GLOBAL CHALLENGES

Arsenic and New Membranes

John Jainschigg | Date: 06-15-09 | Comments

New membrane technology for low-energy water detoxification may help the developing world, and the rest of the world as well.

Arsenicosis�gradual poisoning by arsenic, a common groundwater contaminant�is an urgent issue for tens of millions of people worldwide. The problem is especially severe in regions such as Bangladesh where the majority of residents obtain drinking water directly from shallow wells and other naturally contaminated ground sources.

In developed countries too, patterns of migration, agricultural and urban development and industrialization are increasing the need to mitigate arsenic in potable water supplies, drawing legislators, business and agricultural stakeholders, and communities into debates over regulation and cost. In 2001, the U.S. EPA first proposed and then withdrew an initiative to lower the maximum allowed arsenic levels in drinking water from 50 mg/L to the 10 mg/L mandated by the World Health Organization. Several bills now before House committees seek permission to exempt small municipal water supplies from compliance with max-level regulations for arsenic and other single contaminants.

Meanwhile, around the world, the growing water crisis is raising the stakes�rapidly increasing the need of populations in water-stressed regions, increasing contention over shared water supplies, and magnifying the cost of obtaining potable water to meet short-term needs. Arsenic�a gradual, though cumulative poison at low concentrations, causing slow onset of neurological symptoms and contributing to certain cancers�stands in danger of being de-prioritized where day-to-day availability of drinking water and irrigation of subsistence agriculture are on the table.

Several means are available to remove arsenic from drinking water. Purely chemical methods include coagulation and post-filtration with ferrous chloride, alum or with more expensive ion-exchange resins. Arsenic can also be mitigated by distillation and via active membrane technologies such as reverse osmosis, though at substantial energy cost. None of the current methods for arsenic mitigation can easily be applied, at scale, by communities with limited resources. The least energy-costly must be implemented with close attention to contingent variables such as water Ph (acid/base balance) and require extra steps, such as pre-oxidation of uncharged arsenites to arsenates using chlorine, sunlight or other methods, depending on the nature of contamination in specific instances. Methods requiring energy are intrinsically costly. The most energy-efficient methods, such as membrane reverse osmosis, face other problems: Conventional membranes, for example, are quickly degraded by chlorine used for biological decontamination, and clog in the presence of oil and other hydrocarbon contaminants.

As a long-term threat�and as a more urgent threat to the poor�arsenic has received scant investment over the past decade by traditional wellsprings of business and technical innovation. This has been only partially compensated for by R&D from independent green tech and biopharma companies, and by universities and non-profit entities subsidized with funds provided by WHO, by governments and by NGOs, and further promoted by incentives offered by organizations such as the National Academy of Engineering and the Grainger Foundation, which offer annual awards totaling over $1 million for arsenic abatement advances.

Another collaborative model, now being explored by diverse institutions, couples university-level R&D with primary innovations "open-sourced" by industry. This seems to be the model employed by a partnership among IBM, the King AbdulAziz City for Science and Technology (Saudi Arabia) and Tokyo-based Central Glass, which recently announced the creation of a new low-energy membrane process with application both to de-salination and highly efficient arsenic abatement.

The new technology emerges as a by-product of chip manufacturing. Made of fluorine materials, the new membranes are highly robust: resistant both to chlorine degradation and clogging by oils. In balanced-Ph conditions, the membranes are hydrophobic, thus highly resistant to the passage of water�but boosting Ph (making the solution more basic, as opposed to more acidic) turns them hydrophilic, permitting rapid transport of water freed of most dissolved contaminants. Fortuitously, high Ph also promotes conversion of arsenic compounds to more easily coagulated arsenate forms, meaning that the process removes arsenic as well as salts efficiently.

It is anticipated that initial runs of the new membranes will be incorporated in adapted reverse-osmosis equipment, which uses electricity to force contaminated water through the membranes under pressure. The "water-supertransporting" nature of these membranes under basic conditions, however, suggests that it may be possible to design more energy-efficient reverse-osmosis apparatus, using solar or other location-available energy sources to process water at scale, where needed.