Optimizing wastewater treatment processes requires a thorough understanding of how various parameters affect the efficiency of adsorbent materials. For the removal of dyes like Direct Blue 86 (DB86) using cellulose hydrogels (CAH), the solution's pH is a particularly crucial factor. The pH influences both the surface chemistry of the adsorbent and the state of ionization of the dye molecules in the solution, thereby dictating the strength and nature of their interaction.

Cellulose hydrogels, when synthesized and prepared for adsorption, possess surface functional groups, primarily hydroxyl groups. The behavior of these groups, and thus the overall surface charge of the hydrogel, is highly dependent on the pH of the surrounding medium. In acidic conditions, the hydroxyl groups can become protonated, leading to a net positive charge on the CAH surface. Conversely, in alkaline conditions, these groups can deprotonate, resulting in a net negative surface charge.

Direct Blue 86, being an anionic dye, carries a negative charge in aqueous solutions. This inherent characteristic is central to its interaction with adsorbents. When the cellulose hydrogel is submerged in an acidic solution (low pH), its surface becomes positively charged. This positive surface charge creates a strong electrostatic attraction towards the negatively charged DB86 dye molecules. This electrostatic force is a primary driver for efficient adsorption, facilitating the transfer of dye from the water column to the hydrogel surface. Consequently, studies have shown that the highest removal efficiency for DB86 dye using CAH is observed at an optimal pH of 2.

As the pH of the solution increases towards neutral and then alkaline conditions, the surface charge of the cellulose hydrogel begins to shift. The increased concentration of hydroxide ions (OH-) in alkaline solutions can lead to the deprotonation of the hydroxyl groups on the CAH, making its surface increasingly negative. This negative surface charge then causes electrostatic repulsion between the adsorbent and the anionic DB86 dye molecules. As a result, the adsorption capacity and removal efficiency decrease significantly as the pH moves away from the acidic optimal range.

The research findings indicate that as the pH increases from 2 to 6, the removal percentage of DB86 dye by CAH drops considerably. While there might be slight fluctuations or a minor increase in removal between pH 6 and 8, the trend clearly shows a reduction in efficiency in alkaline environments. This behavior is consistent with the electrostatic interactions governing the adsorption process. Understanding this pH dependency is vital for designing effective wastewater treatment systems. By carefully controlling the pH of the influent water, the performance of CAH as a dye adsorbent can be maximized, ensuring a more efficient and sustainable approach to managing textile dye effluents.

In summary, the pH of the solution plays a pivotal role in the adsorption of Direct Blue 86 onto cellulose hydrogels. Acidic conditions favor adsorption due to electrostatic attraction, while alkaline conditions hinder it. This critical insight allows for the optimization of treatment processes, making cellulose hydrogels a highly effective and adaptable solution for tackling dye pollution in water.