AFAIK "silica sand" is quartz, i. e. highly crystalline SiO2, while silica gel is amorphous. It would help if you could sprecify the material you used more precisely, and also what were you trying to adsorb? I am astonished that quartz should even have noticeable adsorptive properties to the extent you describe.

Deactivation of silica is generally accomplished by transforming its surface Si-OH groups to silanol groups. There are many methods to achieve this goal. I recommend  to youthe following work:

https://pure.tue.nl/ws/files/1596009/617897.pdf - full text. publishers version; Open access

@Monica: The work you cited relates to chromatography gels, but silica sand would have next to no surface hydroxy groups, or am I wrong there?

The cas number of the product i use is 14808-60-7.

I am trying to adsorb a mix of antibiotics ( penicilins, quinolones, sulfonamides, tetracyclines)

Dear colleagues,

My experience with silica is as catalyst support.

In an old work we "deactivated" or at least reduced the activity of silica related with its relatively weak acidiy (associated to silanols), incorporating by dry impregantion little amounts of sodium, and then drying and calcining the solid. This worked reasonably well. Perhaps you can try some thing similar. I'm attaching the paper.

Kind regards,

Luis