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Change Hexavalent to trivalent chrome? Chrome Reduction process




Q. After converting hexavalent to trivalent using sodium metabisulfite (SMBS) [affil link] with pH somewhere between 2-3, and adding sodium hydroxide [affil link] to neutralize the effluent, not able to remove blue colour from treatment effluent. Can someone guide me how to remove this colour>

Thyagaraj Varadasetty
- ballari karnataka INDIA
September 15, 2023


A. Hi Thyagaraj,
The blue color is the trivalent chromium ... to remove the blue color you have to remove the trivalent chromium by converting it to chromium hydroxide and settling it out.

Do some jar tests and see if the pH you are attempting to hold is too low or too high for full precipitation, or exactly what is the problem.

Readers: please do your wastewater treatment experiments with beaker [affil links to item on Ebay & on Amazon] size volumes not with hundreds or thousands of liters.
Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




↓ Closely related postings, oldest first ↓



Q. I machine (Electro Chemical) high Nickel/Chromium components and my solutions contains metal hydroxides and chromium VI. I remove them as waste. They have a pH of > 8, (pH is high to aid machining and is essential.) is it possible to use sodium metabisulfite at this high pH to change Hexavalent chrome to trivalent chrome? If it's possible is there a market for my waste product?

Peter G [last name deleted for privacy by Editor]
- Middlesbrough, England, UK
2002


A. Hexavalent chrome can be converted to trivalent chrome at a pH range 8.0 to 9.0 using sodium hydrosulfite [affil link].

Karl Weyermann
Lebanon, Kentucky


! It is important to keep hydrosulfite dry and sealed up, it can combust.

Jon Quirt
Minneapolis, Minnesota


! Watch out with storing sodium hydrosulfite, though. It's water reactive.

Guy Lester
Ontario, California


Chromium VI Handbook
by Guertin, Jacobs, Avakian
from Abe Books
or Amazon
[affil links]

A. Hi, Peter. There is only "a market" for the sludge in the sense that there may be companies who will recycle it (at greater or less price than landfilling) so you reduce your chance of being stuck with the liability. But I've never heard of anyone paying for this waste product.

Sodium metabisulfite only works at pH of about 5.0 and less (and quite slowly at 5.0). Karl's suggestion sounds good to me if you are doing integrated treatment (using the treated effluent as the coolant). If you are treating end-of-pipe it may be more practical to reduce the pH and use the sodium metabisulfite./P> Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


A. Hi,

You can reduce Cr+6 to Cr+3 with ferrous sulphate ferrous sulphate [affil link] ferrous sulphate [affil link]; the reaction is very fast.

Moshe Yaakov
Lod, Israel


A. I have used sodium metabisulfite to neutralise hexavalent chromium solutions from a chrome plating bath. This was done to both make the waste bath safer and easier to handle and to neutralise any chromic acid that may have crept into nooks and crannies in the plating jig or the workpiece. All I can say is that the yellow/orange hexavalent chromium solution went green, which suggests it does work. However, metabisulfite gives off sulphurous fumes, so be warned!

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


i. Fumes are a problem only if the pH gets too low! You need to adjust the pH of the treatment tank.

tom & pooky   toms signature
Tom Pullizzi
Falls Township, Pennsylvania


A. Both sodium metabisulfite and ferrous sulphate ferrous sulphate [affil link] ONLY reduce in acid conditions, pH <3, and will be of no use in this case. sodium hydrosulfite [affil link] is the material of choice but as warned keep absolutely dry. Just dampening Hydros can cause fires. Add only amount required to take yellow colour away. Cr+3 should precipitate as hydroxide but will be of no commercial value.

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia


A. Although Geoff is correct that metabisulphite is of no value under alkaline conditions, I agree with Tom. Very low pH in the chrome treatment tank allows instantaneous reaction, but thereby generates substantial fuming; my experience has been that if we allow for longer reaction times, chrome can be reduced at a mildly acid pH, maybe 4.5-5.0, generating no serious sulfur small.

Run jar tests to see how high you can keep the pH for sodium metabisulphite reduction of chrome if you allow extended treatment time.

Luck & Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


A. Sodium Metabisulfite does an excellent job of precipitating Cr+3 as the hydroxide. In general, the procedure is:
1. if there is any ferricyanide ion present, add a little laundry bleach [affil link] - maybe 1 oz./drum of wastewater.
2. Lower pH to about 3 with sulphuric acid.
3. Add some SMBS - use trial and error to find out how much is needed. Agitate by bubbling air.
4. Raise pH with NaOH. Be careful. I use NaOH pellets in a wire basket. When everything goes green, reaction is complete. Problem is that the precipitate is gelatinous and difficult to filter. Just let it settle and decant the supernatant liquid, leaving the precipitate behind. I can't imagine there would be anyone interested in buying the precipitate.

Dennis Kirsch
San Antonio, Texas


A. At elevated pH only hydrosulfite can reduce the hexavalent chromium to trivalent.

sara michaeli
sara michaeli signature
Sara Michaeli
Tel-Aviv-Yafo, Israel


A. The problem with sodium hydrosulfite is that it will only bring the Cr+6 down to about 5 - 10 ppm, under alkaline conditions.

I believe the best solution to treating a hex chrome waste without bringing the pH down into the basement is ferrous sulphate ferrous sulphate [affil link]. It will work at pH 5 - 6. The problem with it is that it is only a one electron reductant, so that 3 moles of ferrous is required per mole of hex chrome. This means that it will make a great deal of sludge if your hex chrome is at all high.

Perhaps a combined approach? Use hydrosulfite to reduce the bulk of the chrome, then polish with ferrous.

Hope this is of some help.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


A. WHILE PRECIPITATING, IF INSTEAD OF NaOH, MgO IS USED, IT FORMS STABLE PRECIPITATE. SIMILARLY, BRING pH TO AROUND 2 TO GET BETTER RESULTS AT pH 3 IT TAKES LONGER TIME FOR REDUCTION
AKJAIN

Alok kumar jain
- India
2006




Q. I have a question to ask, if ferric sulphate would be used as the primary precipitant and then the remaining chrome metal be reduced by sodium hydrosulfide, would this work? Has anyone tried using a combination of iron and hydrosulfide?

Rick Fudalewski
Mississauga, ON Canada
2002




sidebar

A. We have developed an insoluble sulfide technology that reduces Hexavalent chrome to the trivalent state at a near neutral pH. This technology produces less sludge than the traditional SMBS and ferrous treatment process and is less costly and safer than the Hydrosulfite method. It also doesn't generate the corrosive SO2 odors that come from the traditional SMBS treatment methods. We have also found that the TDS of the treated supernate is much lower making the treated water more susceptible for water reuse.

Larry B
Dallas, Texas [last name deleted by Editor]
2003


A. My company has developed a proprietary reducing agent which is effective at a pH range of 3-4, and does not give off any sulfur dioxide fumes when converting CR6 to CR3. If is a completely non-hazardous product, with no combustibility problems. If your interested contact us at (xxx) xxx-xxxx.

A.J. R
East Rutherford, New Jersey


Larry and A.J.'s products may indeed by an improvement over conventional treatments. Readers are encouraged to google the subject! And Larry & A.J. are welcome to advertise here or elsewhere.
But hopefully readers who saw the sci.chem newsgroups destroyed by spam, and who have seen so many websites lost to 'comment spam', and so many sites lose credibility due to thinly disguised 'sponsored posts' will understand why we can't let comment spam invade this one :-)
pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

"Internet price/demand curve: Infinite demand at zero cost, zero demand at infinitesimal cost" -- T. J. Pullizzi, 1995"

A. No one has mentioned using calcium polysulfide for Cr(VI) reduction. We use that to treat hex chrome in groundwater in situ. No pH adjustment is required. Sometimes a ferrous iron salt is added to facilitate immobilization of the Cr3+ as the sulfide or with iron oxyhydroxide.

Jim Rehage
Austin, Texas
May 5, 2017




Q. Would there be a way to induce compaction of the gelatinous Cr(OH)3 formed? I am trying to find a chemical that can induce the compaction to lower the disposal costs.

Thanks,
Shee

Shee Pagsuyoin
Quezon City, Philippines
2007


A. Hi, Shee.

No chemical can do that to my knowledge. But "difficult to filter" doesn't mean impossible to filter. A filter press preloaded with filter aid is one way to filter it. Good luck.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




Q. We have tried treating hexavalent chromium with SMBS but our problem is the green color of the solution. Is there a way to convert this into a clear solution ?

Teddy Fabic
Manila, Philippines
August 8, 2009


A. Hi, Teddy. Hexavalent chromium reduction with SMBS is only the first step in the treatment. After that, the pH must be brought back up and the green trivalent chrome converted to chromium hydroxide and precipitated out.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




Q. In Dennis Kirsch's response where would the ferricyanide come from in a chrome solution. We have just recently had this happen and would be interested to know the chemistry.

Phil nelson
Victoria, BC, Canada
November 17, 2010


"Plating Waste Treatment"
by Kenneth F. Cherry
from Abe Books
or Amazon
[affil links]

A. Hi, Phil. I think Dennis was referring to the treatment of a mixed waste which contained chrome but was not exclusively chrome; rather, it also included some cyanide waste.
There is no cyanide in chromic acid, but some older hexavalent chromate conversion coating chemistries included cyanide as an accelerator.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




Q. Dear All
I have gone through this conversation of yours, I understand that chrome trivalent sludge is problem for disposal.

I have a application which runs into approximately 1000 mt per month which requires chromium. but the metals that should not be there in the sludge are Ca, Na, K, Zn, Al, Mg.
Metals that are acceptable in the sludge are Fe, Ni, and Co in any percentage levels from 70% to 1% , but Cu, Mn less than 1%

is it possible to have this sludge prepared with ferrous sulphate ferrous sulphate [affil link] and use ammonia [affil link on Ebay & Amazon] to increase the pH?

If you all can contribute to this idea I think we can get good environmental solution to this.

Mr Pratik Sanghvi
Mumbai, India
April 15, 2011




Q. Hi dear. Please help me to save humans from chrome+6.

I'm try to convert Cr+6 to Cr+3 please give advice and method for my help. Thanks.

muhammad amir
- Karachi, Pakistan
December 14, 2012


A. Hi Muhammad. This entire thread is on that subject, but you get a more specific answer without a more specific question. I don't understand what makes your situation different from everyone else's because you haven't told us yet :-)

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


Q. Hi, Ted Mooney.

Dear sir ,
thanks for your answer.
Please give me the procedure to convert Cr+6 to Cr+3. Thanks a lot.

Muhammad Amir [returning]
- Karachi, Pakistan
December 18, 2012


A. Hi Muhammad. Sorry, but we may be having a language difficulty. It's already been explained that:
Step 1 is to reduce hexavalent chrome to trivalent with a reducing compound; usually this is sodium metabisulfite (SMBS) [affil link] at a pH of below 5.
Step 2 is to raise the pH to precipitate the trivalent chrome as chromium hydroxide so it will settle or can be filtered.

But you must explain your situation before anyone can help you further.

Are you trying to convert a beaker of dilute wastewater, which contains water and hexavalent chromium into trivalent chromium for some reason? That can be done by lowering the pH to below 5.0 and slowly adding a small amount of sodium metabisulfite, under a lab hood, until the faintly amber color of the water turns to faintly blue-green, indicating the conversion is complete.

Are you trying to convert fasteners that already have a hexavalent chromium conversion coating on them to RoHS-compatible trivalent chromium? It is not possible; the old coating must be removed.

Are you trying to convert a hexavalent chromium electroplating bath to a trivalent chromium plating bath? It is not possible; you must discard the old bath and start new.

Are you trying to design an industrial wastewater treatment plant for treating the effluent from a hexavalent chromate conversion coating line or a hexavalent chrome electroplating line? That may involve additions of sulfur dioxide or sodium metabisulfite after lowering the pH, or employing the previously discussed sodium hydrosulfite. But both hexavalent chromium and the sulfur-bearing chemicals like sulfur dioxide, sodium metabisulfite, and sodium hydrosulfite are quite hazardous chemicals. You can't add them to an unknown waste stream without severe hazards. If you over-acidify sodium metabisulfite, for example, it releases hazardous quantities of poisonous gasses.

If you are designing an industrial wastewater treatment plant for plating wastes, please start with a good book on the subject, such as Operation and Maintenance of Surface Finishing Wastewater Treatment Systems [affil link on Amazon or on AbeBooks]. Best of luck!

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




Q. I've taken a new job where the water treatment says that they can't treat chrome with the acid waste stream. At my old shop we treated it all the same, first with Sodium Metabisulfite at a higher pH then drop pH to 3-4 with Sulfuric Acid. At that pH we would add Zinc Sulfite and Ferrous Sulfite.

Kevin Westbrook
Portsmoth Virginia
April 17, 2013


A. "There's more than one way to skin a cat" -- keeping the barnyard metaphors going in our digital age.

It's simplest to mix the chrome wastewater with other acidic rinse waters, but it's most effective to treat wastes before mixing them with anything else.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




Q. Dear friends,

I'm currently working on nickel mine here in Indonesia. The only problem is our waste water that came from run-off's contains Cr6+ (average 0,5 mg/l). We have made a couples of "extra large" sediment basins for precipitating TSS , but we're still searching for the best solution on Cr6+ problem. With daily rainfall 30 to 45 mm, the water discharge is about 471 m3/hour which could potentially pollute the environment when the sediment basin becomes full.

Do you have any suggestion? is it possible using the SMBS method with such big sediment basins?

Thanks in advance, I'm sorry for my bad English :)

Regy Kurniawan
- Kawasi, North Maluku, Indonesia
January 8, 2014

Ed. note: Hopefully someone will help you, Regy. The editors are from the metal finishing field, with no nickel mining experience, and are far too likely to misunderstand the overall situation to make any suggestions :-(



Q. Hi
Dear sir, I am working in an electroplating plant. Here we have already setup an ETP for plant waste water. For chrome neutralization we have passed out this waste water from a process in which chrome is neutralized by SMBS and precipitated by NaOH. Then we do coagulation by alum for reduce the pH to 7.0; then we do flocculation by polyelectrolyte, and after that we do clarification & settling process. Finally we inject the slurry into a filter press for separation of liquid & solid, and then we get the sludge in cake form.

Here my problem is the large amount generation of sludge. I am searching about any procedure for disposing of this sludge. Can we recycle it by chemical reaction and generate potassium dichromate crystal from this sludge

Umesh kaushik
- Chandigarh, Hariyana, India
August 30, 2015


A. Hi Umesh. Wastewater treatment and sludge disposal is expensive. It will almost surely prove impractical to make a product from this sludge. The better approach is to focus on retaining/recycling the chromium before it enters the waste stream. Good luck.

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




How to waste treat one gallon of Alodine

Q. I have read through the thread several times and find it terrifically helpful and interesting. I am planning to use a small quantity (a gallon of Alodine 1201 [linked by editor to info/product on Amazon] to help with the refinishing of about 100 square feet of a 24 ft. aluminum tug style cruiser. I intend to gather as much as possible of the waste water including the rinse in plastic barrels. I will try to keep the rinse to a bare minimum but will still end up with a very dilute waste. Of all the alternatives suggested what would be the best way to treat such dilute waste? Should I keep the concentrated drippings from the initial application separate from the rinse water? obviously much more difficult but possible.

With thanks in advance,

hans elfert
Hans Elfert
- Richmond BC Canada
October 26, 2015


A. Hi Hans. There is no need to separate the more concentrated treatment chemical from the rinse water; you can collect and treat it all together. Reduce the pH of the collected waste water to about 4.5-5.0 with dilute sulfuric acid, slowly add small amounts of sodium metabisulfite (SMBS) [affil link] powder until all trace of amber/yellow/orange/honey color is gone and you have only a blue/green color. Then raise the pH to about 8.5-9.0 with hydrated lime and filter out / settle out any sediment/precipitate before discharging the water.

There may be local or national regulations prohibiting you from doing this, but neutralizing hex chrome with metabisulphite seems far better than discharging hex chrome. There are certainly hazards if you're not familiar with the chemistry, so keep studying. Wear rubber gloves [affil links to item on Ebay & on Amazon] and goggles [affil links to item on Ebay & on Amazon] at a minimum. The reaction will be slow, but safer at pH 4.5-5; lower pH is used in industry, but can release a good amount of sulfur dioxide gas -- better to keep the pH just low enough to reduce the chrome and just take your time. Good luck.

Readers: the internet is a giant one-room schoolhouse where it is impossible to keep people from reading about stuff which they may not be trained & qualified to do. So just because readers may read some steps here does not mean they have the background and skillset to safely and properly follow them :-)

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


A. A small amount of Thiol Red will reduce any Chromium VI and precipitate the metals so that they can be settled or filtered.

Lyle Kirman
consultant - Cleveland Heights, Ohio

Ed. note: The editor doesn't know what 'Thiol Red' is. If anyone has a good link explaining it, we'd welcome it.

A. Do what Mr. Mooney says. That will work.

Or, since you have so little, you could reduce the Cr6+ to Cr3+ with ferrous sulphate ferrous sulphate [affil link]. pH in the neutral range. Then get the pH up to 9 or so, let the solids settle, and dump the free liquid (should be legal) down the drain.

Dewater the settled solids, the filtrate ought to be legal to sewer. Then, put the solids in the trash.

dave wichern
Dave Wichern
Consultant - The Bronx, New York




Q. From the above conversation, I understood about the steps to eliminate the Cr+6 to Cr+3. But my problem is that what are the steps to eliminate trichrome (Cr+3) from an effluent?
What are statutory limits designed for trichrome (Cr+3)?
Kindly suggest .

Satish Kasar
-Pune, Indiana
April 18, 2016


Hi Satish. I can't tell you the limits because every country has their own, many states and zones within states have their own, and it may also depend on whether you are discharging to surface waters vs. a public treatment works.

But trivalent chrome can be precipitated. That is, at neutral to alkaline pH the chrome will not stay dissolved but will become a floc which you can settle or filter depending on your situation. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey




High sulphates in wastewater after chrome treatment

Q. I am working in effluent treatment plant, this plant is designed to treat chrome water from chrome plating plant.
We are facing a problem of high sulphate content (>1500 ppm) in ETP product water.

shahrukh ahmed
- Karachi,Pakistan
January 30, 2018


A. Hi shahrukh. You are only at about 1% of the solubility of sodium sulphate, so if you use sulfuric acid for pickling and sodium hydroxide for treatment, the problem may not be solvable. But if you use lime for treatment instead of sodium hydroxide, this number might sharply decline. Jar testing is definitely needed. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


A. Rather good read on the subject of using lime successfully in Chromium treatment (tannery effluent) done in 2010 by an Egyptian research team:

https://www.researchgate.net/publication/307137491

I love this paper in that it is quite well written in such a manner as to be understandable to non-academics, and found it quite useful some years back while doing a bunch of jar testing on spent trivalent conversion coating bath solution. It's a fantastic resource!

rachel_mackintosh
Rachel Mackintosh
- Greenfield, Vermont
February 22, 2018




Hexavalent chrome from high temperature working of high-chrome steel

Q. Great thread thank you. I have a slightly different situation to the others (solids vs. liquid):

In this case while doing maintenance on heavy duty machinery that operates at high temperatures we are discovering Chrome-6 deposits in a couple of areas. The first is in areas where high chrome steels may have been assembled with the aid of some form of anti-seize product and run at high temp. The second is a case where chrome steel is covered with personnel protection insulating blankets (mainly calcium) and were there was contact and high temps and extended time periods

In both cases the material has been tested to contain chrome-6 and is yellowish. They are relatively dry to slightly pasty. Is there a recommendation for treatment to prevent airborne risk in handling and neutralizing directly from a solid (deposits on a metal part) form or do we need to turn the full volume into a liquid?

Jerry Gorski
Orlando, Florida
November 7, 2018


A. Hi Jerry. I have never heard of that situation and had never even contemplated the possibility, so thanks for this warning! I have seen dried chromate powders and they can certainly be yellow and resemble powdered sulfur. So if it looks like hex chrome and analyzes as hex chrome, I guess it is.

I don't think it will prove practical to treat these dusts and powders in-situ. Dissolving them into water will give you the opportunity for quantitative analysis, reduction & precipitation, and confirmation of what you have accomplished. Further, you will have a widely accepted treatment protocol to follow.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


Q. Thank you, I was hoping to find a way to treat in-situ since these machines are distributed all over, and there may be a risk of it getting it locally air bourn.

Thanks again.

Jerry Gorski [returning]
Orlando, Florida
November 7, 2018




Q. We have traditionally used Nitric Acid to lower the pH of the hexavalent chromium solution before adding the reducing agent. Is there another acid that can be used? I know acetic acid [affil link on Ebay & Amazon] is not a good choice.

Chris Dwyer
Columbus, Nebraska
July 18, 2019


A. Hi Chris. Sulfuric acid is the usual choice and surely a better choice than nitric acid. Good luck.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


thumbs up sign Ted is correct in that Sulfuric is better than Nitric; this is because Nitric acid is an oxidizer and will absolutely have the opposite effect of what you are trying to achieve in a reduction.

rachel_mackintosh
Rachel Mackintosh
- Greenfield, Vermont




Q. Thank you all for all the explanations which has really aided me in a project I am involved in and I really appreciate all of your help.

I have a couple of questions regarding transporting and filtering/dewatering of the sludge.

1) what pump would you recommend for transporting the chromium hydroxide sludge? (diaphragm, progressive cavity etc.?)

2) the trade waste stream has about 60 ppm of hexavalent chromium and discharge flow rate of 2L/s. would a bag filter be sufficient to do the job? (Any low cost alternative filtration suggestion to remove the sludge?)

Anand Krish
- India
July 4, 2021


A. Hi Anand. Air powered diaphragm pumps are usually employed for such applications. They are well suited because they will do no shearing damage to the floc and nothing is liable to happen in the process that can damage the pump.

I think a bag filter would 'work', but poorly, because with no pressure to compact the sludge you won't get beyond a very loose mud. I have seen small installations distribute thin chrome sludges in their boiler room in a thin layer to evaporate the moisture, but thermal drying of filter bags doesn't sound practical for a 30 GPM system. Usually a clarifier (settling tank) followed by a polypropylene filter press, often with some amount of filter aid, is the only practical approach.

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey





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