Hydrothermal carbonization (HTC) is a promising technology for producing char material (hydrochar) from waste biomass. In the present work, a two-stage process was applied and optimized to obtain a composite Fe-loaded hydrochar effective in removing arsenic from water. The first stage of carbonization of the biomass in acid conditions was followed by loading Fe3O4 in the second stage into the hydrochar by alkaline co-precipitation. The effect on the kinetics and on the final yield of HTC induced by a variation of the initial acid pH (5.6, 2.0, and 0.5) was tested. Biomass hydrolysis initially decreased the hydrochar yield and released soluble organic species, responsible for the observed pH variation. This effect was more remarkable at the lower initial pH tested. Soluble organic compounds eventually underwent polymerization, with secondary char formation, an inversion of the pH trend and an increase of hydrochar yield and C%. The final pH attained was linearly related to the hydrochar C%, O/C ratio, and initial pH. Better carbonization performances were achieved at pH = 0.5, 200 °C, and 30 min reaction time, which resulted in 53% mass yield and 72 C%. This value is larger than those previously reported for processes conducted at higher temperatures, and it shows how the addition of acid allows working at lower operative temperatures. Fe loading gave better yields at lower hydrochar concentrations, producing an adsorbent with up to 74% Fe3O4, which adsorbed 2.67 mg/g arsenic. Its adsorption capacity was remarkably affected by the stirring method used, indicating that particle-to-particle interactions considerably influence the process. This effect should be better studied for improved applications in fixed-bed columns.