Abordagem experimental e numérica aplicada na dissolução do calcário no processo de dessulfurização de gases de combustão

Wet FGD technologies account for around 87% of FGD systems worldwide (SOUD, 2000), particularly that which u ses limestone as the absorbent reagent. This technique is widely used in large thermal power stations. The limestone reactivity is one of the parameters that most influence the yield of the FGD process. To evaluate limestone reactivity experiments were conducted in one agitated batch reactor, with pH and temperature control and CO2 sparging. With the aim of improving the design and operation of desulfurization units, many studies have been carried out to investigate the rate of limestone dissolution. This proposed model considers the grain-sized distribution of particles, composition and pH of the liquid phase. This model was validated with experimental results, allowing predicting limestone dissolution kinetics in acid pH, with a maximum error of 18%.A tecnologia de dessulfurização úmida abrange aproximadamente 87% do mercado mundial (SOUD, 2000), especialmente a que utiliza como reagente absorvente, sendo particularmente empregada em grandes centrais termelétricas. A reatividade do calcário é um dos parâmetros que mais influem no rendimento do processo de dessulfurização de gases de combustão. Neste trabalho foram realizados experimentos em um reator de batelada agitado, com controle de pH e temperatura, com borbulhamento de CO2, para avaliação da reatividade do calcário, bem como a elaboração de um modelo para simular a dissolução das partículas de calcário. O modelo proposto leva em consideração a distribuição granulométrica das partículas, sua composição e o pH da matriz líquida onde se realiza a dissolução. Este modelo foi validado utilizando os resultados obtidos experimentalmente, permitindo predizer a cinética de dissolução do calcário em pH ácido, sendo o erro máximo entre os resultados experimentais e os numéricos de 18%

Integrated reduction/oxidation reactions and sorption processes for Cr(VI) removal from aqueous solutions using Laminaria digitata macro-algae

The main goal of this work was the valorization of seaweed Laminaria digitata, after acid pre-treatment, for the remediation of hexavalent chromium solutions. The Cr(VI) removal efficiency by the protonated biomass was studied as a function of different parameters, such as contact time, pH, biomass and Cr(VI) concentration, and temperature. Cr(VI) removal is based on a complex mechanism that includes a reduction of Cr(VI) to Cr(III), through the oxidation of biomass at acidic medium, and further chemical binding of Cr(III) to the negatively charged binding groups, mainly carboxylic groups. The optimum pH for chromium removal, using protonated L. digitata algae, was 2.5. The maximum amount of Cr(VI) reduction by the algae was around 2.1 mmol/g. The uptake capacity of Cr(III) by the oxidized biomass, after Cr(VI) reduction, was higher than by the algae in its original form (protonated algae). Results suggest that the oxidation of the biomass during Cr(VI) reduction, turns other active sites available for Cr(III) binding. Also, the Cr(III) binding from a solution of reduced Cr(VI) was much lower than from a pure Cr(III) solution. The result suggests the presence in solution of Cr(III) complexes with the organic matter released from the algae surface during Cr(VI) reduction. The activation energy obtained for the Cr(VI) reduction by L. digitata was 45 ± 20 kJ mol 1. A kinetic model based on the redox reaction between Cr(VI) species and organic compounds from the biosorbent surface was able to fit well the hexavalent chromium concentration. Trivalent chromium equilibrium biosorption was well described at different chromium concentrations, considering the interaction between carboxylic groups present in the surface of the biomass and Cr(III) in solution

Removal of hexavalent chromium from electroplating wastewaters using marine macroalga Pelvetia canaliculata as natural electron donor

This paper reports a treatment strategy for an electroplating wastewater containing high amounts of hexavalent and trivalent chromium and zinc, and residual iron. The brown macroalga Pelvetia canaliculata was used as a natural electron donor for the reduction of Cr(VI) to Cr(III) at acidic pH, and as a natural cation exchanger for zinc, iron and trivalent chromium sequestration. The strategy adopted for the wastewater treatment involves: (i) the reduction of Cr(VI) to Cr(III) using the macroalga as electron donor; (ii) trivalent chromium, zinc and iron precipitation at pH 8.5; and (iii) the removal of residual zinc ions (13 mg/L) by cation exchange at pH 8.5, using the negatively charged functional groups present at the surface of P. canaliculata. The Cr(VI) reduction was evaluated as a function of the biomass and Cr(VI) concentration, pH and temperature. The reaction was promoted through biomass oxidation in acidic medium. The Cr(VI) reduction capacities of raw and protonated P. canaliculata were around 1.8 and 2.3 mmol/g and the values for the Cr (III) uptake capacity of the oxidized biomass were 0.8 and 1.9 mmol/g, respectively. The results suggest that the oxidation of the biomass during Cr(VI) reduction generates new negatively charged active sites for cation binding. The continuous treatment of the wastewater containing Cr(VI) was evaluated in a column packed with raw P. canaliculata, and a maximum Cr(VI) reduction capacity of around 2.1 mmol/g was achieved. After the Cr(VI) removal step, 100% (below the detection limit) and 95% of the remaining trivalent chromium and zinc, respectively, can be eliminated by precipitation at pH 8.5

Abordagem experimental e numérica aplicada na dissolução do calcário no processo de dessulfurização de gases de combustão

A tecnologia de dessulfurização úmida abrange aproximadamente 87% do mercado mundial (SOUD, 2000), especialmente a que utiliza como reagente absorvente, sendo particularmente empregada em grandes centrais termelétricas. A reatividade do calcário é um dos parâmetros que mais influem no rendimento do processo de dessulfurização de gases de combustão. Neste trabalho foram realizados experimentos em um reator de batelada agitado, com controle de pH e temperatura, com borbulhamento de CO2, para avaliação da reatividade do calcário, bem como a elaboração de um modelo para simular a dissolução das partículas de calcário. O modelo proposto leva em consideração a distribuição granulométrica das partículas, sua composição e o pH da matriz líquida onde se realiza a dissolução. Este modelo foi validado utilizando os resultados obtidos experimentalmente, permitindo predizer a cinética de dissolução do calcário em pH ácido, sendo o erro máximo entre os resultados experimentais e os numéricos de 18%

Enhancement of a solar photo-Fenton reaction with ferric-organic ligands for the treatment of acrylic-textile dyeing wastewater

Literature describes a kinetic mineralization profile for most of acrylic-textile dyeing wastewaters using a photo-Fenton reaction characterized by a slow degradation process and high reactants consumption. This work tries to elucidate that the slow decay on DOC concentration is associated with the formation of stable complexes between Fe3+ and textile auxiliary products, limiting the photoreduction of Fe3+. This work also evaluates the enhancement of a solar photo-Fenton reaction through the use of different ferric-organic ligands applied to the treatment of a simulated acrylic-textile dyeing wastewater, as a pre-oxidation step to enhance its biodegradability. The photo-Fenton reaction was negatively affected by two dyeing auxiliary products: i) Sera (R) Tard A-AS, a surfactant mainly composed of alkyl dimethyl benzyl ammonium chloride and ii) Sera (R) Sperse M-IW, a dispersing agent composed of polyglycol solvents. The catalytic activity of the organic ligands toward the ferrous-catalysed system followed this order: Fe(III)Oxalate > Fe(III)-Citrate > Fe(III)-EDDS, and all were better than the traditional photo-Fenton reaction. Different design parameters such as iron concentration, pH, temperature, flow conditions, UV irradiance and H2O2 addition strategy and dose were evaluated. The ferrioxalate induced photo-Fenton process presented the best results, achieving 87% mineralization after 9.3 kJ(UV) L-1 and allowing to work until near neutral pH values. As expected, the biodegradability of the textile wastewater was significantly enhanced during the photo-Fenton treatment, achieving a value of 73%, consuming 32.4 mM of H2O2 and 5.7 kJ(UV) L-1

Marine macroalgae Pelvetia canaliculata (Phaeophyceae) as a natural cation exchanger for cadmium and lead ions separation in aqueous solutions

This work aims to add value to marine brown macro-algae Pelvetia canaliculata (Linnaeus) Decaisne and Thuret through its use as cation exchanger for separation and recovery of cadmium and lead ions in aqueous solution, as a cost effective and environmental friendly process. Raw algae was established as a cation exchanger, in which cadmium and lead ions present in the solution exchange with Na+, K+, Ca2+ and Mg2+, bound to the negatively functional groups existing on the algae surface, with a stoichiometric ratio of 1:1 between ions of the same charge and 2:1 between monovalent and divalent ions. Batch equilibrium and kinetic experiments were conducted at different pH values using Na-loaded algae. The main functional groups present on the surface of the algae responsible for binding metals, as determined by Fourier Transform Infrared (FTIR) analysis, are weakly acidic carboxylic groups and strongly acidic sulfonic groups. Considering the potentiometric titration and biomass esterification results, the amount of sulfonic and carboxylic groups is 1.0 mmol/g and 1.5 mmol/g, respectively, which is in agreement with the total amount of Na+ present at the surface of Na-loaded biomass (2.5 mEq/g) and total amount of light metals present at the surface of raw biomass (2.5 mEq/g). Maximum biosorption capacity of Pb2+ at pH 4.0 was 1.25 mmol/g (2.5 mEq/g; 259 mg/g) and for Cd2+ at pH 4.5 was 1.25 mmol/g (2.5 mEq/g; 140 mg/g). The mass action law for the ternary mixture was able to predict the equilibrium data, with the selectivity coefficients alpha(Cd)(Na) = 337 and alpha(Pb)(Na) = 941 for carboxylic and alpha(Cd)(Na) = 38 and alpha(Pb)(Na) = 1695 for sulfonic groups, revealing a higher affinity of the biomass towards lead ions. A mass transfer model, considering equilibrium given by the mass action law, and a linear driving force model for intraparticle diffusion, was able to fit well the batch kinetic data

Ion exchange prediction model for multi-metal systems obtained from single-metal systems using the macroalga Pelvetia canaliculata (Phaeophyceae) as a natural cation exchanger

The aim of this study was to investigate the cation exchange capacity of the macroalga Pelvetia canaliculata (Linnaeus) Decaisne & Thuret in a multi-metal system containing Cd2+, Pb2+, Cu2+ and Zn2+. The Naloaded alga was established as a cation exchanger, in which cadmium, lead, copper, zinc and hydrogen ions present in the liquid phase exchange with sodium ions bound to the functional groups on the algal surface, mainly weakly acidic carboxylic groups and strongly acidic sulfonic groups. A mass action law for the senary system (Na+/H+/cd(2+)/Pb2+/Zn2+) was able to predict the equilibrium data using the selectivity coefficients determined for the single-metal systems. Multi-metal equilibrium results, in agreement with the selectivity coefficients, showed a higher preference (affinity) of the biomass toward lead ions followed by copper, cadmium and zinc ions. A mass transfer model, considering equilibrium given by the mass action law, and a linear driving force model for intraparticle diffusion, was able to fit well the batch kinetic experimental data for all chemical species in the liquid and solid phase. The results indicate that the biomass of P. canaliculata is an efficient natural cation exchanger for multi-metal systems