The presence of potentially toxic elements (PTEs) in drinking water and the food chain is a well-known hazard to human health. Among PTEs, mercury is particularly dangerous for humans and other living organisms due to its wider effects on internal organs. Hg contamination is a critical issue for water bodies used for aquaculture, making its elimination mandatory. Among the techniques proposed for Hg removal, adsorption is advantageous because of its versatility, absence of secondary pollution, and relatively low cost, especially when adsorbents can be obtained from waste materials. In this article, adsorbent materials are synthesized by introducing thiols and primary amino groups into cellulose fibers isolated from soybean hulls. After characterization, the ability of the materials to remove mercury from both ultrapure and aquaculture water solutions is tested. The results confirm the affinity of Hg for thiol groups, leading to the adsorption of 44 mg(Hg)/g in a wide pH range. The amino-modified material adsorbs ≈50% Hg less than the thiol-functionalized one. Test in real water shows that organic matter and salts influence the Hg adsorption process, without affecting the overall efficiency. Finally, in real water, a final concentration below the Hg legal limit for human consumption (1 μg L) is found.