Arsenic levels in Synthetic Salts? >.<

[shardakar]

I've been in contact with a friend who works as a chemical supplier to both Pharmaceutical and Industrial Manufacturers.

I was sourcing for options other than Natural Seawater for a 20,000L tank. My rational was that by mixing my own seawater, I wouldn't have to worry about any pathogenic organisms (specifically, parasites like C.Irritans,A.Ocellatum) tagging a ride in the seawater. I also live far enough from the Sea that transporting that much Seawater, means the costs of mixing my own salts isn't that much higher.

The big shocker came when I received the Chemical Analysis of the salts. Apparently, Technical grade Magnesium Sulphate has about 3.5ppm Arsenic. Obviously this is a no-go. So my friend got me the chemical analysis for Pharmaceutical grade Magnesium Sulphate, which surprisingly, still has something like 0.2ppm Arsenic and about 0.5ppm Heavy metals.

After running come calculations,
considering that I mix the salts in the right proportions and with a final salinity of 35,000ppm. I will end up with ~150ppb to ~10ppb Arsenic in the water(depending on the grade of salt used)...

So the question is, what are my options for arsenic removal?
I read some research that stated activated carbon can bring levels of Arsenic down to as low as 10ppb. Is it possible that activated carbon can bring down this level even lower? Perhaps about 5 ppb?
Also, with a skimmer running, will the arsenic get caught in the skimmate?

I guess this could also serve as a public service announcement of sorts. I highly highly doubt that synthetic marine salt manufacturers would use something as expensive as Pharmaceutical salts (which still isn't the highest grade available). So it'd probably be a good idea to be cautious when you buy synthetic salts for your aquariums. You don't really know whats in there.

[Randy Holmes-Farley]

Arsenic is probably mostly present as arsenate, which looks just like phosphate. It won't be skimmed, and I would not count on any binding to GAC from seawater.

GFO is very good at arsenic removal. That is what it was first made for: dropping potable water down into the ppb range. That said, whether it removes from those levels down to even lower levels in seawater I do not know.

Alumina also works.

All that said, I do not know how much might be a problem, or how much might be in typical salt mixes.

[HighlandReefer]

You may want to get some prices for salt mix at the pallet price. It is my understanding the cost is much cheaper getting it delivered in pallets. Many of the municipal aquariums are doing this.

[shardakar]

It seems like iron impregnated GAC has the potential to remove arsenic, what do you make of this study?
http://pubs.acs.org/doi/abs/10.1021/bk-2005-0915.ch020

Abstract:

Eight different types and brands of household filtration systems were purchased at area stores and assessed for their efficiency to remove arsenic from drinking water. These systems were using primarily activated carbon as filter media. The results showed the arsenic removal by the activated carbon in these household filtration systems was minimal. A new procedure was developed to impregnate iron onto granular activated carbon based on treatment of virgin activated carbon with ferrous chloride, followed by oxidation of ferrous to ferric iron. The iron impregnation onto activated carbon significantly enhanced the material's ability for arsenic uptake. In a test with tap water containing 518 µg/L of arsenic, hardness of 260.3 mg/L (as CaCO3), alkalinity of 110 mg/L (as CaCO3), and pH 7.85, arsenic can be removed to below 10 µg/L for 2,950 empty bed volumes, and at saturation, total amount of arsenic removed reached 3.92 mg As per g of adsorbent.

Or is this thing the same as GFO?

Are there also resins that can adsorb heavy metals? The fact that they are relatively simple to recharge seems promising.

I did consider a bulk purchase of synthetic premixed salts, but considering that most of it is going to be over priced sodium chloride, mixing it on my own is actually the cheapest option by far.
My concern is that if even pharmaceutical grade still contains significant amounts of arsenic and the logical assumption that few or no synthetic marine salt manufacturers are going to use pharmaceutical grade salt, we could actually have arsenic in the 100ppb range in our aquariums.

[Randy Holmes-Farley]

That product is similar to GFO in how it works.

Polyfilters can bind heavy metals, as can cuprisorb, but arsenic is not a metal and I would not assume these will have much impact.

Think of arsenic (arsenate) like phosphate, as it is almost chemically identical.

The only test I saw of aquarium water showed 20 ppb arsenic and I do not know if it was reliable or not. But we have no information on what levels might pose a problem, so I would not assume there is an issue.

[shardakar]

Thank you randy. You've help me understand so much about seawater chemistry.
The analysis papers I got indicate the arsenic content as As2O3 or arsenic trioxide. According to wiki, its solubility Is 20g/L.

Any idea what happens to this arsenic trioxide in a seawater environment? Does some reaction happen in the water that causes to become arsenate?

[Randy Holmes-Farley]

In normal seawater, arsenic is present primarily as arsenate, HAsO4--, just like phosphate (HPO4--).

If it comes in in other forms, it may take a while to become arsenate. Some of those forms moght include arsenite (AsO2-).