Removal of Pb2+ using a biochar–alginate capsule in aqueous solution and capsule regeneration

• This study investigated removal of Pb2+ by biochar–alginate capsule (BAC).
• Combining biochar and alginate greatly improved the sorption of Pb2+ (263.2 mg/g).
• The BAC showed a high desorption feasibility ranging from 90 to 95%.
• BAC maintained a sorption capacity of 70% even after 10 instances of recovery.
• BAC was showed high economic efficiency for Pb2+ removal as compared to other adsorbents.

A biochar–alginate capsule (BAC) was successfully synthesized by dropping a mixture solution of biochar (BC) and calcium chloride (CaCl2) into an alginate (AG) solution, with adjusting the mass ratio of BC to AG. The adsorption characteristics of the BAC were investigated by measuring the removal of lead ions (Pb2+) from an aqueous solution. The maximum adsorption of Pb2+ was found at a pH of 5.0 and an adsorption time of 120 min. The lead adsorption equilibrium data of the BAC fit well with the Langmuir and Freundlich equations. Based on the Langmuir isotherm, which was a better fit than the Freundlich isotherm, the maximum sorption capacity of Pb2+ by BAC (BC:AG = 8:1) was 263.158 mg/g, which is much higher than the capacity for other adsorbents. The sorption adequately followed the pseudo second-order kinetic expression (R2 = 0.997). The BAC was easily collected after the Pb2+ adsorption process and highly recoverable (90–95%) using a HNO3 solution. The adsorption efficiency of Pb2+ by the recovered BAC remained around 70% of the initial adsorption capacity even after being recycled 10 times (repeated adsorption–desorption).

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