[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Clarification: silver-bearing wastes and Chemgon



Title: Clarification: silver-bearing wastes and Chemgon

Background
I had contacted Marlyn Aquilar directly concerning her e-mail posted 8/2/01, "Does anyone know anything about a polymer called Chemgon?  Dentists are using it as an alternative to silver recovery."   We had a telephone conference to discuss my experiences with Chemgon, which are a little complicated for e-mail. 

Then on 8/15/01 Judy Kennedy responded, "I hope Alice will post a message on this topic" so I'll give it a shot.  It's been a while since Judy and I talked about Chemgon so some of the details she mentioned need clarification, and I have a new test result she was not aware of.  Anyone with an interest in managing silver-bearing wastestreams is welcome to give me a call to discuss these issues further.

I'll start the discussion with general pollution prevention options for silver-bearing wastestreams from film developing.  Next I'll review problems encountered when testing for silver in wastes derived from photoprocessing and ways to avoid getting false negative test results.  Finally, I'll summarize my knowledge of the Chemgon product.

General Pollution Prevention Options for Silver-Bearing Wastestreams
Change the process so no silver waste is generated.  Graphic arts shops may consider going with "computer to plate" digital technology, and dentists can use digital x-rays.

Large operations (like the typical snapshot developer or hospital x-ray facilities) install treatment systems to recover silver for refining, and discharge treated water to the sewer.  Several different units in series are usually necessary to achieve sewer discharge limits (electrolytic, two steel wool cannisters, etc.)  The quantity and value of silver recovered provides an economic incentive to invest in treatment systems and the regular service they require to operate properly.

Small operations (dentists, screen printers, plate-developing, graphic arts, small clinics) may generate just half a liter of silver-bearing waste per month.  Economically, an investment in on-site treatment equipment for silver recovery is often not justified and the equipment will not perform well with periods of inactivity.  To get the silver recycled, I'm finding it's better for small operations to collect silver-bearing wastes in a container and have it picked up by a recycler or broker.  In my area, these services require a minimum of 5 to 25 gallons collected on site for pickup.  The receiving facilities report that silver is difficult to recover from solutions that are stored for long periods of time.  These barriers can make it difficult for small operations to implement a silver recycling.

Photoprocessing solutions are not all equal.  The chemistries used in graphic arts, color photography, and x-ray developing are different.  Treatment systems may not operate properly with some solutions or if the wrong solutions are mixed together.

Testing Photoprocessing Wastes
Wastes discharged to the sewer or treated residues disposed into the garbage should be tested to ensure compliance with environmental regulations.  However, we have experienced multiple difficulties analyzing samples for silver.

One of the first lab steps in analyzing a solution for total silver is nitric acid digestion, intended to dissolve metals in the sample.  However, with photographic solutions, adding nitric acid breaks up a complex that was keeping silver in solution, causing silver to precipitate.  The sample is then filtered (i.e., the silver precipitate is removed), then tested for silver and a false negative is the result.  This testing interference is well documented and experienced chemists can make appropriate method adjustments.  The obstacle to overcome is primarily communication. 

Our most dramatic example of a false negative was in an untreated sample of waste "Black Dot" activator from a screen print shop.  With nitric acid digestion the total silver result was 0.28 ppm; with alternate sample preparations (four alternate methods were tried) the results were 2.6 ppm, 40 ppm, 968 ppm and 83,500 ppm.  Remember that these results are all from the same sample.  These analytical problems can occur with any photographic or film-developing waste test and are not uniquely associated with the Chemgon product.

To improve silver testing, these are my suggestions.  I attach a copy of these suggestions to the chain of custody when I submit a sample for silver analysis:

Our Knowledge of Chemgon
I cannot provide any definitive statement about the Chemgon product's performance.  I suggest anyone that uses Chemgon have the treated waste properly tested following the guidance above.  Then, check with your local solid waste utility or health department to ensure it meets their criteria for disposal as regular garbage.  In King County, silver is not the only issue to resolve (free liquids, persistence and toxicity were also evaluated).  Requirements may differ in municipalities where wastes are incinerated rather than landfilled.

In 1995 our program worked with screen printers, who develop black and white film as part of the screen making process.  Chemgon was marketed by local suppliers as a way for small quantity generators to treat and dispose of waste photo chemicals to the garbage.  Like Marlyn, we wanted to know if this stuff really worked.  Test results presented in Chemgon product literature state "0 ppm" without providing the untreated solution silver concentration.  Phone calls to the manufacturer did not yield any further data.

Samples of spent "Black Dot" activator from screen printers were tested for total silver, then treated with Chemgon, then the treated samples were tested for silver again (total and TCLP).  Most of these sample results did exceed TCLP levels.  However, we do not view this data as reliable because of problems with lab testing and false negatives.

In December 2000 we sampled a used Chemgon kit filled with spent photographic activator and stabilizer from a local print shop (making plates).  I disposed of leftover sample in the garbage with a clearance from our local Health Department as it met solid waste disposal criteria, including TCLP limits for metals.  For this testing we followed the guidance presented above.

We have not tested the Chemgon kit on other sources of silver-bearing wastes such as x-ray developing or photoprocessing.

Here are some other observations about Chemgon as a treatment method:

Alice I. Chapman, PE
King County Hazardous Waste
130 Nickerson St, Suite 100
Seattle, WA  98109-1658
206-263-3058 (ph)
206-263-3070 (fax)

http://www.metrokc.gov/hazwaste