Characterization and application of microalgae hydrochar as a low-cost adsorbent for Cu(II) ion removal from aqueous solutions

Hydrochar prepared from the hydrothermal liquefaction of microalgae is characterized and investigated for copper removal from aqueous solution. Two hydrochars were prepared at 210 °C (HD210) and 250 °C (HD250). The effect of the initial solution pH, the initial Cu(II) concentration, the contact time, and the temperature will be investigated. According to the elemental analysis, the volatile matter in the hydrochars was lower and ash content was higher than those of microalgae. Also, pore characteristic analysis revealed that the surface area of the HD250 was higher than that of the HD210 suggesting a higher potential for the adsorption process. FTIR analysis and Boehm titration showed that both hydrochars contained oxygen-containing functional groups (OFG) on the surface which were effective for the copper removal. The adsorption experiments indicated that the amount of copper adsorbed reached a maximum value at the pH of 5 which was considered as the optimum solution pH. In addition, HD250 had a higher amount of copper adsorption than that of HD210 at all values of the solution pH. The adsorption data at the optimum solution pH was well fitted by the Langmuir’s isotherm model and the adsorption process could be well described by the pseudo-2nd order kinetic model. Moreover, thermodynamic analysis revealed that copper adsorption onto the hydrochar was a physical endothermic process.