Permeable reactive barriers
Introduction
Acid rock drainage (DAR) (in English Acid Rock Drainage, ARD) is a natural and complex phenomenon associated with the existence of rocks that contain sulfide minerals. This process generates acidic solutions with heavy metals, as a result of weathering and biochemical reactions that occur in the presence of water and oxygen. Unlike acid mine drainage (AMD), which also produces acidic solutions due to the presence of sulfide minerals, AMD originates exclusively in contexts linked to mining activity.
Acid rock drainage generates sulfuric acid and dissolves heavy metals, reducing the pH of the water to values between 1.5 and 5. The presence of ferric or ferrous ions produces reddish-brown or greenish-blue colors, respectively. When acidic and non-acidic waters mix, surface deposits of reddish iron oxides are formed. This process deteriorates the quality of water, soil and ecosystems, with impacts that can extend downstream. [1].
Factors for the generation of acid rock drainage
Direct factors
The formation of ARD occurs in the presence of a sulfide mineral and oxidants:[2].
Indirect factors
Temperature, water availability and pH are key factors in the generation of acid rock drainage (ARD). An increase of approximately 10 °C doubles the rate of chemical reactions, accelerating the formation of DAR. Rainfall and water sources influence soil saturation and the transport of oxidation products, especially in materials with high hydraulic conductivity. In acidic conditions (pH less than 5), heavy metals remain dissolved and favor the activity of acidophilic bacteria such as Acidithiobacillus ferrooxidans. However, as the pH increases, the metals precipitate as ferric hydroxides, forming orange deposits visible in the channels.
Stages of DAR generation
The DAR formation process includes 3 stages[4]:.
Treatment technologies
There are various technologies to treat heavy metal contamination, grouped into three types according to their level of intervention: passive, active and in situ. Two passive treatment methods are described below.
References
- [1] ↑ a b Montano, Yeidy; Tapia, Pedro M.; Quispe, Cristian; Fuentealba, Beatriz (2022-12). El Drenaje Ácido de Roca y sus potenciales impactos ambientales. Consultado el 28 de octubre de 2025.: https://hdl.handle.net/20.500.12748/441
- [2] ↑ a b Huang, Zhu; Pan, Xiao-Dong; Wu, Ping-Gu; Han, Jian-Long; Chen, Qing (1 de febrero de 2014). «Heavy metals in vegetables and the health risk to population in Zhejiang, China». Food Control 36 (1): 248-252. ISSN 0956-7135. doi:10.1016/j.foodcont.2013.08.036. Consultado el 28 de octubre de 2025.: https://www.sciencedirect.com/science/article/pii/S0956713513004283
- [3] ↑ Jacobs, James A., ed. (25 de abril de 2014). Acid Mine Drainage, Rock Drainage, and Acid Sulfate Soils (en inglés). Wiley. ISBN 978-0-470-48786-0. doi:10.1002/9781118749197. Consultado el 28 de octubre de 2025.: https://onlinelibrary.wiley.com/doi/book/10.1002/9781118749197
- [4] ↑ Egiebor, Nosa O.; Oni, Ben (2007). «Acid rock drainage formation and treatment: a review». Asia-Pacific Journal of Chemical Engineering (en inglés) 2 (1): 47-62. ISSN 1932-2143. doi:10.1002/apj.57. Consultado el 28 de octubre de 2025.: https://onlinelibrary.wiley.com/doi/abs/10.1002/apj.57
- [5] ↑ Franco, Luis Fernando Londoño; Muñoz, Paula Tatiana Londoño; Garcia, Fabián Gerarado Muñoz (22 de agosto de 2016). «Los riesgos de los metales pesados en la salud humana y animal». Biotecnología en el Sector Agropecuario y Agroindustrial 14 (2): 145-153. ISSN 1909-9959. doi:10.18684/BSAA(14)145-153. Consultado el 29 de octubre de 2025.: https://revistas.unicauca.edu.co/index.php/biotecnologia/article/view/489