Arsenic in drinking water is a problem for a number of places around the world (not, however, in the Netherlands). This poisonous element is released in mines where ore is extracted and in smelting works where ore is processed. The arsenic is found in the waste water, by which means it is released into the environment in high concentrations and subsequently makes its way into the ground water. Drinking this water can pose a danger for public health.
The population of Bangladesh has unhealthy drinking water because arsenic is leached out of the rocks where it naturally occurs and ends up in the groundwater. Because it is not possible to reuse arsenic, the waste water containing it must be safely processed. Some businesses filter their water. The drinking water in Bangladesh is also filtered. The waste produced by this process, however, is unstable, because the arsenic can also escape from this waste.
Mineral
In the Environmental Technology sub-department at Wageningen University, a part of Wageningen UR, doctoral student Paula González-Contreras is working on a new procedure by which arsenic can be safely stored. ‘We have already proved that the principle works,’ she says.
The process involves using bacteria to help create mineral crystals, called scorodite, which absorb arsenic. These crystals are created by adding iron to water contaminated with arsenic. The iron then oxidises in a bioreactor and reacts with the arsenic to form scorodite. The scorodite adheres to the surface of the bacteria which are present in the bioreactor. In this way the first crystal is formed, which then grows in regular patterns and forms a clump.
‘We call this bioscorodite, because we have made the mineral using biological processes. But it has exactly the same composition and properties as the naturally occurring, very stable scorodite,’ says González-Contreras.
Advantages
There are many advantages to this innovative process. It is less expensive, easier, requires far fewer additives than conventional techniques, and results in a safer product. To begin with, this crystallisation technique requires 80% less iron than the chemical process also used to remove arsenic, and needs no other crystals in order to start the process. It is also possible to utilise the new procedure at far lower temperatures than the conventional method: instead of 150 degrees, it only needs 80 degrees Celsius. ‘Now we can even get it to work at 70 degrees,’ says González-Contreras. ‘The reaction works with just one gram of arsenic per litre of water, which is the pollution level often found in the industrial sector. Furthermore, we use bacteria which are well-known and available to everyone. These bacteria occur in nature near mines and hot sulphur springs.’ And finally, tests show that bioscorodite is quite safe.
Control
Now that the principle has been established, the doctoral student is working on gaining more control over the process in the bioreactor. ‘The bacteria grow under acidic conditions on the oxidised iron-arsenic compounds. We want to improve the biological conditions such that maximum production is possible and oxidation and crystallisation are simultaneous. We hope to develop a reactor in which the conditions are such that it’s possible to use a constant flow of water and get a solid substance remaining at the bottom.’ The goal of her research is to develop a trial system – probably in 2011 – which will be available to the metallurgical industry for testing, in countries like the Netherlands, Germany, or Brazil. The filters used in Bangladesh to remove arsenic from the water could also be placed in the reactor. The process’s waste product has no economic value, but because it is very stable, it can be safely stored. The natural mineral is also very popular among collectors because of its rarity.
González-Contreras expects that it will be possible within a few years to apply the new technology on a larger scale and commercially.
The Chilean researcher Paula González-Contreras came to Wageningen University on a personal scholarship and is conducting her doctoral research in cooperation with Paques BV, a company in Balk, the Netherlands, which develops biotechnology-based water and gas purification systems, and the zinc producer Nyrstar, located in Budel, the Netherlands. Her research is partly financed by SenterNovem, a division of Agentschap NL, itself a department of the Dutch Ministry of Economic Affairs, Agriculture and Innovation. González-Contreras hopes to obtain her doctorate in April of 2012 on the basis of this research.
by Yvonne den Hilster