Adsorption Kinetics of Cadmium and Lead by Biochars in Single- and Bisolute Brackish Water Systems.

Cd(II) and Pb(II) adsorption from brackish water by activated carbon (AC) and biochars derived from bamboo (BB), palm shell (PSB), and mangrove wood (MB) in single- and bisolute systems was investigated. Physicochemical characterization including SEM, FTIR, pH, pHPZC, elemental analysis, proximate analysis, XRF, iodine number, BET surface area analysis, and TGA was carried out. The adsorption of Cd(II) and Pb(II) was in the following order: AC > BB > MB > PSB and was higher in single-solute than bisolute systems with greater Pb(II) adsorption efficiency than Cd(II). Salinity negatively affected metal sorption, particularly for Cd(II), but higher pH enhanced removal. Upon increasing the salinity from 0 to 25 ppt, the removal efficiency of BB was reduced from 75.9 to 52.2% (Cd) and 91.1 to 80.5% (Pb) in the single-solute system. In addition, the removal efficiency was decreased from 71.6 to 41.3% for Cd(II) and 90.9 to 76.3% for Pb)(II) in the bisolute system. The removal trend of the adsorption system of BB with 0 ppt salinity responded positively upon increasing pH from 5 to 8, and the removal of Cd(II) was increased from 54.4 to 75.8% and that of Pb(II) was increased from 66.3 to 91.0% in the single-solute system. The adsorption kinetic data are well explained by the pseudo-second-order model suggesting that chemisorption is the rate-limiting step. The key results of the present work suggest the applicability of BB as an alternative adsorbent to AC due to its comparable physicochemical properties, such as surface area (191.95 m2/g), pore volume (0.1038 cm3/g), pHPZC (9.27), iodine number (104.2 mg/g), and the presence of hydroxyl (-OH), amine (-NH), and COO- groups necessary for metal bonding. The adsorption performance of BB is promising, and hence, it can be considered to remove the Cd(II) and Pb(II) ions from brackish water as statistically it is the least impacted by change in salinity at a confidence level of P ≤ 0.05 compared to MB and PSB.

Assessing arsenic redox state evolution in solution and solid phase during As(III) sorption onto chemically-treated sewage sludge digestate biochars.

This work aimed to determine the arsenic redox state distribution during As(III) sorption onto chemically-modified biochars. A solid-liquid extraction protocol using phosphoric (0.3 M) and ascorbic (0.5 M) acids at 80 °C for 20 min was established to ensure a quantitative recovery and stability of As(III) during the extraction. During sorption experiments, the redox conversions of As occurred and As(III) was either stable or partially oxidized in solution. The As distribution strongly varied depending on the biochar chemical treatment performed as well as the selected washing procedures (batch versus column washings). As(III) oxidation was favored with the KOH-modified biochar washed in batch mode. This oxidation was mostly induced by the biochar solid compounds rather than by soluble compounds released in solution. The As redox state distribution of As sorbed onto the biochars was successfully assessed using the extraction procedure. Arsenic was predominantly sorbed as As(III) (76-92%) onto the biochars.

Lead sorption by biochar produced from digestates: Consequences of chemical modification and washing.

The main objectives of this work are to investigate the consequences of different chemical treatments (i.e. potassium hydroxide (KOH) and hydrogen peroxide (H2O2)) and the effect of biochar washing on the Pb sorption capacity. Biochars derived from sewage sludge digestate and the organic fraction of municipal solid waste digestate were separately modified with 2 M KOH or 10% H2O2 followed by semi-continuous or continuous washing with ultrapure water using batch or a column reactor, respectively. The results showed that the Pb adsorption capacity could be enhanced by chemical treatment of sludge-based biochar. Indeed, for municipal solid waste biochar, the Pb maximum sorption capacity was improved from 73 mg g-1 for unmodified biochar to 90 mg g-1 and 106 mg g-1 after H2O2 and KOH treatment, respectively. In the case of sewage sludge biochar, it increased from 6.5 mg g-1 (unmodified biochar) to 25 mg g-1 for H2O2 treatment. The sorption capacity was not determined after KOH treatment, since the Langmuir model did not fit the experimental data. The study also highlights that insufficient washing after KOH treatment can strongly hinder Pb sorption due to the release of organic matter from the modified biochar. This organic matter may interact in solution with Pb, resulting in an inhibition of its sorption onto the biochar surface. Continuous column-washing of modified biochars was able to correct this issue, highlighting the importance of implementing a proper treated biochar washing procedure.