Activated Sand

Activated Sand is used for filtration. In short, the sand grains are reacted with calcium hydroxide or other concentrated alkali/alkaline mineral over heat then packed in a column. This produces calcium silicate on the surface of the grains. After you add this alkaline sand into the column that you want to use for filtration, a dilute acid like dilute phosphoric acid is run through the sand. This dissolves the calcium silicate into silica gel. Therefore the surface of the sand grains are coated in silica gel which is negatively charged and can bind cations. If you did it right, the water should flow through the column much slower than before the activation.

Slow sand filter

Slow sand filters actually work by creating a miscroscopic silica gel layer on the sand grains. This is why they say the filter has to "mature" before it starts working, because this gel layer needs to form. Slight acidity from microbe/algal growth in the top of the filter is what causes it. Also mucus like polysaccharides emitted from the microbes also contributes to filtration.

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