Waste-to-wealth application of wastewater treatment algae-derived hydrochar for Pb(II) adsorption

Hydrochar, as an energy-lean solid waste, is generated from an advanced biofuel conversion technique hydrothermal liquefaction (HTL) and always leads to environmental pollution without appropriate disposal. In this study, HTL-derived hydrochar is recycled and prepared as adsorbent used for Pb(Ⅱ) removal from wastewater. As the original porous structure of hydrochar is masked by oily volatiles remained after HTL, two types of oil-removal pretreatment (Soxhlet extraction and CO₂ activation) are explored. The result shows that CO₂ activation significantly enhances the adsorption capacity of Pb(Ⅱ), and the maximum adsorption capacity is 12.88 mg g^-1, as evaluated using Langmuir adsorption model. Further, apart from oily volatiles, most inorganic compounds derived from wastewater-grown algae is enriched in hydrochar, causing a smaller surface area of hydrochar. An ash-removal alkali treatment following CO₂ activation is introduced to dramatically increase the adsorption capacity to 25.00 mg g^-1 with an extremely low Pb(II) equilibrium concentration of 5.1×10^-4 mg L^-1, which is much lower than the maximum level of Pb concentration in drinking water (set by World Health Organization). This work introduces an approach to reuse HTL-hydrochar as an inexpensive adsorbent in Pb-contaminated water treatment, which not only provides another possible renewable adsorbent candidate applied in the field of lead adsorption, but also finds an alternative route to reduce solid waste effluent from HTL process.