Speciation of Aluminum in Acidified Waters

By Cindy M. Lee, Environmental Engineering and Science, Clemson University, 2002.

One of the symptoms of aquatic systems affected by acid rain is high concentrations of dissolved aluminum. The acid precipitation increases the dissolution of minerals containing aluminum and releases aluminum from ion exchange sites. The dissolved aluminum is of concern because it is toxic to fish and can harm the root hairs of terrestrial vegetation. The conventional wisdom is that only the free ion contributes to the toxicity, although some studies show evidence that questions this assumption (Parent et al., 1996). This problem looks at how complexation affects the speciation of aluminum in waters affected by acid rain and how the speciation could influence toxicity.

Acid rain will leach Al3+ from soils due to the dissolution of aluminous minerals such as kaolinite (Al2Si2O5(OH)4)and gibbsite (Al(OH)3). The dissolved aluminum then acts as a buffer keeping the pH of the soil solution acidic due to reactions such as

Al(OH)3 + 3H+ = Al3+ + 3H2O

The acidity of the system can be characterized in the following manner

Acidity (base neutralizing capacity) = [H+] - [HCO3-] + 3[Al3+] + 2[Al(OH)2+] + [Al(OH)2+]

The amount of fluoride in natural waters is often controlled by the solubility of the mineral fluorite (CaF2). Fluorosis is a disease caused long-term ingestion of high concentrations of fluoride. Symptoms can appear at concentrations of as low as 1 to 2 mg/L (52 to 105 micromolal) (Wang, 1999). Fluoride concentrations in groundwater from Thomas County, Kansas, for example, range from 0.05 to 2 mg/L (KGS, 2001). In Ethiopia, fluoride concentrations in groundwater range from 0.1 to 23.3 mg/L (Haile, 1999). To learn more about high levels of fluoride in drinking water see the report by Hurtado et al. (2000).

Tabulated in Table1 is the hypothetical analysis of dissolved species (in micromolal) in porewater from the C horizon of a soil that has been affected by acid rain. The pH is 5.65 and the temperature is 10° C.

Enter all the data except for total Al (aq) in Visual MINTEQ. Make kaolinite an infinite solid. Include a table in Excel to support your answers to the questions below.

1.What percentages of total Al (aq) are present as Al3+ and Al-OH complexes, and as fluoride complexes? If you change the concentration of fluoride to 1 micromolal what is the distribution among the differ species of Al?

2. How does the distribution change if you add 15 mg/L of DOM to the soil water with 5 micromolal of fluoride?

3. What is the saturation state of the water with respect to quartz, fluorite, Al(OH)3(am), and gibbsite for the three scenarios (5 um F-, 1 um F-, and 5 um F-plus DOM)?

4. What suggestions would you have for decreasing the toxicity of waters with high Al concentrations based on your modeling results? Test your ideas with MINTEQ.

References

Haile, G. 1999. Hydrogeochemistry of waters in Lake Ziway area. 25th WEDC Conference. Addis Ababa, Ethiopia http://www.lboro.ac.uk/wedc/papers/25/286.pdf

Hurtado, R.; Gardea-Torresdey, J.; and Tiemann, K. J. 2000. Fluoride occurrence in tap water at "Los Altos de Jalisco" the central Mexico region. Proceedings of the 2000 Conference on Hazardous Waste Research. http://www.engg.ksu.edu/HSRC/00Proceed/gardea_torredey1.pdf

Kansas Geological Survey. 2001. Geohydrology of Thomas County (Kansas) http://www.kgs.ukans.edu/General/Geology/Thomas/05_gw7.html

Langmuir, D. 1997. Aqueous Environmental Geochemistry. Upper Saddle River, NJ:Prentice-Hall. ISBN: 0-02-367412-1.

Parent, L.; Twiss, M. R., and Campbell, P. G. C. 1996. Influences of natural dissolved organic matter on the interaction of aluminum with the microalgae Chlorella: A test of the free-ion model of trace metal toxicity. Environ. Sci. Technol. 30:1713-1720.

Wang, X. 1999. Fluoride removal from water using geomaterials http://www.science.uwaterloo.ca/earth/waton/s9912.html

Answers:

Your answers may differ somewhat from the ones provided, but the essence should be similar.

1. According to the analysis by MINTEQ with fluoride of 5 micromolal, about 1% of the aluminum is present as Al3+; about 2% is present as Al-OH complexes, and about 97% is present as fluoride complexes (see Table 2 ). When the fluoride is reduced to 1 micromolal, the percentages change to about 7% for the free alumunum, about 16% for hydroxy complexes, and about 75% for the fluoride species. But note that the concentration of free aluminum has not essentially changed, just the relative proportion of the free ion compared to the other complexes. The reason that the concentration of dissolved aluminum is not changing is because it is maintained at equilibrium with the infinite solid specified (kaolinite). What happens if you change the specified infinite solid to halloysite?

2. With the addition of 15 mgC/L DOM using the Gaussian DOM model available in MINTEQ, the distribution is about 1% free aluminum, about 2% Al-OH, and about 3 % fluoride species. The Al complexed with DOM accounts for about 11% of the total aluminum (see Figure 1). The change is the percent complexed by fluoride decreased dramatically. Again note that the concentration of free aluminum has changed very little.

3. Quartz is supersaturated while Al(OH)3(am), gibbsite, and fluorite are undersaturated. The saturation indices do not change for the three scenarios. The oxyhydroxides of aluminum such as amorphous Al(OH)3 and gibbsite tend to dissolve at low pH values to give cationic aluminum species and at high pH values to give anionic Al(OH)4- (Langmuir, 1997). You can test this observation by increasing the pH and re-running MINTEQ.

4. For waters that are acidified but that are not used as drinking waters, high concentrations of fluoride could be added. MINTEQ shows that at 100 um fluoride, there is no free aluminum present, at 50 um there is only 0.025%. Or the DOM concentrations could be increased. MINTEQ shows that about 0.7% of the aluminum is present as the free ion at 50 mgC/L and at 100 mgC/L. about 0.5%. See the problem titled, "The Influence of Aluminum Speciation on Fish," for additional information.

Al-DOM 11.04

Comments to Instructors

This problem was adapted from an example problem presented in Chapter 7 of Aqueous Environmental Geochemistry by Donald Langmuir (see references). It was used in a simplified form (questions 1 and 3) in a senior level chemistry class called "Chemistry of Aqueous Systems" that used Geochemistry of Natural Waters by James I. Drever (ISBN 0-13-272790) as the text. The students had worked several problems using Visual MINTEQ in previous homework assignments. At the point this problem was assigned, complexation and chemistry of acidified waters had been covered in lecture. The majority of students were able to answer questions 1 and 3 successfully. Contact the author, Cindy Lee (LC@clemson.edu), for further information.