Hard to say without data. Generally when I've performed adsorption experiments in past (for metals, not dyes, granted), when observing that some species did not adsorb well I tried to analyze it from the properties of the biochar (which you also need to study). Biochar pH, SSA, mineral content, functional group content, and zeta potential can all influence adsorption when it comes to metals, other factors apply for dyes of course. I would at least say from that list that zeta potential and functional groups could play an important role in determining adsorption capacities for dyes however.

Thanks for your consent.

there could be many reasons for the adsorption process depending on many factors. one simplest reason can be the surface charge, if your biochar has a highly negative surface it will rarely adsorb anionic dyes or molecules.

Pre-treatment: The preparation and pre-treatment of biochar can also impact its adsorption capabilities. Puffed rice biochar might not have undergone suitable activation or modification processes to enhance its adsorption properties.

Chemical Properties: The chemical composition of the biochar can influence its adsorption capacity. Some biochars may have functional groups on their surface that enhance their ability to adsorb certain pollutants. Puffed rice biochar may lack these functional groups or have an unfavorable chemical composition for dye removal.