Adsorption of lead (II) in a single and multi-metal system by hydrochar from pulp and paper sludge

Abstract

Abstract Lead (II) is one of the most toxic heavy metals found in South African wastewater due to various mining activities. Hence, effective and cheaper methods are needed to treat this wastewater before it is released to the environment. The effectiveness of hydrochar, synthesised from pulp and paper sludge, as an adsorbent for lead (II) removal from synthetic wastewater was determined. The effect of adsorbent dosage, solution pH, lead (II) initial concentration and time on the adsorption of lead by the hydrochar was investigated. The hydrochar was synthesised at 180, 220 and 250 °C. The most favourable hydrothermal carbonisation (HTC) temperature to synthesise pulp and paper sludge hydrochar for lead (II) adsorption was 180 °C based on the temperatures investigated. The hydrochar synthesised at 180 °C were modified with sodium hydroxide to improve its metal uptake. FTIR results showed that the NaOH modification increased the concentration of oxygen containing functional groups. Furthermore, the BET surface area of the hydrochar increased after modification. The hydrochar had a cellulose crystalline structure. The sodium hydroxide modification increased the maximum adsorption capacity from 24.72 to 37.32 mg.g −1 . Moreover, the modified hydrochar synthesised at 180 °C (HC-180-NaOH) was effective in the adsorption of lead (II) in the presence of copper (II), cadmium (II) and zinc (II) where the affinity of the heavy metals towards the hydrochar was lead (II) > copper (II) > cadmium (II) ≈ zinc (II). Hence, the hydrochar, synthesised from waste material, is a potential adsorbent for the adsorption of lead (II) in industrial wastewater. Graphical abstract