Clarifying or filtering buffered 98% sulfuric acid?

Question

A pal's business recently bought 55gal of surplus 98% sulfuric acid. Unknown buffers or impurities give it a dark brown color (clear not cloudy) .

The problem: The dark coloring makes the product unusable for their application. It must be as colorless as possible. To avoid wasting expensive product and disposal fees, they want to try to remove some or all of the color. Distillation won't work as 55gal is too much for them to process safely & quickly. They're willing to try any reasonable method that might possibly help.

1: Basically, they need to know what readily-available filter media can withstand 98% H2SO4 (and might clarify the product even slightly).

2: If filtering isn't possible, what chemical reactions might clarify the product? Again, even a modest improvement might be enough.

This is a small shop that sunk lots of cash into this product. I'd hate to see them lose money if there's a chance the product can be salvaged.

10 pts. await for the best answer. Thanks!

Update: Today they test-filtered through silica gel, zeolite granules and activated charcoal. They are unsure of the results.

Silica Gel: Slow, exothermic reaction (perhaps created silicic acid?). Filtrate turned a dark brownish-red, and almost black.

Activated Charcoal (unwashed): Filtrate turned dark, purplish red, almost black.

Zeolite: Filtrate lightened to a faint pinkish-red.

They think the charcoal may have impurities, and are washing & drying some tonight
to test again tomorrow.

The best visual results were with the zeolite, and they don't have any fritted glass filters at present.

Does anyone know what could cause those color changes? Also, why their highly pure charcoal didn't work? Will their zeolite accomplish the same thing without reacting?

Thanks again for your time, Answerers. These guys are VERY desperate for a solution. :/

jaygee: Re: letting it settle. When the drum came in and they discovered the problem, they poured a sample in a gallon glass jar and left it for a week to see if that would work. Nothing changed. They also tried chilling another sample, and again, no luck. Whatever that buffer is, it *really* wants to stay in solution :)

Also, the MSDS was no help. The product was supposed to be pure, so no buffers or adulterants are listed.

Jay: I agree, but any idea why the silica gel also seemed to react? It's brand new, perfectly clear, and has no cobalt chloride or anything. Yet after contact, the H2SO4 turns from dark brown to a dark blood red. Kinda the opposite direction from their intended result :)

It almost seems like this color is some kind of indicator. Neutralizing an H2SO4 sample with Ca(OH)2, KOH or NaOH turns it almost clear, but silica gel and activated charcoal turn it a darker reddish brown.

Weird, no?

Answer 1

First, I think the best way to do this is not by filtration but by settling.

If filtration really is the key, I can say that the best filtration media that made of glass or polymers like HDPE. Maybe there is a thin film or fiber that can withstand sulfuric acid.

Overall I think Glass/polymer beads or film should be used as a filter medium.

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I see so settling cannot resolve it. I think the color change was caused by zeolite. Zeolite has many metallic elements that will react to Sulfuric Acid. I think Silica is the best one. Activated Carbon will react.

Answer 2

Acid Filter

Answer 3

Activated charcoal should clarify the acid, and it won't react with the acid at all. It can be left to settle.