TUTCRIS - Tampereen teknillinen yliopisto


Biochars from solid digestates as sorbing materials for metal(loid)s removal from water



KustantajaTampere University
TilaJulkaistu - 23 toukokuuta 2019
OKM-julkaisutyyppiG5 Artikkeliväitöskirja


NimiTampere University Dissertations


Sewage sludge digestate (SSD) and the organic fraction of municipal solid waste
digestate (OFMSWD) are currently considered as alternative feedstocks for biochar production due to the high amount of the organic solid waste remaining at the end of the treatment. The pyrolysis of solid digestate is known as an alternative to promote the recycling of organic wastes and generate added-value bio-products (e.g. biochar). Generally, the digestate biochar has a much lower sorption capacity for metal(loid)s compared to activated carbons. Therefore, chemical treatment is considered as a potential option to improve the biochar surface properties and thus inducing a better sorption ability for metal(loid)s on the biochar surface.

In this present work, the SSD and OFMSWD derived biochars were treated with 2 M KOH or 10% H2O2 followed by batch washing or batch and subsequent column washings with ultrapure water. The physicochemical properties including the pH of point of zero charge (pHPZC), the Brunauer-Emmett-Teller surface area (SBET) and cation exchange capacity (CEC) were determined for all the biochars in order to link their improved surface properties to the enhanced sorption ability for metal(loid)s. All the biochars were then used to study the influence of chemical treatment and biochar washing procedure on the sorption behavior of Pb(II), Cd(II) and As(III, V) through the batch sorption kinetics and isotherms. Moreover, the As redox state distribution (i.e. As(III) and As(V)) during the As(III) sorption onto the biochar surface and in liquid solution was determined by using solid-liquid extraction followed by liquid chromatographic analysis.

Results showed increases of the pHPZC, SBET and CEC after chemical treatment of the biochar, in accordance with the enhanced sorption ability for Pb(II), Cd(II) and As(V). For instance, the maximum sorption capacity (Qm) was increased from 1.6 µmol g−1 (As(V)) and 15.4 µmol g−1 (Cd(II)) on the raw SSD biochar to 8.1 µmol g−1 (As(V)) and 306.1 µmol g−1 (Cd(II)) after the H2O2 and KOH treatment, respectively (at initial pH 5.0). Similarly, the Qm of Pb(II) was also increased from 31.4 µmol g⁻1 (raw SSD biochar) to 121.9 µmol g⁻1 on the H2O2 modified SSD biochar. However, the sorption capacity for Pb(II) was not determined after KOH treatment due to the failing of the Langmuir isotherm model to fit the experimental data. This indicates that insufficient washing of
the KOH-modified SSD biochar can hinder the Pb(II) sorption due to the release
dissolved organic compounds from this biochar that may interact with Pb2+ and thereby forming Pb-ligand complexes in the solution. In addition, the As redox distribution showed a large oxidation (70%) of As(III) to As(V) in KOH-modified SSD biochar with batch washing, while As(III) was partially oxidized (7%) in the KOH-modified SSD biochar with batch and subsequent column washings. This highlights an important role of washing procedure for sorption of metal(loid)s, particularly for Pb(II) and As(V).

The As extraction followed by liquid chromatographic analysis was successfully
established to quantitatively recover and preserve As(III) oxidation with the use of ascorbic acid. During the sorption kinetics, As(III) may be stable or partially oxidized depending on the biochar treatment. In addition, the oxidation of As(III) was strongly induced by the biochar material and to a lesser extent by the release of dissolved compounds from the biochar.

In summary, digestate biochars with the chemical treatment followed by a proper
biochar washing procedure can be successfully used as potential sorbents to enhance the Pb(II), Cd(II) and As(III, V) sorption capacity. Moreover, the determination of As redox distribution on the biochars and in liquid phase during the sorption process can be achieved through the As extraction and chromatographic analysis, providing a better understanding of the transformation between As(III) and As(V) in the biochar-liquid sorption system.