Synthesis of nitrogen-purged biochar and modification with hydrothermal activation: comparative assessment as adsorbents for effective elimination of Pb(II) and Cd(II) from wastewater

For sustainable development, wastewater treatment and recycling are inevitable; using adsorbents to treat wastewater is widely accepted. This article is aimed to synthesize adsorbents (novel biochars) from agricultural wastes and to investigate the adsorption behaviours of Pb(II) and Cd(II) on novel biochars. Nitrogen purged biochar (NPBC) was synthesized by pyrolysis with nitrogen purgation of wheat straw; whereas, steam activated biochar (SABC) was synthesized by pyrolysis-cum-hydrothermal (steam) activation. Synthesized NPBC and SABC were characterized by physical, chemical, XRD, FTIR, and SEM–EDX methods. SABC was more porous and amorphous in nature with higher specific surface area (SSA), but consisted relatively less polar surface functional groups than that of NPBC. The external layer of both NPBC and SABC were negatively charged (pH > pHPZC). Results revealed that SABC exhibited 97% and 80% recovery efficiency of Pb(II) and Cd(II), respectively, while NPBC demonstrated Pb(II) and Cd(II) removal efficiencies of 87.6% and 69.4%, respectively, at initial concentrations of 50 and 5 mg L−1. Adsorption of Pb(II) and Cd(II) upon SABC and NPBC occurred spontaneously, also exothermically, but decreased with increased reaction temperature. The maximum adsorption capacities (qmax) of NPBC and SABC for Pb(II) were 452.13 and 1117 mg g−1, whereas for Cd(II) those were 314.9 and 470.43 mg g−1, respectively, according to the Langmuir isotherm approach (R2 > 0.989). So, SABC performed better than NPBC for removing both metals from wastewater. Both NPBC and SABC exhibit substantial prospects for regeneration and consequent adsorbent recovery. Hence, the designed adsorbents can be very effective towards cationic toxic heavy metal removal from wastewater.