This study investigated how the particle size of the modified adsorbent diatomite earth & chitosan (DE&C) affects the removal of sunset yellow (SY) from water using a fixed bed column. Fourier Transform Infrared Spectroscopy (FTIR) analysis indicated that the amino group (–NH₂) played an important role during the adsorption of SY. The calculated surface area of the DE&C absorbent was found to be 69.68 m² g⁻¹ and SEM images showed that DE&C is a superior porous adsorbent. Four dimensions of DE&C were measured in a fixed bed column. The breakthrough time of SY increased as the size of DE&C decreased and the effluent concentration showed a trend of first increasing and then decreasing. It was also found that the nature of the variation in the effluent concentration with time was parabolic and that the adsorption data fitted the parabolic equation at C_t/C₀ = 0–0.1. Due to the different combination ability with hydrogen bonding, the phenomenon of parabolic variation is quite disparate from other adsorbents. But, compared with the breakthrough time, the saturation time was close to other particle sizes, except for 2.86–3.35 mm in size. pH is an important parameter for the adsorption of SY and the main mechanism is adsorption with each other between positive and negative charges. The Thomas model shows lower correlation with the smaller sizes and R² with an average of 0.9413. The Thomas model indicated that the maximum value of q₀ was 97.06 mg g⁻¹ and with an increase in the initial SY concentration, the bed height and the values of q₀ decreased as k_t increased. The Yoon–Nelson model describing the theoretical required time was close to the experimental values and k_YN increased as C₀ increased.