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Arsenic V in Drinking Water

How To Treat Arsenic V in Water


What is arsenic V and how does your type of arsenic impact water treatment?

Arsenic–Most of us are familiar with the human carcinogen that’s long and insidious history has made it an infamous toxin. But not all arsenic is created equal... 

Arsenic can exist in water in several forms which together add up to equal total arsenic. Your water’s total arsenic concentration generally consists of two common oxidation states (or forms): arsenic III (aka arsenite) and arsenic V (aka arsenate).

In this Tips For Taps article, we take a deep dive into the subject of arsenate in water as we ask:

  • Why does the form of arsenic in water matter?
  • What is arsenic V and how does it differ from arsenic III? 
  • How to test for arsenic V in drinking water?
  • How to treat arsenic V in drinking water?
  • What Is Arsenic and Why Does Its Form Matter in Water?

    Arsenic is naturally occurring and enters the water supply mostly through the weathering of rocks. Due to its natural ability to enter the water system and widespread presence, about 77 million people in the United States, and over 140 million people globally, have arsenic levels above the Maximum Contaminant Level (MCL) recommended level of 10 parts per billion (PPB). 

    It is a known carcinogen and can lead to a host of health effects such as skin damage, cardiovascular issues, reproductive issues, and endocrine system disruption. 

    Arsenic can exist in two forms–organic and inorganic

    What Is Organic Arsenic?

    Organic arsenic molecules are arsenic molecules attached to complicated carbon structures. It is most often found in seafood and shellfish and is generally considered non-toxic or at least considerably less toxic than its inorganic counterpart.

    What Is Inorganic Arsenic?

    Inorganic arsenic molecules are simple arsenic compounds without carbon. Contrary to organic arsenic, the inorganic form is considered highly toxic and unsafe for human consumption. Inorganic arsenic naturally occurs in the environment and is commonly found in soil, sediment, and groundwater. Arsenic III and arsenic V are both forms of inorganic arsenic.

    While both arsenic III and arsenic V share the fact that they are the inorganic form of arsenic, they have some very important differences. 

    What Is Arsenic V and How Does It Differ From Arsenic III? 

    Arsenic III and arsenic V are two different oxidation states of arsenic. The oxidation state of an atom describes how many electrons it can gain, lose, or share with other atoms when forming chemical bonds. Arsenic can take on four different oxidation forms

  • Arsenic +3
  • Arsenic +5
  • Arsenic 0
  • Arsenic -3
  • The oxidation state is important in determining what kind of compounds it can form and what kind of reactions it can undergo.

    For example, arsenic +3 is the only oxidation state that can form the oxyanion compound arsenate (ArO43-) which is soluble in water. The most common form of arsenic V is arsenate. It is the most soluble and prevalent form of arsenic in water. 

    The inherent difference in shape and charge that each different oxidation states (and their compounds) have helps determine how it can be removed from your tap water. This is why testing your water is crucial. You can only make informed treatment decisions if you know what you need to treat in the first place.

    How To Test for Arsenic V in Drinking Water

    Typically, step one is to test for total arsenic. If total arsenic results come back high, it’s worth considering a follow up test to determine what form of arsenic dominates the total arsenic detection in order to make the best choice when it comes to choosing a filter. 

    The test for determining the oxidation state, or form, of your water’s arsenic is called a “speciation test” and it’s the only way to know the concentration of arsenic III versus arsenic V in your water sample. 

    Arsenic Speciation Water Test: This Tap Score test uses standard EPA methods in certified laboratories to determine the oxidation state of your water's arsenic so that you can accurately determine how to treat your water to the safest possible level. Like all Tap Score Reports, this test will provide an easy-to-understand breakdown of detections and quantified results, along with personalized treatment recommendations tailored to your water’s unique chemistry.

    While the water source may provide some hint as to the dominant form of arsenic in your water, there are some trends. In general, arsenic V is more prevalent than arsenic III in water. 

    Arsenic V will usually dominate in urban water systems, especially when the water is treated with chlorine disinfection, as chlorine quickly converts most of arsenic III to arsenic V. 

    If you are getting your water from a private well, on the other hand, it is possible that arsenic III levels are high due to reducing conditions of groundwater.

    That said, water chemistry and proper treatment is incredibly complex, so investing in testing can help you choose the best treatment unit with confidence.

    How To Remove Arsenic V From Drinking Water?

    While no amount of arsenic in drinking water is considered a good amount of arsenic, there is some good news if you learn that the majority of the arsenic in your water is arsenic V. 

    Ideally, you would want to remove all forms of arsenic from your tap, but as mentioned above, different oxidative forms of arsenic respond differently and have varied removal efficiency to various treatment technologies. While arsenic III is notoriously difficult to treat, the outlook is more promising when it comes to removing arsenic V from water. 

    Arsenic V can be successfully removed from drinking water by reverse osmosis, ultrafiltration, and distillation without the necessary pretreatment arsenic III needs to be successfully removed.

    Talk to water testing expert

    What’s the Takeaway?

    Arsenic is odorless and tasteless and therefore nearly impossible to detect in tap water without actually testing. If you want to test for this sneaky and serious contaminant, Tap Score is here to help. 

    If you are concerned about all forms of arsenic in your drinking water, you can test for total arsenic with a city water or well water testing package. Each standard package tests for a wide array of heavy metals, minerals, general chemistry, physical properties and more. 

    Because choosing a removal mechanism largely depends on the species of arsenic you have, testing for Arsenic Speciation is the best followup to ensure your treatment solutions will tackle your concerns.

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