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Strange particles color in water

Strange particles color in water

Strange particles color in water

Hi everybody.
Hope all is well.

I have a seawater with hardness of 470 °dH (Mg=1730 mg/l and Ca=506 mg/l) at pH 7.9 with a lot of fine particles that settle down quickly. Negligible organics content. The initial color of the particles are white/yellowish. Yellowish/light brownish on membrane surface (after some aeration). The Fe and Mn (total and dissolved) are lower than measurement limits (0.002/0.003 mg/l).
When I adjusted pH to 6.9 and aerated shortly, the particles color on membrane turned dark gray/black.

I was thinking that it could be Mn or Fe, but frequent external lab analyses showed no trace.
Any idea what could be happening here?

Very much appreciated!

RE: Strange particles color in water

Most likely the particles are calcium and magnesium. Filter the particles on a filter paper and have them analyzed in a laboratory.

Seawater is supersaturated with respect to calcium carbonate.

RE: Strange particles color in water

Thanks. but how come grey/black particles?
As far as I know, the Ca and Mg and their oxides have light color (white,...). I am confused here.

RE: Strange particles color in water

You are not giving much to go on. What is the capacity, the intake, the pretreatment, the complete water analysis, etc.

Are you chlorinating the raw water?

Black could be a number of things, iron reducing bacteria, sulfur, manganese.

Again, the best thing is to filter the material out a Whatman paper and have the laboratory analyze.

RE: Strange particles color in water

Fe and Mn are lower than measurement limits (almost nothing. sulfur, I do not know. the major parameter was hardness.

Experiment was 2 stages.
1. direct seawater filtration (without any pretreatment or chemicals, without pH adjustment) which resulted in yellow/brownish fouling on membrane and in the tank.
2. direct seawater filtration (without any pretreatment, but with pH adjustment to 6.9)

RE: Strange particles color in water

Color is not an intrinsic material property; it is related to the surface texture and how it interacts with light.
At lower pH you may be forming a different morphology and changing the reflection of the surface.
It could even just be surface texture of the crystals.
Though microbiological activity could be a part of this also.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

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