The main objectives of this work are to investigate the consequences of different chemical treatments (i.e. potassium hydroxide (KOH) and hydrogen peroxide (HO)) 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% HO 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 for unmodified biochar to 90 mg g and 106 mg g after HO and KOH treatment, respectively. In the case of sewage sludge biochar, it increased from 6.5 mg g (unmodified biochar) to 25 mg g for HO 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.
This work seeks to extend the knowledge on the effect of chemical treatments of sewage sludge digestate (SSD)-derived biochar for As(III and V) and Cd(II) sorption ability using potassium hydroxide (KOH) or hydrogen peroxide (H 2 O 2). Results showed the increases of pH of point of zero charge, the Brunauer-Emmett-Teller (BET) surface area and cation exchange capacity (CEC) after chemical treatments of biochar. The sorption ability was enhanced from 1.6 µmol g-1 (As(V)) and 16.1 µmol g-1 (Cd(II)) on raw biochar to 8.5 µmol g-1 (As(V)) and 318.5 µmol g-1 (Cd(II)) on KOH-modified biochar. Furthermore, arsenic redox distribution showed a large oxidation (70%) of As(III) to As(V) in KOH-biochar with batch washing, while a partial oxidation (7%) was observed in KOH-biochar with batch and subsequently column washing. The washing procedures after KOH treatment play an important role on arsenic sorption, due to the release of phosphate (PO 4 3-) as well as organic matter from the biochar that may subsequently lead to the oxidation of As(III) to As(V). Our findings highlight the potential influence of biochar on the redox transformation of As(III) to As(V) and therefore require a careful assessment while investigating the fate of As in aquatic environments.
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