Strange particles color in water 2

[Seda6334]

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!

 

[bimr]

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.

 

[Seda6334]

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.

 

[bimr]

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.

 

[Seda6334]

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)

 

[EdStainless]

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.

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P.E. Metallurgy, consulting work welcomed