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Effects of Activated Carbon on Aquatic Trace Element Supplements - I

Shawn Keslar, Chemist, WVU NRCCE Analytical Laboratory
keslar/wvu.edu

In the recent past, I have observed several discussions on the AQUATIC-PLANTS mailing list regarding the use of activated carbon for aquarium water decolorization, and the effects of such on trace element supplements, primarily in the context of marine/reef systems. There has been very little discussion, however, on the subject of planted freshwater systems. As of late, the subject has piqued my curiosity, and the following is a short study I recently conducted.

Experimental background

  1. Materials
    1. Plantex CSM trace element mix.
      Specfications:

      Total Magnesium:1.50%
      Water Soluble (Chelated) Magnesium1.50%
      Manganese (Chelated)2.00%
      Copper (Chelated)0.10%
      Zinc (Chelated)0.40%
      Iron (Chelated)7.00%
      Molybdenum0.06%

      NOTE: Experimental solution was prepared by dissolving 1 teaspoon Plantex CSM in 1L of deionized water. This dosage is approximately what is used in a 50g tank (as per John Hadyt, the supplier of the mix). I used a 50g dosage for both Plantex and activated carbon to exaggerate any trends and make them easily observed. Two batches of solution were prepared; one served as a control sample and the other was treated with activated carbon to check for metal uptake.

    2. Hartz Activated Filter Charcoal Blend. As of yet, I do not have any data on this carbon. I will contact Hartz to attempt to obtain whatever data I can get as to source, carbon type, etc. This data will be added when I receive it. Three tablespoons of carbon (~41g) (once again dosing for a 50g tank, as per Albert Thiel, 1992). (I'll post a reference to this paper when I can locate it again).

  2. Procedure

    Both the carbon sample and the control sample were sealed into 2L polyethylene bottles. The bottles were then loaded onto a continuous agitator and rotated for 96h. Samples were taken at 0, 0.5, 1,4,8,24,48,72, and 96 hours. The samples were filtered with polysulfone 0.45 micron syringe filters and preserved with nitric acid until analysis by inductively coupled plasma emission spectrometry. Samples were tested for iron, magnesium, manganese, copper, zinc, and molybdenum.

II. Results

At present, I only have reliable data for iron, manganese, and magnesium. The samples will be retested for copper and molybdenum at a later date on a more sensitive instrument when I have time. The iron and manganese data, however, clearly show that this particular brand of activated carbon has a signficant effect on added trace metals.

As time permits, I'll post my raw data to this page; for now, though, I will include the graphs of iron and manganese concentration versus time, as well as depletion attributed to the carbon.

[Iron Depletion Graph]

[Manganese Depletion Graph]

III. Discussion

From the graphs above, it is clearly evident that adsorption of the chelated iron and manganese is occurring. The control samples show little change in the metal concentrations, ruling out both adsorption to the bottle material as well as oxidation of the chelated species.

When the background information on the carbon is made available to me, I will post it. This information is vital for a full explanation of the above results. I feel that these results may be more typical of a microporous carbon, which contains smaller pores and is generally used to remove small organic molecules (such as our chelates) from solution. I would like to run a comparable series in the future with a known macroporous carbon to compare the results. My feeling is that the offending yellowing compounds in our tanks are most likely humic and fulvic acid derivatives produced by incomplete breakdown of organic matter. These compounds have molecular weights and sizes that are quite large relative to the metal chelates that we are adding to feed our plants. I feel that the use of a macroporous carbon may alleviate some of the problems with adsorption of our expensive fertilizers; macroporous carbons should preferentially adsorb the larger humic and fulvic acids.

IV. Future lines of experimentation

As above, I'd like to do a study with a known macroporous carbon to check out my theory. In addition, if anyone reading this would like me to test a particular type of carbon, let me know. If you can send me a sample, I can probably work it in somewhere.

I'm also keeping in mind a study on nitrogen and phosporus trapping by activated carbon, and maybe even an attempt to identify some of the nasty yellowing compounds that accumulate in our tanks over time. I've got a couple of other things in mind, too.
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This page was last updated 29 October 1998