The mechanisms of calcium-catalyzed graphenization of cellulose and lignin biochars uncovered.

A recent study has shown that highly crystalline graphene-based materials can be obtained from poorly organized carbon precursors using calcium as a non-conventional catalyst. XRD and TEM analyses of calcium-impregnated cellulose and lignin biochars showed the formation of well-ordered graphenic structures (Lc > 7 nm, d002 < 0.345 nm) above 1200 °C, far below the standard graphenization temperatures (T > 2000 °C). Herein, we propose new insights on the mechanism controlling the formation of highly graphenic biochars using Ca as a catalyst. We postulate that the calcium-catalyzed graphenization occurs through the formation of a metastable calcium carbide by reaction between CaO particles and amorphous carbon between 1000 and 1200 °C. CaC2 decomposes into calcium vapor and a graphenic shell covering the CaC2 particles as confirmed by TEM analysis. The thickness and planarity of the graphenic shell increase with the CaC2 initial particle size (between 20 and 200 nm), and its growth is controlled by the diffusion of the calcium vapor through the graphene layer. A much effective graphenization was obtained for the lignin biochars compared to cellulose, with Lc > 10 nm and d002 < 0.340 nm, attributed to the insertion of sulfur in the graphenic shells, which favors their ruptures and the decomposition of CaC2 into graphene. We believe that these findings would enable the reduction of costs and environmental impact of graphene-based materials synthesis using cheap and abundant renewable feedstocks and catalysts as well.

Phosphorus recovery from municipal sludge-derived hydrochar: Insights into leaching mechanisms and hydroxyapatite synthesis.

Hydrothermal liquefaction has the potential to exploit resources from municipal sewage sludge. It converts most organics into a liquid biofuel (biocrude), concentrates P in the solid residue (hydrochar), and consequently enables its efficient recovery. This study thoroughly evaluated the effects of extraction conditions on P and metal release from hydrochar by nitric acid. Among assessed factors, acid normality (0.02-1 N), liquid-to-solid ratio (5-100 mL/g), and contact time (0-24 h) had positive effects while decreasing eluate pH (0.5-4) improved leaching efficiencies of P and metals. Notably, eluate pH played a dominant role in P leaching and pH < 1.5 was crucial for complete extraction. P and metal leaching from hydrochar have strong interactions and their leaching mechanism was identified as product layer diffusion using the shrinking core model. This suggests that the leaching efficiency is susceptible to agitation and particle size but not temperature. Using 10 mL/g of 0.6 N HNO3 for 2 h was considered the best extraction condition for efficient P leaching (nearly 100%) and minimization of cost and contaminants (heavy metals). Following extraction, adding Ca(OH)2 at a Ca:P molar ratio of 1.7-2 precipitated most P (99-100%) at pH 5-6, while a higher pH (13) synthesized hydroxyapatite. The recovered precipitates had high plant availability (61-100%) of P and satisfactory concentrations of heavy metals as fertilizers in Canada and the US. Overall, this study established reproducible procedures for P recovery from hydrochar and advanced one step closer to wastewater biorefinery.

New insights on mercury abatement and modeling in a full-scale municipal solid waste incineration flue gas treatment unit.

Modeling approaches are generally used to describe mercury transformations in a single step of flue gas treatment processes. However, less attention has been given to the interactions between the different process stages. Accordingly, the mercury removal performance of a full-scale solid waste incineration plant, equipped with a dry flue gas treatment line was investigated using two complementary modeling strategies: a thermochemical equilibrium approach to study the mercury transformation mechanisms and speciation in the flue gas, and a kinetic approach to describe the mercury adsorption process. The modeling observations were then compared to real-operation full-scale data. Considering the typical flue gas composition of waste incineration facilities (high concentrations of HCl compared to Hg), it was found that a process temperature decrease results in better mercury removal efficiencies, associated with a higher oxidation extent of Hg in HgCl2, and the enhancement of the sorbent capacity. Improvements can also be attained by increasing the sorbent injection rate to the process, or the solid/gas separation cycles. An empirical correlation to predict the mercury removal efficiency from the main operating parameters of dry flue gas treatment units was proposed, representing a useful tool for waste incineration facilities. The presented modeling approach proved to be suitable to evaluate the behavior of full-scale gas treatment units, and properly select the most adequate adjustments in operating parameters, in order to respect the increasingly constraining mercury emissions regulations.

Integrating alum sludge with waste-activated sludge in co-conditioning and dewatering: a case study of a city in south France.

The unique geographical location of waterworks and wastewater treatment plant (WWTP) in Graulhet (France) profited the environmental resource integration and "Circular Economy." Alum sludge from a local waterworks was introduced to co-conditioning and dewatering with waste-activated sludge from a nearby WWTP to examine the role of the alum sludge in improving the dewaterability of the mixed sludge. Experiments demonstrated that the optimal mixing ratio was 1:1 (waste-activated sludge/alum sludge, v/v). Alum sludge has been shown to beneficially enhance mixed sludge dewaterability, by decreasing both the specific resistance to filtration (SRF) and the capillary suction time (CST). Moreover, the optimal polymer (Sueprfloc-492HMW) dose for the mixed sludge (mix ratio 1:1) was 200 mg/L, highlighting a huge savings (14 times) in polymer addition without alum sludge involvement. In addition, cost-effective analysis of its potential full-scale application has demonstrated that the initial investment could be returned in 11 years. The co-conditioning and dewatering strategy can be viewed as a "win-win" strategy for the Graulhet, France, water and wastewater industry. Graphical abstract.

Alum sludge as an efficient sorbent for hydrogen sulfide removal: Experimental, mechanisms and modeling studies.

This paper firstly reported a systematic study of using alum sludge (waterworks residue) for H2S adsorption. Various trials were performed at ambient temperature in a fixed bed column to study the effects of H2S flow rate, sorbent bed depth on the alum sludge adsorption efficiency of H2S. The Breakthrough Curves were simulated by the Thomas model, Bed Depth Service Time model and Yoon-Nelson models. The mechanisms of H2S adsorption onto alum sludge was examined by different physiochemical characterizations of exhausted and raw alum sludge. Moreover, the mass transfer coefficients were determined from mathematical descriptions of breakthrough curves. The alum sludge adsorption capacity was determined to be 374.2 mg of H2S/g, slightly decreasing with the increasing flow rate and increasing with the increasing bed depth. All the three models successfully predict breakthrough curves which could be used for scaling-up purposes. The microporous structure, alkaline pH and the inherent metal species of the alum sludge promoted the formation of metal sulphate species. This study demonstrated that alum sludge could be used as cost-effective, largely available, and efficient sorbent for H2S removal.

Hydroxyapatite-based sorbents: elaboration, characterization and application for the removal of catechol from the aqueous phase.

Hydroxyapatite (HAP) is highly considered as good sorbent for the removal of metals from the aqueous phase. However, soluble metals co-exist with organic pollutants in wastewaters. But little work has been devoted to investigate the reactivity of HAP for the removal of organic compounds. The main objective of this work is to study the reactivity of HAP-based sorbents for the removal of catechol as a model organic pollutant from an aqueous solution. Thus, HAP sorbents were firstly synthesized using calcium carbonate and potassium dihydrogen phosphate under moderate conditions (25-80°C, atmospheric pressure). A zinc-doped HAP was also used as sorbent, which was obtained from the contact of HAP with an aqueous solution of zinc nitrate. All the sorbents were characterized by different standard physico-chemical techniques. The sorption of catechol was carried out in a batch reactor under stirring at room temperature and pressure. Zinc-doped HAP sorbent was found to be more reactive than non-doped HAP sorbents for the fixation of catechol. The highest sorption capacity was of 15 mg of C per gram of zinc-doped HAP sorbent. The results obtained suggest the reaction scheme of HAP sorbents with metals and organic pollutants when HAP sorbents were used for the treatment of complex wastewaters.

Modification of Hydroxyapatite with Ion-Selective Complexants: 1-Hydroxyethane-1,1-diphosphonic Acid.

Hydroxyapatite (HAP) was modified with 1-hydroxyethane-1,1-diphosphonic acid (HEDP), and its effect on divalent metal ion binding was determined. HAP was synthesized from calcium hydroxide and phosphoric acid. After calcination, it was modified with HEDP, and the influence of time and temperature on the modification was investigated. HEDP incorporation increased as its initial solution concentration increased from 0.01 to 0.50 M. Unmodified and modified HAP were characterized using Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and specific surface area analysis. Ca/P ratios, acid capacities, and phosphorus elemental analyses gave the effect of modification on composition and surface characteristics. A high reaction temperature produced new phosphonate bands at 993, 1082, and 1144 cm-1 that indicated the presence of HEDP. HAP modification at a high temperature-long reaction time had the highest HEDP loading and gave the sharpest XRD peaks. The emergence of new HAP-HEDP strands was observed in SEM images for treated samples while EDS showed high phosphorus contents in these strands. Modified HAP had a high acid capacity from the additional P-OH groups in HEDP. The P(O)OH groups maintain their ability to bind metal ions within the HAP matrix: contacting the modified HAP with 10-4 N nitrate solutions of five transition metal ions gives an affinity sequence of Pb(II) > Cd(II) > Zn(II) > Ni(II) > Cu(II). This result is comparable to that of commercially available di(2-ethylhexyl)phosphoric acid, a common solvent extractant, and the trend is consistent with the Misono softness parameter of metal ion polarizabilities.