By Nick Nicholas
In water and wastewater treatment, arsenic is classfied as a heavy metal. It is well known for its toxicity and it can cause numerous health problems in humans if consumed at high enough doses. Arsenic can affect the skin, liver, kidneys, bladder, and prostate as well as the nervous, respiratory, cardiovascular, immune, and endocrine systems. Due to the many health risks associated with arsenic, the U.S. EPA has set the drinking water standard at 10 ppb as the maximum allowable concentration in drinking water. Therefore, this is the goal of all treatment systems for arsenic removal from water.
Commonly found in groundwater, arsenic mainly exists in two oxidation states, arsenite (As(III)) and arsentate (As(V)). As(III) is the more toxic of the two, and is unfortunately the more difficult one to remove due to its higher solubility. In most, if not all removal methods, iron has proven to be the most effective medium. A few methods of removal include ion exchange, reverse osmosis (RO), oxidation, and coagulation, which is one of the more popular methods. Improving on the success of coagulation is its electrochemical counterpart, electrocoagulation (EC).
Below, we will discuss the benefits of electrocoagulation for arsenic removal from water in comparison to other treatment methods.
Improved Removal of As(III)
With other treatment methods, there is typically an issue with the removal of As(III) as it is much more soluble than its counterpart As(V). Ion exchange, RO, and coagulation by themselves cannot remove As(III) to acceptable levels, so a process is added prior to those steps to make it more easily reactive with the medium of removal. With EC, some unknown reaction mechanism allows As(III) to coagulate and precipitate well enough for sufficient removal with that one process alone. EC removal of As(V) is comparable to coagulation and other treatments, being quite efficient, but it is its higher removal rates of As(III) that sets it apart from the others.
May Not Require Oxidation Step Beforehand
The theory behind the improved removal of As(III) by EC is that during EC, oxidation of As(III) occurs in situ. The oxidation results in the formation of As(V), which is much easier to precipitate out of solution as it is less soluble. This oxidation from As(III) to As(V) is done purposefully before other treatment methods, but may not be required before an EC step. Regardless of whether or not the theory is true, studies (like this research study) have proven that removal rates of As(III) are higher in EC treatments on their own versus other treatment methods without a previous oxidation process. Therefore, if the removal rate of As(III) by this single process is high enough based on water testing laboratory results analysis, there likely will not need to be an oxidation step prior to EC.
Faster Removal Rate
If an oxidation step is not needed before EC, then the overall removal process can happen much faster. Also, some oxidation processes can take much longer to occur, further extending the reaction time required for proper treatment. With optimized power supplied to the electrodes, EC removal of arsenic can take an hour at most to complete and additional oxidant can potentially be introduced prior to the process to reduce reaction time.
Simple and Lower Lifecyle Cost
Other treatment methods for arsenic can be complex and costly, which can make it difficult for less financially stable communities to be able to afford a system that will treat drinking water to safe levels for human use. EC systems, on the other hand, have a lower lifecycle cost and are relatively easy to operate. These systems are easy to install, easy to clean, and easy to maintain. The main costs associated with EC are electrode replacement when they become too corroded, chemicals for pH adjustment, and the cost of power. Luckily, iron electrodes and pH chemicals are typically readily available at a relatively low cost.
Arsenic removal from drinking water is paramount to the health and safety of people all over the world. Previous methods work, but a different method could be easier, faster, efficient, and have a lower lifecycle cost. Electrocoagulation is one such method.