillation rate (rmp/min) / Removal rate (%)
1 / 1Yunnan red soil 1） / 1（0.025） / 1（15） / 1（200） / 64.1
2 / 1 / 2（0.030） / 2（30） / 2（225） / 76.5
3 / 1 / 3（0.035） / 3（45） / 3（250） / 84.8
4 / 2Yunnan red soil 2） / 1 / 2 / 3 / 47.0
5 / 2 / 2 / 3 / 1 / 55.8
6 / 2 / 3 / 1 / 2 / 54.2
7 / 3Yunnan red soil 4） / 1 / 3 / 2 / 70.7
8 / 3 / 2 / 1 / 3 / 69.6
9 / 3 / 3 / 2 / 1 / 78.8
Ⅰ（Sum of thehorizontal test results） / 225.4 / 181.8 / 187.9 / 198.7
Ⅱ（Sum of the horizontal test results） / 157.0 / 201.9 / 202.3 / 201.4
Ⅲ（Sum of the horizontal test results） / 219.1 / 217.8 / 211.3 / 201.4
Ⅰ/3 / 75.1 / 60.6 / 62.6 / 66.2
Ⅱ/3 / 52.3 / 67.3 / 67.4 / 67.1
Ⅲ/3 / 73.0 / 72.6 / 70.4 / 67.1
Range R / 22.8 / 12.0 / 7.8 / 0.9

Through comparing the factor ranges in Table 3, it can be seen that the difference of soil type impact on average arsenic removal rate reaches 22.8%, while the difference of oscillation rate impact on the average arsenic removal rate is 0.9%, indicating that the impact of soil type on arsenic removal rate is much more important than that of oscillation rate. The soil quantity and oscillation time have similar impact on arsenic removal, with the range value of 12.0% and 7.8% respectively.

It can be seen from the orthogonal test results that in arsenic adsorption study, priority should be given to the selection of soil type, followed by the consideration of soil quantity and oscillation time, and finally the oscillation rate. The test results show that the red soil in Yunnan can be better used in the treatment of arsenic contaminated water.

2.6 The mechanism of arsenic adsorbed on Soil

Many studies have shown that the iron and aluminum in the soil played an important role in arsenic adsorption. Arsenate can form insoluble arsenide with iron and aluminum cations and produce co-precipitation with amorphous iron and aluminum hydroxide. The more amorphous Fe and Al oxides contained in the soil, the stronger the arsenic adsorption capacity is [10、11]. Arsenic can be fixed on the surface of metal oxide through specific adsorption and non-specific adsorption. Specific adsorption refers to the entrance of anions into the metal atom coordination shell on the oxide surface to re-coordinate with the ligand hydroxyl or hydrated to combine on the solid surface directly through covalent bond or coordinate bond [12]. Spectroscopic studies have confirmed that arsenic (Ⅲ) and arsenic (V) can conduct ligand exchange through OH+ or OH2+ on the iron surface to generate the inner ligand [13]. By Raman spectroscopy and Fourier transform infrared technology, and combined with electrophoretic mobility testing as well as surface complexation model, etc, Goldberg et al. held that arsenic (V) formed inner ligand on amorphous aluminum oxide, whereas arsenic (Ⅲ) was adsorbed in the form of outer ligand.

In this study, infrared spectrum has been used in the research of AsO43-adsorption by Yunnan red soil 4 (see Figure 4). It can be seen in Figure 4 that the band intensities of infrared spectra before and after AsO43-adsorption by Yunnan red soil 4 are significantly different, indicating that red soil samples before and after adsorption have differences in composition. In the research of iron hydroxide arsenic adsorption and precipitation mechanism, Liu Huili [15], et al found that the infrared spectra of hydroxide solid sample before and after arsenic adsorption had –OH bending vibration absorption bands at 1625~1641cm-1, indicating that the arsenate ion might exist on the red soil surface in the form of protonated bidentate surface complexation of -FeO2As(O) (OH) - and FeO2As (O) 2-. In this study, the infrared spectra also have 1641~1686 characteristic absorption band, indicating that re-coordination has been conducted between the arsenate with the hydroxyl or hydrated base in the metal oxides, and combined on the surface of soil particles via coordination bonds. At the same time, Liu Huili et al also found in the study that when the initial arsenic concentration was 500mmol/L, the ferric hydroxide solid after adsorption showed the characteristics bands in the wavenumber 821.54cm-1; when the arsenic initial concentration was 50mmol/L, the characteristics bands of ferric hydroxide solid after adsorption appeared in the wavenumber 806.11cm-1. Roddick et al (2002) also reported that when the pH was from 2 to 6, the As-O stretching vibration band was about 825cm-1. Jia et al [16]researched and reported that when pH=3, the arsenic formed the precipitation of ferric arsenate on the surface of ferric hydroxide; the infrared spectrum analysis showed that the As-O stretching vibration band was about 825cm-1. It can be seen from the previous study results that, the As-O stretching vibration band was about 825cm-1, and decreased with the decreasing arsenic concentration. In this study, the arsenic initial concentration is low (0.1mmol/L) with complex soil particle composition, and the metal oxide content is much lower than the pure product, so there is less content of formed ferric arsenate. The As-O stretching vibration band reduces to 793.68cm-1in Figure 4. The results show that AsO43-has formed a small amount of metal arsenide on the red soil surface, consistent with previous findings.

Fig. 4 IR spectra of red soil before and after As (V) adsorption

3 Conclusions

(1) There are great differences in the arsenic adsorption effect of different soil types, and the red soil in Yunnan has good arsenic adsorption effect and can be better applied in the treatment of arsenic contaminated water.

(2) Soil quantity, the initial concentration and oscillation rate of arsenic solution have great impact on arsenic adsorption effect. Within a certain range, the larger the soil quantity is and the faster the oscillation rate is, the greater the capacity of arsenic adsorption is.

(3) Soil type, soil particle quantity, oscillation rate and arsenic solution initial concentration have different impacts on arsenic removal effect, of which the soil type has the greatest impact, followed by the soil quantity, oscillation time and oscillation rate.

(4) The IR spectra analysis showed that the stretching vibration band of As-O key in the red soil IR spectra after arsenic adsorption was located in 793.68cm-1, showing there was a small amount of metal arsenide crystal precipitate on the red soil surface after adsorption. The red soul solids after adsorption had water-OH bending vibration absorption band in 1625~1641cm-1, indicating that the arsenate ion might exist on the red soil surface in the form of protonated bidentate surface complexation of -FeO2As(O)(OH)- and FeO2As(O)2-.

(5) The test results showed that the red soil in Yunnan could be better applied in the treatment of water contaminated by arsenic, but the release of adsorbed arsenic still required further study.

References:

[1] United States Environment Protection Agency (USEPA). National primary drinking water regulations: Final rule. WashingtonDC: Federal Register. 30 Jan, 2001, U.S. Gov. Print Office, 1991, 56 (20): 3526.