Simulated geochemical weathering of a mineral ash-rich biochar in a modified Soxhlet reactor.

Although there are many studies on the characterization of C in biochar and its C sequestration potential, there is little knowledge on the mineral fraction in biochar and its weathering. The latter, however, can have powerful implications on nutrient availability. In the present study, a modified Soxhlet reactor was used to simulate the long-term geochemical weathering of an ash-rich biochar produced from sewage sludge of a non-industrial area in New Zealand. The weathering process took place during a period of 300 h, with and without the addition of humic acid (1.00 g added to 20.00 g of biochar), and the treatments were referred to as treatment BC-HA and BC-B, respectively. Both the leaching kinetics and the transformations within the solid phase were studied. The results revealed that substantial amounts of K (8.5-10.2%) and S (20.2-28.3%) were recovered in the weathering solutions. Noticeable Ca (17.9-20.7%) and P (15.4%) in the solid were released but only a few were recovered in the weathering solutions because of the precipitation. The presence of humic acids increased this dissolution and thus the availability of K, S, Ca, Mg and P, but induced N immobilization. Nitrogen availability was already very low (<1.0% of the total N) due to the probable recalcitrant heterocyclic N structure. The pH of the biochar samples dropped from 8.4 to 7.5; this was mainly attributed to loss of base cations through leaching and probable carbonation of the system. The XPS spectra evidenced the oxidation of C in biochar during the weathering process with the formation of carbonyl and carboxylic functional groups. The results obtained in this study showed some promise for the positive use of modified Soxhlet extractor in simulating the geochemical weathering in ash-rich biochars and providing a better understanding on the kinetics of nutrient release. This will be key information in assessing the added value of biochars as soil amendments.

Laboratory appraisal of organic carbon changes in mixtures made with different inorganic wastes.

Mixtures of organic and inorganic wastes were incubated to examine the changes in organic C (OC) contents. An anaerobic sludge and a CaO-treated aerobic sludge, with OC concentrations of 235 and 129 gkg(-1), were used. The inorganic wastes used - referred to as "conditioners" - were shot blasting scrap, fettling, Linz-Donawitz slag, foundry sand (FS), and fly ash from wood bark combustion (FA). The total OC (TOC) and KMnO(4)(-) oxidized OC were determined. DTA-TGA profiles and FTIR spectra were also obtained. Mixtures made with the FS contained significantly lower (P<0.05) amounts of TOC (45 gkg(-1)) than the rest of mixtures, which was attributed to the non-existence of reactive surfaces in the conditioner and the increased aeration induced by this material. Those made with FA contained significantly higher (P<0.05) amounts of TOC (170 gkg(-1)), which was attributed to: (i) the addition of an extra source of C - black carbon (BC) - in the FA, and (ii) the inhibition of mineralization from the compounds present in this conditioner (e.g., amorphous aluminosilicates, BC). The results highlight the importance of the characteristics of the conditioners on the fate of the OM originating from the sludges.

Reductive dechlorination of alpha-, beta-, delta-, and gamma-hexachlorocyclohexane isomers by hydroxocobalamin in the presence of either dithiothreitol or titanium(III) citrate as reducing agents.

The effect of the reducing potential on the reductive dehalogenation of the different HCH (hexachlorocyclohexane) isomers has not yet been studied. In the present study, the potential for dehalogenation of (alpha-, beta-, delta-, and gamma-HCH isomers by the dithiothreitol (DTT) and titanium(III) citrate (reducing potential at pH 7, -0.33 and -0.48 V, respectively), with and without the addition of hydroxocobalamin was investigated. In the presence of DTT without catalyst, there was no disappearance of any of the HCH isomers studied after 1 h of treatment. However, disappearance of the gamma- and alpha-HCH isomers was observed during the same time period when titanium(III) citrate was used as the reductant in the absence of catalyst (62.9 and 16.6% disappearance, respectively). Addition of the hydroxocobalamin to the DTT system favored mainly the disappearance of gamma- and alpha-HCH (92.9 and 30.8% disappearance after 1 h, respectively); disappearance of delta-HCH and beta-HCH was small (11.9%) or negligible, respectively. Addition of the hydroxocobalamin to the titanium(III) citrate system favored the degradation of all HCH isomers under study: beta- and alpha-HCH completely disappeared to undetectable levels (<0.1%) after 1 and 2 min, respectively; degradation of delta-HCH and beta-HCH was slower than that of the other two isomers, although they had almost completely disappeared (99.9 and 99.6% disappearance, respectively) after 10 and 60 min, respectively. The order of disappearance, gamma-HCH > alpha-HCH > delta-HCH > beta-HCH, coincided with a decreasing order of the axially positioned Cl atoms of these isomers (considering their thermodynamically most stable configuration). This study is the first description of the rapid degradation of delta- and beta-HCH under abiotic conditions, and the results demonstrate the effect of the reducing potential on the reductive dehalogenation of HCH isomers.