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Re: kitty litter and all that

> Dave Gomberg takes issue with the statement:
> > >Natural, amorphous soils commonly contain significant amounts of iron
> > >(typically 5%
> Thanks for the correction Dave. I rechecked my reference and it should
> have been 0.4% not 5%. My error. :%]

Gee, I checked my old basic geochemistry text ("Geochemistry" by Mason)
and he quotes Arie Poldevaart when placing the average iron content of
sediments (aquatic soils are after all sediments) at 4.97% FeO+Fe2O3.
This characteristic would be so variable that an average like is of
rather little value.  Even in most cases where there is 5% iron in a soil
or sediment only a fraction of the iron will be in a chemically active
state.  Much will be in refractory minerals or bound up in inactive sites
in clays.

> > > It's probably not wise to combine
> > > really high light levels with significant amounts of chelated iron in
> > > solution as this seems to promote many types of algal growth especially
> > > filamentous algae.
> I will qualify really high lighting to mean 4+ wpg or partial sunlight
> and significant chelated iron to be greater than 0.2 ppm. I stand by my
> statement. I just want people to be aware that higher light intensities
> may mean making adjustments.

Actually, I can say from experience that with high light conditions you
don't need to add *any* chelated iron to have problems with hair algae

> Thanks for the new insights on other adsorption characteristics Roger.
> I'd be very interested to learn more about metal hydroxide binding of
> phosphates and the other things you mention.

You're welcome.  There are lots of geochemistry sources out there where
you can read about it.  Surface adsorbtion is one of the most important
processes involved in the transport of solutes in streams and in ground

I'd like to point out a complicating factor.  There is a very grey area
between cation exchange and surface adsorbtion.  Metal hydroxides provide
very little cation exchange capacity, but have a high tendency to adsorb
solutes on their surfaces. If the solute bound by the metal hydroxide
(phosphate for instance) happens to carry a negative charge, then the
surface of the hydroxide will gain a negative charge.  The negative charge
is balanced by an accumulation of positively charged ions (cations) in the
water near the surface of the hydroxide.  Different ions can be exchanged
in this positively charged layer, thus hydroxides that have little or no
native CEC might over time gain CEC through the solutes adsorbed on their

When we build aquarium substrates from processed end-member materials
(like sieved sand or gravel, kitty litter, vermiculite - even laterite or
peat) we are starting with simple components with simple chemical
characteristics.  The simple components gain complexity over time in our
aquariums, so the chemical characteristics of an aquarium substrate that
has aged for a year or so might be very different from the characteristics
of the same substrate when it was first built.

I'm a big proponent of patience.

Roger Miller