Adsorption has been used to treat ground water and drinking water containing arsenic. This technology typically can reduce arsenic concentrations to less than 0.05mg/L, and, in some cases, has reduced arsenic concentrations to below 0.01mg/L. Its effectiveness is sensitive to a variety of untreated water contaminants and characteristics. It is used less frequently than precipitation/coprecipitation and is most commonly used to treat ground water and drinking water, or as a polishing step for other water treatment processes.
In adsorption, solutes (contaminants) concentrate at the surface of a sorbent, thereby reducing their concentration in the bulk liquid phase. The adsorption media usually is packed into a column. As contaminated water is passed through the column, contaminants are adsorbed. When adsorption sites become filled, the column must be regenerated or disposed of and replaced with new media.
Dissolved organic and metal contaminants in ground water and drinking water use these types of sorbents to treat arsenic:
- activated alumina
- activated carbon
- copper-zinc granules
- granular ferric hydroxide, ferric hydroxide-coated newspaper pulp, iron oxide-coated sand, iron
filings mixed with sand
- greensand filtration
- proprietary media
- surfactant-modified zeolite
Activated alumina (AA) is the sorbent most commonly used to remove arsenic from drinking water and also has been used for ground water. The reported adsorption capacity of AA ranges from 0.003 grams to 0.112 grams of arsenic per gram of AA. AA is available in different mesh sizes, and its particle size affects contaminant-removal efficiency. Up to 23,400 bed volumes of wastewater can be treated before AA requires regeneration or disposal and replacement with new media. AA regeneration is a four-step process:
The regeneration process desorbs the arsenic. The regeneration fluid most commonly used for AA treatment systems is a solution of sodium hydroxide. The most commonly used neutralization fluid is a solution of sulfuric acid. The regeneration and neutralization steps for AA adsorption systems might produce a sludge because the alumina can be dissolved by the strong acids and bases used in these processes, forming an aluminum hydroxide precipitate in the spent regeneration and neutralization fluids. This sludge typically contains a high concentration of arsenic.
Activated carbon (AC) is an organic sorbent that is commonly used to remove organic and metal contaminants from drinking water, ground water and wastewater. AC media normally are regenerated using thermal techniques to desorb and volatilize contaminants. However, regeneration of AC media used for the removal of arsenic from water might not be feasible. The arsenic might not volatilize at the temperatures typically used in AC regeneration. In addition, off-gas containing arsenic from the regeneration process might be difficult or expensive to manage.
An AC system impregnated with metals such as copper and ferrous iron has a higher reported adsorption capacity for arsenic.
Iron-based adsorption media include granular ferric hydroxide, ferric hydroxide-coated newspaper pulp, iron oxide-coated sand and iron filings mixed with sand. These media primarily have been used to remove arsenic from drinking water. Processes that use these media typically remove arsenic using adsorption in combination with oxidation, precipitation/coprecipitation, ion exchange or filtration. The media require periodic regeneration or disposal and replacement with new media. The regeneration process is similar to that used for AA and consists of rinsing the media with a regenerating solution containing excess sodium hydroxide, flushing with water and neutralizing with a strong acid.
Surfactant-modified zeolite (SMZ) is prepared by treating zeolite with a solution of surfactant to form a stable coating on the zeolite surface. SMZ must periodically be regenerated with surfactant solution or disposed of and replaced.
Adsorption can be operated using multiple beds in series to reduce the need for media regeneration. Beds first in the series will require regeneration first, and fresh beds can be added at the end of the series. Multiple beds also can allow for continuous operation because some of the beds can be regenerated while others continue to treat water.
There are several key factors that can affect the performance of the adsorption process:
- Wastewater pH – The optimal pH to maximize
adsorption of arsenic by activated alumina is acidic (pH 6). Therefore, pre-treatment and post-
treatment of the water could be required.
- Arsenic oxidation rate – Adsorption is more effective removing some types of arsenic than others.
- Flow rate – Increasing the rate of flow through the adsorption unit can decrease the adsorption of contaminants.
- Fouling – The presence of suspended solids, organics, silica or mica can cause fouling of adsorption media.
Very high concentrations of competing contaminants may require frequent replacement or regeneration of adsorbent. The capacity of the adsorption media increases with increasing contaminant concentration. High arsenic concentrations can exhaust the adsorption media quickly, resulting in the need for frequent regeneration or replacement. Spent media that can no longer be regenerated might require treatment or disposal.