Submerged nanoporous anodized alumina structure for solar-powered desalination.

Development of nanoporous structures utilizing a single step of anodization technique is well recognized as a cost-effective and straightforward approach for several applications. In the current work, anodized alumina was developed with nanoporous structure by utilizing oxalic acid as an electrolyte with a continuous voltage of 40 V. The formed nanoporous structure was subjected to desalination application because of its high absorbance of broadband solar spectrum energy. The desalination setup consists of two solar stills namely conventional and modified. The developed structure is placed in the modified still to examine its performance. It was observed that the structure distributing heat to surrounding water by absorbing photon energy from the sun through the nanopores and giving an efficient pathway to the water vapours for developing effective desalination. The nanoporous structure having ~ 45 nm average diameter. Furthermore, the band gap energy of nanoporous structure was found to be ~ 2.5 eV (absorption spectrum fitting) and ~ 2.8 eV (Tauc plot). The nanoporous structure possess the visible light spectra in solar region which helps the band gaps of nanoporous structure to provide an additional supply of energy for generating more water to evaporate. Moreover, the Urbach energy of the structure is 0.5 eV which reveals less defects in the modified still. The overall distillate yield of modified still was increased to 21% in contrast to conventional. Water quality analysis was also carried out before and after the desalination experiments, and the results were within acceptable limits set by World Health Organization (WHO).

Biofiltration of n-butyl acetate with three packing material mixtures, with and without biochar.

Two cost-effective packing materials were used for n-butyl acetate removal in lab-scale biofilters, namely waste spruce root wood chips and biochar obtained as a byproduct from a wood gasifier. Three biofilters packed with spruce root wood chips: without biochar (SRWC), a similar one with 10% of biochar (SRWC-B) and that with 10% of biochar impregnated with a nitrogen fertilizer (SRWC-IB) showed similar yet differing maximum elimination capacities of 206 ± 27, 275 ± 21 and 294 ± 20 g m-3 h-1, respectively, enabling high pollutant removal efficiency (>95% at moderate loads) and stable performance. The original biochar adsorption capacity was high (208 ± 6 mgtoluene g-1), but near 70% of it was lost after a 300-day biofilter operation. By contrast, the exposed impregnated biochar drastically increased its adsorption capacity in 300 days (149 ± 7 vs. 17 ± 5 mgtoluene g-1). Colony forming unit (CFU) and microscopic analyses revealed significant packing material colonization by microorganisms and grazing fauna in all three biofilters with an acceptable pressure drop, up to 1020 Pa m-1, at the end of biofilter operation. Despite a higher price (14 vs. 123 €m-3), the application of the best performing SRWC-IB packing can reduce the total investment costs by 9% due to biofilter volume reduction.

Pyrolysed sewage sludge for metal(loid) removal and immobilisation in contrasting soils: Exploring variety of risk elements across contamination levels.

Efficient treatment of sewage sludge may transform waste into stable materials with minimised hazardous properties ready for secondary use. Pyrolysed sewage sludge, sludgechar, has multiple environmental benefits including contaminant sorption capacity and nutrient recycling. The properties of five sludgechars were tested firstly for adsorption efficiency in laboratory solutions before prospective application to soils. A wide variety of metal(loid)s (As, Cd, Co, Cr, Cu, Ni, Pb, Sb, and Zn) was involved. Secondly, the sludgechars (3 % v/v) were incubated in five soils differing in (multi)-metal(loid) presence and the level of contamination. The main aim was to evaluate the metal(loid) immobilisation potential of the sludgechars for soil remediation. Moreover, nutrient supply was investigated to comprehensively assess the material's benefits for soils. All sludgechars were efficient (up to 100 %) for the removal of metal cations while their efficiency for metal(loid) anions was limited in aqueous solutions. Phosphates and sulphates were identified crucial for metal(loid) capture, based on SEM/EDS, XRD and MINTEQ findings. In soils, important fluctuations were observed for Zn, being partially immobilised by the sludgechars in high-Zntot soils, while partially solubilised in moderate to low-Zntot soils. Moreover, pH showed to be crucial for material stability, metal(loid) adsorption ability and their immobilisation in soils. Although metal(loid) retention was generally low in soils, nutrient enrichment was significant after sludgechar application. Long-term evaluation of the material sorption efficiency, nutrient supply, and ageing in soil environments will be necessary in future studies.

Waste to energy: Trending key challenges and current technologies in waste plastic management.

Due to the 'forever' degrading nature of plastic waste, plastic waste management is often complicated. The applications of plastic are ubiquitous and inevitable in many scenarios. Current global waste plastics production is ca. 3.5 MMT per year, and with the current trend, plastic waste production will reach 25,000 MMT by 2040. However, the rapid growth in plastic manufacture and the material's inherent nature resulted in the accumulation of a vast amount of plastic garbage. The current recycling rate is <10 %, while the large volumes of discarded plastic waste cause environmental and ecological problems. Recycling rates for plastic vary widely by region and type of plastic. In some developed countries, the recycling rate for plastics is around 20-30 %, while in many developing nations, it is much lower. These statistics highlight the magnitude of the plastic waste problem and the urgent need for comprehensive strategies to manage plastic waste more effectively and reduce its impact on the environment. This review critically analyses past studies on the essential and efficient techniques for turning plastic trash into treasure. Additionally, an attempt has been made to provide a comprehensive understanding of the plastic upcycling process, the 3Rs policy, and the life-cycle assessment (LCA) of plastic conversion. The review advocates pyrolysis as one of the most promising methods of turning plastic trash into valuable chemicals. In addition, plastic waste management can be severely impacted due to uncontrollable events, such as Covid 19 pandemic. Recycling and chemical upcycling can certainly bring value to the end-of-life plastic. However, the LCA analysis indicated there is still a huge scope for innovation in chemical upcycling area compared to mechanical recycling. The formulation of policies and heightened public participation could play a pivotal role in reducing the environmental repercussions of plastic waste and facilitating a shift towards a more sustainable future.

Adsorption of common greywater pollutants and nutrients by various biochars as potential amendments for nature-based systems: Laboratory tests and molecular dynamics.

Spruce wood and Typha (wetland plant) derived biochars pyrolyzed at 350 °C and 600 °C were tested for their sorption affinity for organic pollutants (diclofenac, methylparaben, benzotriazole and sodium 1-decanesulfonate) and nutrients (nitrate, ammonium, phosphate and boron) commonly found in greywater. Batch and column studies combined with molecular dynamics modelling determined the sorption capacity, kinetics, and described the underlying mechanisms. The spruce biochar (600 °C) exhibited the highest sorption capacity mainly for the tested organics. The dynamic test performed for spruce biochar (600 °C) showed that the magnitude of desorption was low, and the desorbed amount ranged between 3 and 11 %. Molecular dynamics modelling (a computational tool for elucidating molecular-level interactions) indicated that the increased sorption of nitrate and boron on spruce biochar (600 °C) could be attributed to hydrophobic interactions. The molecular dynamics shows that predominant adsorption of organic pollutants was governed by π-π stacking, with a minor role of hydrogen-bonding on the biochar surface. In summary, higher pyrolysis temperature biochar yielded greater adsorption capacity greywater borne contaminants and the reaction temperature (10-34 °C) and presence of anionic surfactant had a limited effect on the adsorption of organic pollutants, suggesting efficacious application of biochar in general for greywater treatment in nature-based systems.

Complex Study of Straw Suitability for the Production of Nonindustrial Straw Pellets.

Nonindustrial straw pellets should comply with limitations on the content of ash, chlorine, nitrogen, sulfur, and heavy metals, and have a high melting temperature of ash. To produce such pellets, the properties of straw can be improved by leaching. In known papers, the completion of chlorine washing-out was not controlled. Aims of the paper were to study ash solubility at leaching of straw until completion of chlorine removal and to make a conclusion on studied straw suitability for the production of nonindustrial pellets. Aims were achieved by straw soaking with heating to 100 °C and subsequent plug flow flashing with control of leaching completion by the absence of chlorine in leachate; studying the ash, chlorine, nitrogen, sulfur, and heavy metals content of straw; studying the thermal behavior of ash at heating; determining the initial deformation temperature (IDT) of ash; and comparing the properties of original and leached straw with the specification of straw pellets. Straw leaching until completion of chlorine washing-out provided decreasing chlorine, nitrogen, and sulfur contents below limitations, and the ash content decreased from 7.15 to 3.93% at water leaching to 4.29% at leaching with a 10% solution of acetic acid. In the ternary diagram, the composition of straw ash shifted from a zone of low melting eutectics to zones of high-melting tridymite and cristobalite. The IDT of the original straw ash was 847, 1250 °C after water leaching, and above 1275 °C after leaching with an acetic acid solution. Monitoring the absence of chlorine in the leaching liquid can be applied as a control parameter for straw leaching completion. The original straw was not suitable for the production of nonindustrial pellets because of the high contents of Cl, S, and Cr and the low IDT of ash. All indexes of straw were improved due to leaching, but the Cr content was above limitation. Producers of pellets need to assess straw suitability as to heavy metal content both in the original and leached states.

Sewage sludge treatment methods and P-recovery possibilities: Current state-of-the-art.

With the growing emphasis on environmental protection, the ways of sewage sludge treatment are changing. In this review, we analyse different methods of sewage sludge treatment in terms of potential environmental risk and raw materials recovery. The review begins with a comparison and assessment of existing reviews on this topic. Then, it focuses on the properties and current utilisation of sewage sludge in agriculture and a brief description of sludge thermal treatment methods (mono- and co-incineration, pyrolysis, and gasification). The final part of the review is devoted to technologies for treating sludge ash from mono-incinerators to recover phosphorus, a substance listed as a critical raw material by the EU. Our results show that direct use of sewage sludge likewise composts containing sewage sludge should no longer be considered as a direct source of nutrients and organic matter in agriculture, because of its pollutant content. Co-incineration and landfilling represent a dead-end in sludge treatment due to the loss of raw materials, whereas pyrolysis is sustainable for remote locations with low heavy metal content sludge. Heavy metals also pose a problem for the direct use of sludge ash and must be therefore removed. There are already sludge ash processing technologies that are capable of processing ash to form a variety of raw materials such as phosphorus. These regeneration approaches are currently in their infancy, but are gradually being introduced. The sewage sludge treatment industry is rapidly evolving, and we have attempted to summarise and discuss the current state of knowledge in this review, which will provide a baseline towards the future of sewage sludge suitable treatment.

Thermal plasma gasification of organic waste stream coupled with CO2-sorption enhanced reforming employing different sorbents for enhanced hydrogen production.

In the past few years, rising concerns vis-à-vis global climate change and clean energy demand have brought worldwide attention to developing the 'biomass/organic waste-to-energy' concept as a zero-emission, environment-friendly and sustainable pathway to simultaneously quench the global energy thirst and process diverse biomass/organic waste streams. Bioenergy with carbon capture and storage (BECCS) can be an influential technological route to curb climate change to a significant extent by preventing CO2 discharge. One of the pathways to realize BECCS is via in situ CO2-sorption coupled with a thermal plasma gasification process. In this study, an equilibrium model is developed using RDF as a model compound for plasma assisted CO2-sorption enhanced gasification to evaluate the viability of the proposed process in producing H2 rich syngas. Three different classes of sorbents are investigated namely, a high temperature sorbent (CaO), an intermediate temperature sorbent (Li4SiO4) and a low temperature sorbent (MgO). The distribution of gas species, H2 yield, dry gas yield and LHV are deduced with the varying gasification temperature, reforming temperature, steam-to-feedstock ratio and sorbent-to-feedstock for all three sorbents. Moreover, optimal values of different process variables are predicted. Maximum H2 is noted to be produced at 550 °C for CaO (79 vol%), 500 °C for MgO (29 vol%) and 700 °C (55 vol%) for Li4SiO4 whereas the optimal SOR/F ratios are found to be 1.5 for CaO, 1.0 for MgO and 2.5 for Li4SiO4. The results obtained in the study are promising to employ plasma assisted CO2-sorption enhanced gasification as an efficacious pathway to produce clean energy and thus achieve carbon neutrality.

Biochar reduces the toxicity of silver to barley (Hordeum vulgare) and springtails (Folsomia candida) in a natural soil.

The use of biochar in soil remediation is a promising method to deal with metal contamination. In the present study, the influence of biochar amendment on the toxicity of silver (as AgNO3) to terrestrial organisms was assessed. For this, toxicity tests were conducted with terrestrial plant barley (Hordeum vulgare) and invertebrate springtails (Folsomia candida) in the standard natural Lufa soil amended or not with a wood-derived biochar at 5% (w/w). Biochar addition increased root length and mass in barley, compared to unamended soil. However, the effects of Ag on barley growth were masked by a great variation among replicates in biochar-amended soil. Photosynthetic pigment contents (total chlorophyll and carotenoids) were lower in plants exposed to Ag in Lufa soil, but not in biochar-amended soil. Moreover, Ag drastically decreased dehydrogenase activity in Lufa soil. For springtails, the addition of biochar clearly decreased the toxicity of Ag. The LC50 was 320 mg Ag/kg in Lufa soil, while no mortality was observed up to 500 mg Ag/kg in biochar-amended soil. The EC50 for effects on reproduction was significantly higher in biochar-amended soil compared to unamended Lufa soil (315 and 215 mg Ag/kg, respectively). The wood-derived biochar used in this study has shown a potential for remediation of contaminated soils, as a decrease in Ag toxicity was observed in most endpoints analysed in barley and springtails.

Biochar considerably increases the easily available water and nutrient content in low-organic soils amended with compost and manure.

The soil hydraulic properties of two low-organic soils (Fluvisol; Regosol) were investigated following their amendment with biochar alone or in combination with manure, compost and co-composted biochar. Self-irrigating boxes containing the soil and amendment combinations were purposed with a battery of soil moisture sensors as well as soil porewater sampling devices. Static sampling determined bulk density, porosity and derived soil water retention curves. The aim of this study was to identify the most advantageous amendment combinations to enhance soil water retention whilst simultaneously avoiding excessive nutrient leaching arising, primarily, from manure application. Biochar significantly decreased bulk density and increased total porosity when compared to compost in the Fluvisol, whereas manure affected the greatest changes in the Regosol. All of the tested amendments adjusted the shape or extent of the soil water retention curves, but biochar addition resulted in the greatest increase (⁓50%) in easily available water content (for plants) in both soils, when compared to the control. Saturated hydraulic conductivity was, however, not changed by any of the amendments which reflects a lack of influence on infiltration. An enhancement in nutrient retention occurred in some of the soil amendment configurations, such as for co-composted biochar at 2% dosage and 5% manure-biochar mixture, as revealed by porewater analysis. In summary, the application of biochar with and without additional compost and manure can enhance soil water retention in low-organic soils whilst maintaining or enhancing nutrient retention. Such finding supports the application of mixed organic amendments to low-organic (and therefore drought-prone) arable soils.

Biochar in manure can suppress water stress of sugar beet (Beta vulgaris) and increase sucrose content in tubers.

Increased soil drought events threaten the yields of sugar beet (Beta vulgaris L.) and other staples of arable production in central Europe. In this study we evaluated soil moisture and nutrients as impacted by a two and five % (wt) addition of biochar, manure and their blend to a loamy-sand Regosol. Cyclical soil drought was achieved by the controlled reduction of watering by 75% in pot experiments. Ongoing soil moisture and nutrient measurements were taken, and physiological parameters of sugar beet plants were analysed three weeks after the induced drought. At the end of the experiment (16 weeks) plants were harvested and their mass assessed, as well as their nutrient, pigment and sugar contents. In contrast to the addition of manure, soil volumetric water contents were two to three times greater after biochar amendment, compared to the control soil. Porewater analysis revealed that nutrient leaching (e.g., NO3-, K+) from manure addition to soil was reduced when biochar was blended in (by ≤86% compared to manure alone). Crop analysis showed that leaf gas exchanges were moderated during drought following soil amendment, and leaf and tuber yields were increased furthest when combined biochar-manure blends were applied (> 2-times compared to the control). Perhaps most importantly, the advantageous soil conditions induced by the combined biochar and manure addition also resulted in significantly increased sugar contents in plants (2.4-times) pointing to immediate practical applications of these results in the field.

Is application of biochar to soil really carbon negative? The effect of methodological decisions in Life Cycle Assessment.

One of the proposed solutions for improving soil conditions and mitigating climate change, two of the urgent environmental issues of today, is application of biochar to soil. This carbonaceous material made from a large variety of biomass feedstocks, by several different processes, and at various conditions is envisaged to remain stable in soil for centuries and thus effectively keep carbon out of the atmosphere. Nevertheless, a careful analysis of the entire system of biochar production and application is necessary, before such solutions are applied at a large scale, to avoid creating an even bigger environmental problem. One well-established method to do so is Life Cycle Assessment (LCA). This method has already been applied by many authors to assess biochar-to-soil projects, and based on this literature it appears that biochar application indeed contributes to climate change mitigation. However, even with such a standardized and widely used method like LCA, there are vast methodological differences between the LCA studies. Furthermore, whether the climate impact of biogenic carbon should be considered is an unresolved issue common to all assessments of biomass-utilizing projects. The effects of those and other methodological issues are investigated in the case study of the gasifier in Zlatá Olesnice. The results show that whether a project of biochar application to soil is deemed environmentally beneficial largely depends both on the context of the project as well as the methodological decisions, especially regarding biogenic carbon. Nevertheless, it can be concluded that biochar production from waste feedstock in most cases leads to climate change mitigation.

Innovative in situ remediation of mine waters using a layered double hydroxide-biochar composite.

The current demand for alternative water sources requires the incorporation of low-cost composites in remediation technologies. These represent a sustainable alternative to more expensive, commercially used adsorbents. The main objective of this comprehensive field-scale study was to incorporate the layered double hydroxides (LDHs) into the hybrid biochar-based composites and apply an innovative material to remediate As/Sb-rich mine waters. The presence of hydrous Fe oxides (HFOs) within the composite enhanced the total adsorption efficiency of the composite for As(V) and Sb(V). The kinetic data fitted a pseudo-second order model. Equilibrium experiments confirmed that the composite had a stronger interaction with As(V) than with Sb(V). The efficient removal of As(V) from mine water was achieved in both batch and continuous flow column systems, reaching up to 98% and 80%, respectively. Sb(V) showed different behavior to As(V) during mine water treatment, reaching adsorption efficiencies of up to 39% and 26% in batch and column experiments, respectively. The migration of Sb(V) in mine water was mostly attributed to its dispersion before it was able to show affinity to the composite. In general, the proposed column technology is suitable for the field remediation of small volumes of contaminated water, and thus has significant commercial potential.

Enhanced sorption of trivalent antimony by chitosan-loaded biochar in aqueous solutions: Characterization, performance and mechanisms.

Contamination of aquatic systems by antimony (Sb) is a worldwide issue due to its risks to eco-environment and human health. Batch sorption experiments were conducted to assess the equilibrium, kinetics and thermodynamics of antimonite [Sb(III)] sorption by pristine biochar (BC) and chitosan-loaded biochar (CHBC) derived from branches of Ficus microcarpa. Results showed the successful loading of chitosan onto biochar surface, exhibiting more functional groups (e.g., CO, -NH2, and -OH). Langmuir model well described the Sb(III) sorption isotherm experimental data, and the maximum sorption capacity of Sb(III) by CH1BC (biochar loaded with chitosan at a ratio of 1:1) was 168 mg g-1, whereas for the BC it was only 10 mg g-1. X-ray photoelectron spectroscopy demonstrated that CH1BC oxidized 86% of Sb(III) to Sb(V), while BC oxidized 71% of Sb(III). Density functional theory calculations suggested that the synergistic effect of exogenous hydroxyl and inherent carbonyl contributed to the enhanced removal efficiency of Sb(III) by CHBC. Key mechanisms for Sb(III) sorption onto CHBCs included electrostatic interaction, chelation, surface complexation, π-π interaction, and hydrogen bonding. Overall, this study implies that CHBC can be a new, viable sorbent for the removal of Sb(III) from aquatic systems aiding their safe and sustainable management.

Effect of pyrolysis temperature on removal of organic pollutants present in anaerobically stabilized sewage sludge.

Sewage sludge was excluded from the list of component materials for the production of EU fertilizing products and it was banned as feedstock to produce pyrolysis & gasification materials in European Commission's technical proposals for selected new fertilizing materials under the Regulation 2019/1009 (STRUBIAS report). This exclusion of pyrolysis as a viable way to treat sewage sludge was mainly due to the lack of data on the fate of organic pollutants at pyrolysis conditions. In this work, we are addressing this knowledge gap. We studied slow pyrolysis as a potential process to efficiently treat organic pollutants present in stabilized sewage sludge. Sewage sludge was pyrolyzed in a quartz fixed bed reactor at temperatures of 400-800 °C for 2 h and the sludge and resulting sludge-chars were analyzed for the presence of four groups of organic pollutants, namely (i) polychlorinated biphenyls (PCBs), (ii) polycyclic aromatic hydrocarbons (PAHs), (iii) pharmaceuticals, and (iv) endocrine-disrupting and hormonal compounds. Pyrolysis at ≥ 400 °C effectively removed pharmaceuticals (group iii) to below detection limits, whereas pyrolysis at temperatures higher than 600 °C was required to remove more than 99.8% of the compounds from groups i, ii and iv. Based on these findings, we propose, that high temperature (>600 °C) slow pyrolysis can satisfactory remove organic pollutants from the resulting sludge-char, which could be safely applied as soil improver.

Iron-modified biochar and water management regime-induced changes in plant growth, enzyme activities, and phytoavailability of arsenic, cadmium and lead in a paddy soil.

The aim of this study was to evaluate the effect of raw (RawBC) and iron (Fe)-modified biochar (FeBC) derived from Platanus orientalis Linn branches on the plant growth, enzyme activity, and bioavailability and uptake of As, Cd, and Pb by rice in a paddy soil with continuously flooded (CF) or alternately wet and dry (AWD) irrigation in a pot experiment. Application of RawBC (3%, w/w) significantly increased soil pH, while FeBC decreased it. The FeBC was more effective in reducing As and Pb bioavailability, particularly under the AWD water regime, while RawBC was more conducive in reducing Cd bioavailability under the CF water regime. The FeBC decreased As concentration, but increased concentrations of Cd and Pb in the straw and brown rice, as compared to the untreated soil. Soil catalase and urease activities were enhanced by RawBC, but decreased by FeBC treatment. The FeBC increased the grain yield by 60% and 32% in CF and AWD treatments, respectively. The FeBC can be recommended for immobilization of As in paddy soils, but a potential human health risk from Cd and Pb in FeBC-treated soils should be considered due to increased uptake and translocation of the metals to brown rice.

Fertilizer and Soil Solubility of Secondary P Sources-The Estimation of Their Applicability to Agricultural Soils.

The demand for phosphorus (P) sources is increasing with the growing world population, while objections to direct agricultural use of waste P sources, such as sewage sludge, are being raised. Therefore, the need arises to employ safe and efficient secondary P fertilizer sources, originating from P-rich wastes. These recycling sources are commonly tested in accordance with the current fertilizer rules, designed originally for conventional apatite-based P fertilizers. The behavior of sewage sludge ash, an inorganic recycling secondary P source, was investigated under soil-like conditions. Standardized soil P tests, including the soil buffering capacity test and the Olsen, the Mehlich3, and water extraction methods, were employed together with standardized fertilizer P-solubility tests by neutral ammonium citrate and 2% citric acid extraction. In addition, total content and the overall soil mobility of selected metallic elements present in sewage sludge ash were investigated. The suitability of standardized soil tests for the evaluation of recycling P sources was shown. An apparent influence of Ca:Al content ratio on sewage sludge ash behavior under different soil-like conditions shows the inadequacy of the current fertilizer test and the necessity to understand soil-like behavior of secondary P sources, when considering these as possible agricultural P bearers (fertilizers).

Material analysis of Bottom ash from waste-to-energy plants.

Bottom ash (BA) from waste-to-energy (WtE) plants contains valuable components, particularly ferrous (Fe) and non-ferrous (NFe) metals, which can be recovered. To assess the resource recovery potential of BA in the Czech Republic, it was necessary to obtain its detailed material composition. This paper presents the material composition of BA samples from all three Czech WtE plants. It was found that the BA contained 9.2-22.7% glass, 1.8-5.1% ceramics and porcelain, 0.2-1.0% unburnt organic matter, 10.2-16.3% magnetic fraction, 6.1-11.0% Fe scrap, and 1.3-2.8% NFe metals (in dry matter). The contents of individual components were also studied with respect to the BA granulometry and character of the WtE waste collection area.

Possibilities of mercury removal in the dry flue gas cleaning lines of solid waste incineration units.

Dry methods of the flue gas cleaning (for HCl and SO2 removal) are useful particularly in smaller solid waste incineration units. The amount and forms of mercury emissions depend on waste (fuel) composition, content of mercury and chlorine and on the entire process of the flue gas cleaning. In the case of high HCl/total Hg molar ratio in the flue gas, the majority (usually 70-90%) of mercury is present in the form of HgCl2 and a smaller amount in the form of mercury vapors at higher temperatures. Removal of both main forms of mercury from the flue gas is dependent on chemical reactions and sorption processes at the temperatures below approx. 340 °C. Significant part of HgCl2 and a small part of elemental Hg vapors can be adsorbed on fly ash and solid particle in the air pollution control (APC) processes, which are removed in dust filters. Injection of non-impregnated active carbon (AC) or activated lignite coke particles is able to remove mainly the oxidized Hg(2+) compounds. Vapors of metallic Hg(o) are adsorbed relatively weakly. Much better chemisorption of Hg(o) together with higher sorbent capacity is achieved by AC-based sorbents impregnated with sulfur, alkali poly-sulfides, ferric chloride, etc. Inorganic sorbents with the same or similar chemical impregnation are also applicable for deeper Hg(o) removal (over 85%). SCR catalysts convert part of Hg(o) into oxidized compounds (HgO, HgCl2, etc.) contributing to more efficient Hg removal, but excess of NH3 has a negative effect. Both forms, elemental Hg(o) and HgCl2, can be converted into HgS particles by reacting with droplets/aerosol of poly-sulfides solutions/solids in flue gas. Mercury captured in the form of water insoluble HgS is more advantageous in the disposal of solid waste from APC processes. Four selected options of the dry flue gas cleaning with mercury removal are analyzed, assessed and compared (in terms of efficiency of Hg-emission reduction and costs) with wet methods and retrofits for more efficient Hg-removal. Overall mercury removal efficiencies from flue gas can attain 80-95%, depending on sorbent type/impregnation, sorbent surplus and operating conditions.

Geochemical and spectroscopic investigations of Cd and Pb sorption mechanisms on contrasting biochars: engineering implications.

Biochars prepared from nut shells, plum stones, wheat straws, grape stalks and grape husks were tested as potential sorbents for Cd and Pb. Mechanisms responsible for metal retention were investigated and optimal sorption conditions were evaluated using the RSM approach. Results indicated that all tested biochars can effectively remove Cd and Pb from aqueous solution (efficiency varied between 43.8% and 100%). The removal rate of both metals is the least affected by the biochar morphology and specific surface but this removal efficiency is strongly pH-dependent. Results of variable metal removal combined with different optimized conditions explain the different metal sorption mechanisms, where the predominant mechanism is ion exchange. In addition, this mechanism showed very strong binding of sorbed metals as confirmed by the post-desorption of the fully metal-loaded biochars. Finally, these biochars could thus also be applicable for metal contaminated soils to reduce mobility and bioavailability of Cd and Pb.