Changes of sewage sludge digestate-derived biochar properties after chemical treatments and influence on As(III and V) and Cd(II) sorption

Abstract

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.

Pyrolysis of Solid Digestate from Sewage Sludge and Lignocellulosic Biomass: Kinetic and Thermodynamic Analysis, Characterization of Biochar

“…This could explain the lower capacities achieved when HCl modified biochar was used, as it lowered the pH of the solution (despite initial adjustment) compared to KOH modified biochar, which increased the pH of the solution. Another reason for the better performance of KOH modified biochar compared to HCl modified biochar is most likely due to the higher increase in the specific surface area of the biochar, as alkali treatment could significantly increase the specific surface area [108], which is one of the major factors affecting NH 4 + adsorption. The results of adsorption of heavy metals (Cu 2+ and Cd 2+ ) performed at the same experimental conditions as adsorption of NH 4 + and PO 4 3− ions are presented in Figure 5c,d.…”

“…However, the tested biochars exhibit higher biosorption capacity for biosorption of heavy metals than for NH 4 + or PO 4 3− ions. This could be related to the presence of mineral phases such as aluminosilicate, quartz, calcite and metal oxides on the biochar surface, which promote the sorption of metals [108]. The main mechanisms responsible for the heavy metal adsorption on biochar include complexation with oxygen-containing functional groups (-OH, -COOH), coordination of heavy metals with π electrons in unsaturated bonds (-CH, C=O and C=N), precipitation with different minerals such as PO 4 3− and ion exchange with positively charged ions such as K + , Ca 2+ , Na + and Mg 2+ [48].…”

Review of biochar role as additive in anaerobic digestion processes

“…Depending on the agent selected and thermal treatment conditions used, different degrees of activation can be achieved. The activated BC needs to be thoroughly washed with deionized water to neutralize its pH and to remove any remaining chemicals [104], and this procedure can contribute to a negative environmental impact of the technology. Oxidative activation that uses acidic or alkaline agents is among the most common activation methods.…”

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

“…The importance of adding ascorbic acid in the extraction solution to stabilize As(III) was assessed by applying the extraction procedure to a biochar sample with and without spiking with a known amount of As(III) just before performing the extraction step. In this study, the BKOH bat was selected due to its release of dissolved compounds and its ability to oxidize As(III) as previously reported by Wongrod et al (2018aWongrod et al ( , 2018b. 5 µg).…”