Soil Solution Sampling and Water Sampling Filtration

July 19, 2010

There is a requirement to filter samples on site for all metals analysis. This is done with a .45 micron filter. The filtration is a requirement for METALS only and other analyses must be made WITHOUT filtration as some compounds may form colloids which are too large to pass through this pore size.

A further discussion has arisen concerning analyses on soil solutions collected by ceramic or polysulfon plastics (Rhizon) which have a 0.1 micron pore size.

I attach here some (unaltered so sorry for the spelling errors!)  email correspondence and invite responses or comments which may add to these discussions:

Vincent,

Good question, simple answer. Dissolved metals are present as ions (i.e Zn2+), dissotiation of the salts. How much dissolves, depends on the solability product of the salt and the pH (also on pressure en temp. but these are pretty much constant in soils).  

Sampling with rhizons is not a problem at all. The problem is sampling with porous material on which the metal ions can adsorb (metal lattice, or ordinary ceramics)

 Regards,

 Jan Frank Mars

An interesting discussion indeed!

My question related specifically to the Rhizon samplers.

Now forgive my probably very stupid question but what I was getting at is this:

Leaving colloids aside, when metals dissolve in water, are they present as atoms or as molecules or both. I assume that at atomic level anything will pass through 0.1 micron holes but if the metal is also present as part of a molecule; are most molecules small enough to pass through a 0.1 micron slot?

Have a great week-end

Vincent

Gents,
Nice discussion, interesting though! and Gerard is very much on the right track. First TOC and pesticides are large non-polar molecules. They hate water and love to stick together as colloides, and wont pass micropores as Gerard discribed. Therefore sampling TOC with soil moisture samplers is not a good idea, of course 1 or 2 molecules always will pass but reprecentabillity will be poor and volitiles are lost 4-ever. As Gerard wrote you need a monitoring well to sample these compounds with allt the discussions about micro-purging turbidity aso.

All the radioactive elements -minus tritium – are metals. These should be sampled as metals, meaning a 45 micron filtration. A bad or no filtration at all – will over estamate the real concentration. Vincent see example in the presentation of groundwater sampling wich is send to us from this Irish guy. There you see the difference in filtration or no filtration.
Why is this interesting, because at present there is a huge discussion about measuring metals in soil. Some scientist will argue that for example mercurysulphate is so stable it wont dissolve at all and stick to the soil matrix for the next bilion years. However during analyses all the mercury is dissolved and there is no distinction between the oxidation state of the element.

My problem is tritium this is a waste product for nuclear plants it is a hydrogen atom with 1 proton and 2 neutrons. yes it can be found in hydrogen gas (H2), but why not in water H2O, with the heavy hydrogen atom or any othor non organic compound which contains hydrogen? Next question is then what do you analyse in the lab??

That is something I’m trying to found out. But again maybe I’m complety wrong and tritium is only found in hydrogen gas (H2) and so instable it directly falls back into a more stable isotope and emitting radioactive radiation.
I’m not a specialist here, but ttry to find out.

Best regards,

Jan Frank

> Hello Vincent and Jan Frank
> 
> A straw punctured with many many holes of 0,1 (or a bit more in) microns would work like a sampler with which you could take water from the UNsaturated zone of the soil (Rhizons and fine porous ceramics work like that). If the holes are above 1 micron (by heart) applying a vacuum will cause air to pass instead of retaining it and only letting pass water. In other words; a straw punctured with “large” holes will operate like a piezometer; it will collect water only from the saturated zone (and a vacuum does not help). Sorry for the scientist.
> 
> Then the TOC and elements question:
> I am sure that (after letting establish an equilibrium, so throwing away the first samples) the mentioned elements will pass. Dissolved tritium will pass also, except if the vacuum will degass it from the sample (with limited underpressure the recovery on tritium would be higher). Other elements, even the large uranium molecules, are far smaller than the 0,1 micron of the polysulfon plastic. However when adhered to colloids (like tannine which is present in peat soils) they may not pass as the colloids surpass the diameter of the pores. Same counts for pesticides and similar hydrophobic organics like PAH. Their state of emulsion, rather than completely dissolved state, will prevent them from passing through the rhizon membranes. This in fact is, I believe, much underestimated. For  myselfI I can say that this knowledge is very  recent but should be mentioned somewhere in the manual at some time and place. I am slightly astonished that Mr. Frits Meiboom, the former producer of the Rhizons was a real scientist. I will need to discuss this with the new producer and see how we should adapt the manual.
> It is also strange that none of the scientist using the Rhizons ever mentioned (to my knowledge) this limited recovery. On the other hand, where can they compare it with ??
> 
> Hope this info helps. Curious to see if Jan Frank has something to add that could be of more help to me or you.
> 
> Gerard
>
> ________________________________
>
>> Hi Jan Frank
>
> A thought has occurred to me following a site visit on Wednesday with
> some researchers from the University of Manchester. We were discussing
> the use of Rhizon samplers for their experiments and they told me they
> had experienced a lot of problems collecting dissolved organic carbon
> (DOC) after filtration through a .45 micron filter and the Rhizons, from
> literature have .15 micron  (actually this is from the Eijkelkamp
> catalogue but their website says .1 micron as do the instructions for
> the macro rhizons?) pore size. My thoughts are that we should check with
> Rhizosphere to ask whether the radiological elements XXXX  wishes
> to sample for will pass through the membrane or do you know?
>
> I’m copying this email to Gerard in case he has some ideas (Gerard the
> elements we want to extract are Tritium (although I thought this was a
> gas?) Technicium, Strontium 90, Cesium, Uranium)
>
>

> Vincent van Walt
> 01428 661 660
> www.vanwalt.com

I would invite others to participate in this discussion and you can post comments.

Thanks

Vincent van Walt