Simultaneous Removal Of CO2 Research Articles

Effect of H2S on biogas sorption enhanced steam reforming using a Pd/Ni-Co catalyst and dolomite as a sorbent

To achieve net zero carbon emissions from energy systems, biogas has become an attractive renewable resource for hydrogen production. The sorption enhanced steam reforming (SESR) process is proposed to produce high-purity hydrogen from biogas, enabled by combining the catalytic reforming reaction with the simultaneous CO2 removal by sorption in a single reactor. One of the most critical challenges in using biogas in conventional reforming processes is the presence of H2S since it may deactivate the reforming catalyst. Here we experimentally study the effect of the biogas H2S concentration on the H2 production by SESR, i.e., accounting for the presence of a CaO-based solid sorbent. This work was performed in a fixed-bed reactor using a Pd/Ni-Co hydrotalcite-like material (HT) catalyst and dolomite as CO2 sorbent. Biogas (60CH4/40CO2 vol.%) with different concentrations of H2S (150, 350, 500 and 1000 ppm) was evaluated. The catalyst did not deactivate for biogas H2S concentrations of 150 and 350 ppm during five cycles of the SESR process. However, a slight decrease in the catalyst activity was detected under higher sulfur concentrations after the third SESR cycle. Sulfur was detected in the spent catalyst and sorbent materials, with a higher proportion in the sorbent for the highest H2S concentration tested (1000 ppm). H2 yield decreased by 10.8% and 4.5% points for biogas H2S concentrations of 500 and 1000 ppm after five cycles, respectively, while H2 purity decreased by only 3 vol.% and 2 vol.% points, respectively.

Process Simulations of High-Purity and Renewable Clean H2 Production by Sorption Enhanced Steam Reforming of Biogas

Renewable clean H2 has a very promising potential for the decarbonization of energy systems. Sorption enhanced steam reforming (SESR) is a novel process that combines the steam reforming reaction and the simultaneous CO2 removal by a solid sorbent, such as CaO, which significantly enhances hydrogen generation, enabling high-purity H2 production. The CO2 sorption reaction (carbonation) is exothermic, but the sorbent regeneration by calcination is highly endothermic, which requires extra energy. Biogas is one of the available carbon-neutral renewable H2 production sources. It can be especially relevant for the energy integration of the SESR process since, due to the exothermic sorption reaction, the CO2 contained in the biogas provides extra heat to the system, which can help to balance the energy requirements of the process. This work studies different process configurations for the energy integration of the SESR process of biogas for high-purity renewable H2 production: (1) SESR with sorbent regeneration using a portion of the produced H2 (SESR+REG_H2), (2) SESR with sorbent regeneration using biogas (SESR+REG_BG), and (3) SESR with sorbent regeneration using biogas and adding a pressure swing adsorption (PSA) unit for hydrogen purification (SESR+REG_BG+PSA). When using biogas as fuel (Cases 2 and 3), these configurations were studied using air and oxy-fuel combustion atmospheres in the sorbent regeneration step, resulting in five case studies. A thermodynamic approach for process modeling can provide the optimal process operating conditions and configurations that maximize the energy efficiency of the process, which are the basis for subsequent optimization of the process at the practical level needed to scale up this technology. For this purpose, process simulations were performed using a steady-state plant model developed in Aspen Plus, incorporating a complex heat exchanger network (HEN) to optimize heat integration. A comprehensive parametric study assessed the effects of biogas composition, temperature, pressure, and steam to methane (S/CH4) ratio on the process performance represented by the selected key performance indicators, i.e., H2 purity, H2 yield, CH4 conversion, cold gas efficiency (CGE), net efficiency (NE), fuel consumption for the sorbent regeneration step, and CO2 capture efficiency. H2 with a purity of 98.5 vol % and a CGE of 75.7% with zero carbon emissions can be achieved. When adding a PSA unit, nearly 100% H2 purity and CO2 capture efficiency were achieved with a CGE of 77.3%. The use of oxy-fuel combustion during regeneration lowered the net efficiency of the process by 2.3% points (since it requires an air separation unit) but allowed the process to achieve negative carbon emissions.

Design and economic evaluation of a novel amine-based CO2 capture process for SMR-based hydrogen production plants

Chemical absorption using amine solvents is the most commercially recognized technology to capture CO2 from the SMR-based hydrogen production plant. Most studies focused on the use of PSA inlet or tail gas streams to remove CO2 using MDEA solvent which is limited to about 60% of total plant CO2 emitted. Whilst the capture of a flue gas stream has been reported via the MEA solvent to remove 90% CO2 yet at relatively higher costs. A systematic investigation is required to identify the cost-effective use of the absorption processes when multiple positions are available for CO2 capture. In this study, multiple cases of the CO2 absorption process have been evaluated using MEA or MDEA solvents. Especially, a novel design for the simultaneous removal of CO2 from the flue gas of a reformer and the PSA inlet gas stream is proposed and its techno-economic benefits are examined. For this purpose, the rate-based model has been implemented in the MATLAB environment to simulate the absorption process, whereas the solvent regeneration energy and the capital and operating costs of the main equipment have been estimated using the correlations taken from the literature. The new system based on the sequential reuse of MEA solvent for two streams is found to be the most economic process to remove 99% of total plant CO2. The annualized cost of the proposed system is found 17.9% less in comparison to the best relevant conventional studied system of simultaneously removing CO2 from the PSA inlet and reformer flue gas streams using MEA and MDEA solvents.

Interaction investigation of three forest waste biochars and CaO in the process of Ca-L/CARBONOx

Ca-L/CARBONOx technology has realized the simultaneous removal of CO2 and NO in the flue gas from coal-fired power plants. Forest wastes are characterized by wide sources, low cost, high lignin content, and high chemical activity. In this study, three forest waste biochars including elm biochar (YB), fir biochar (SB) and bamboo biochar (ZB) were blended with CaO, and their simultaneous CO2/NO removal characteristics were compared. The interaction mechanism between CaO and biochar (BC) in the Ca-L/CARBONOx process was also discussed. Results show that the addition of biochar inhibits the pore structure collapse of CaO at higher cyclic numbers. CaO can also improves the NO removal efficiency for three biochars by catalyzing and providing active adsorption sites. The removal efficiency of NO by three biochars added with CaO are greater than 97% at 650 ℃ in the chemical control stage of CO2 adsorption. SB showed the best NO removal performance and reaction selectivity among the three biochars. Under the presence of CaO, CaO/YB and CaO/ZB have similar CO2 capture performance, while CaO/SB and CaO/ZB have the highest NO removal performance and the best NO reaction selectivity, respectively.

Amine-grafted mesoporous silica materials for single-stage biogas upgrading to biomethane

The potential of amine-grafted silica materials (i.e., aminosilicas) was investigated for single-stage biogas purification via simultaneous removal of CO2, H2S, and water vapor. Custom aminosilicas were synthesized by covalent tethering of primary amines onto commercially available mesoporous silica. Screening adsorption experiments were completed at 40 °C in the presence of dry 30 vol% CO2 in N2, and performance was measured using thermogravimetric analysis. Selected materials with equilibrium CO2 uptakes greater than 6 wt% were chosen for additional assessments in terms of CO2 adsorption kinetics. The highest-performing aminosilica achieved fast CO2 uptake by reaching 82% of its equilibrium CO2 uptake in one minute. This material was subjected to rigorous 100-cycle testing and retained stable performance as evidenced by maintaining 99% of its initial CO2 uptake throughout cycling, regardless of using air or N2 as desorption purge gas. The final candidate also underwent column-breakthrough tests in the presence of humid 24% CO2, 200 ppmv H2S, balance N2, at 40 °C, and achieved concurrent and complete (100%) removal of all target impurities, namely CO2, H2S, and water vapor, prior to their respective breakthrough times. The results suggest promising potential of aminosilicas as a viable method of biogas purification, particularly in remote areas where access to other purge gases such as steam or N2 is not practical.

CO2-permselective membrane reactor for steam reforming of methane

Steam reforming of methane (SRM) for hydrogen production with simultaneous CO2 removal in a CO2-permselective membrane reactor is studied experimentally and analyzed by a mathematical model. The membrane consists of a thick porous BYS (Bi1.5Y0.3Sm0.2O3-δ)-SDC (Sm0.2Ce0.8O2-δ) support layer and a thin, dense SDC-carbonate dual-phase layer (∼150 μm) for CO2 separation. Experimental results of CO2 permeation in the BYS-SDC membrane and SRM reactions in a fixed-bed reactor were conducted to obtain the CO2 permeation and SRM reaction rate parameters for the model. The model's reliability is confirmed by comparing the simulation results with atmospheric pressure experimental data for SRM in the BYS-SDC membrane reactor. Simulation results are presented to show the effects of permeation number (θ) and Damkohler number (Da) (which respectively represent membrane CO2 permeance and SRM reaction rate), feed pressure in the reaction side, and sweep side conditions on the performance of SRM in the membrane reactor. Overall, increasing θ results in an increase of H2 yield, retentate H2 concentration, CO2 recovery, and a decrease of retentate CO concentration, with such effects being enhanced at higher Da and feed pressure or vacuum in the sweep side. CO2 permeance affects CH4 conversion only at high feed pressures. At feed pressure of 5 atm and applying vacuum at the sweep side of the membrane reactor, the simulation results show that the membrane reactor can achieve over 99% CH4 conversion, H2 yield, and CO2 recovery and produce an essentially pure H2 stream with zero CO concentration at Da >10,000 and θ > 1.

Effects of phosphorus modification on the catalytic properties and performance of CuCeZr mixed metal catalyst for simultaneous removal of CO and NOx

CO oxidation and selective catalytic reduction of NOx using NH3 (NH3-SCR) have been profoundly studied to eliminate CO and NOx by catalytic routes, respectively. However, the research about proceeding above two reactions together over a single catalyst has not ever been reported for now. Herein, we demonstrate one phosphorus-modified CuCeZr catalyst for the simultaneously catalytic removal of CO and NOx, which realizing up to 90% conversion of both CO and NOx in the temperature of 200–350 °C. Based on the physical–chemical characterizations, it is believed that the introduced phosphorus has a closer affinity with Ce and Zr than Cu. More importantly, phosphorus modification achieves a trade-off between redox ability and surface acidity over the CuCeZr catalyst, which is beneficial to suppress the over-oxidation of NH3 during SCR process. DRIFTS analyses suggest that the SCR reaction mainly proceeds through the Eley-Rideal mechanism, and the carbonate species originated from absorbed CO and surface lattice oxygen are proved to be the key intermediate species during CO oxidation. This work creates a pioneer on realizing CO oxidation and NH3-SCR reactions on a single catalyst simultaneously, providing a novel strategy for flue gas pollutants synergistic control.

Performance Evaluation of Polyethersulfone Membranes for Competitive Removal of Cd2+, Co2+, and Pb2+ Ions from Simulated Groundwater

This paper presents studying the performance of three types of polyethersulfone (PES) membrane for the simultaneous removal of Co2+ ions, Cd2+ ions, and Pb2+ ions from binary and ternary aqueous solutions. Co2+ ions, Cd2+ ions, and Pb2+ ions with two different initial concentrations (e.g., 10 and 50 ppm) were selected as examples of heavy metals that contaminate the groundwater as a result of geological and human activities. This study investigated the effect of types of PES membrane and metal ions concentration on the separation process. For the binary aqueous solutions, the permeation flux of the PES2 membranes was higher for the separation process of solutions containing 50 ppm of Cd2+ ions and 10 ppm of Co2+ ions (24.7 L/m2·h) and Pb2+ ions (23.7 L/m2·h). All the metals in the binary solutions had high rejection when their initial concentration was lower than the initial concentration of the other metal present in the same solution. Using PES2, the maximum rejection of Cd2+ ions was 61.3% when the initial concentrations were 50 ppm Pb2+ ions: 10 ppm Cd2+ ions and 55.4% for Pb2+ ions when the initial concentrations were 10 ppm Pb2+ ions: 50 ppm Cd2+ ions. For the ternary aqueous solutions, the rejection and the permeation flux of the PES membranes increased with decreasing the heavy metal initial concentration. Using PES2, the maximum permeation flux was 21.6 L/m2·h when the initial concentration of the metals was 10 ppm; and the maximum rejection of the metals obtained at initial concentration of 10 ppm was 50.5% for Co2+ ions, 48.3% for Cd2+ ions, and 40% for Pb2+ ions. The results of the filtration process using PES2 of simulated contaminated-groundwater indicated the efficient treatment of groundwater containing Co2+, Cd2+, and Pb2+ ions.

Simultaneous removal of CO2, NOx and SOx using single stage absorption column

Capturing flue gases often require multiple stages of scrubbing, increasing the capital and operating costs. So far, no attempt has been made to study the absorption characteristics of all the three gases (NO, SO2 and CO2) in a single stage absorption unit at alkaline pH conditions. We have attempted to capture all the three gases with a single wet scrubbing column. The absorption of all three gases with sodium carbonate solution promoted with oxidizers was investigated in a tall absorption column. The absorbance was found to be 100% for CO2, 30% for NO and 95% for SO2 respectively. The capture efficiency of sodium carbonate solution was increased by 40% for CO2 loading, with the addition of oxidizer. Absorption kinetics and reaction pathways of all the three gases were discussed individually in detail.

Renewable hydrogen production from biogas by sorption enhanced steam reforming (SESR): A parametric study

H2 production from biogas (60%CH4 + 40%CO2) by sorption enhanced steam reforming (SESR) was thermodynamically and experimentally studied in a fluidized bed reactor. Biogas is an interesting renewable biomass resource for hydrogen production due to its sustainable nature. SESR combines the catalytic reforming reaction of biogas with simultaneous CO2 removal in a single step. A Pd/Ni–Co hydrotalcite-like material (HT) was used as catalyst and dolomite as CO2 sorbent. The effects of temperature (550–800 °C), steam/CH4 molar ratio (2–6) and gas hourly space velocity (GHSV) (492–3937 mL CH4 gcat−1 h−1) on the process performance were evaluated. CO2 in biogas was effectively removed by the sorbent from the gas phase at 550–700 °C, without influencing the reforming process. H2 yield increased with temperature from 550 to 650 °C, but H2 concentration decreased at temperatures higher than 600 °C, requiring a tradeoff between both parameters to select an optimum operating temperature. H2 purity of 98.4 vol% was obtained at 550–600 °C and H2 yield of 92.7% was reached at 650 °C. Higher steam/CH4 ratios enhance the process, whereas higher space velocities decrease H2 yield. Results demonstrate that high-purity high-yield biohydrogen can be produced by the SESR of a renewable biomass resource as biogas.

Simultaneous removal of Co, Cu, Cd and Ni from zinc sulfate solution by zinc dust cementation

Instead of conventional two or three-stage zinc dust cementation purification, one-stage purification was proposed to remove copper, cadmium and nickel as well as cobalt by CuSb activation simultaneously from solution for an electrolytic zinc production process under development by MMG Limited. Copper was rapidly removed, forming a porous copper shell around a zinc particle. The shell acted as a cathode for the reduction of cobalt, cadmium, and nickel while the zinc particle dissolved anodically. The measurement of hydrogen evolution as well as slurry potential explains how the reduction and re-dissolution of cobalt, cadmium and nickel as a function of time is influenced by temperature, pH, and antimony and zinc dust dosages, providing a deeper understanding of impurity removal. Cobalt, cadmium and nickel were initially removed without back reaction. When zinc dissolved to a certain degree, the shell potential exceeded their redox potentials and the impurities started to re-dissolve. The shell potential was determined by the electrical contact between a zinc particle and the shell, the portion of the remaining zinc and the surface product on the zinc particle. The zinc dissolution weakened the electrical contact and therefore increased the shell potential. Temperature, pH, and antimony and zinc dust dosages not only affect the impurity reduction but also zinc dissolution, the shell potential and finally the impurity re-dissolution in different degrees. Copper, cadmium, cobalt and nickel can be completely removed in one step. The optimal conditions for impurity removal are recommended.

Optimization of photosynthetic biogas upgrading in closed photobioreactors combined with algal biomass production

Photosynthetic biogas upgrading in microalgae photobioreactors constitutes a sustainable and environmentally friendly technology for the simultaneous removal of CO2 and H2S from raw biogas. The potential of algal-bacterial symbiosis for biogas purification has been already studied and demonstrated in open photobioreactors. However, few studies have focused on the implementation of this process in closed photobioreactors, which offer higher photosynthetic efficiencies, enhanced biomass productivities, a limited N2 contamination and improved CO2 mass transfer. In this study, the influence of alkalinity (inorganic carbon concentrations from ∼200 to ∼1700 mg L–1), and nitrogen (N) deprivation strategies (24 or 48 h limitation) on both biomethane quality and biomass composition has been evaluated and optimized in a closed tubular photobioreactor (TPBR) interconnected to a mixing chamber and a biogas absorption column (total working volume of 132 L). The increase in the alkalinity of the cultivation broth mediated an increase in the upgrading performance of the TPBR, reaching CO2 removals of 96.3 ± 0.8 %, and a complete H2S abatement, resulting in an enhanced biomethane composition (CH4 = 97.2 ± 0.9 %). Moreover, process operation under N deprivation did not affect the quality of biomethane, which complied with standards required for injection into natural gas grids or use as vehicle fuel. N-deprivation promoted the continuous storage of intracellular carbohydrates (29.0 ± 4.4 % compared with 14.3 ± 2.1 % obtained under N excess). A maximum biomass productivity obtained was of 15.8 ± 4.3 g m–2 d–1 during N excess vs. 11.2 ± 3.3 g m–2 d–1 during N-deprivation at 26.8 d of hydraulic retention time. During the N feast-famine cycles, the cyanobacterial species Pseudanabaena sp. was dominant over green algae.

Simultaneous removal of CO2 and H2S from coalbed methane in a membrane contactor

Coalbed methane (CBM) is an unconventional natural gas, and vigorous development of CBM has great significance for improving the current status of global coal mine safety production, alleviating market gas supply pressure, and protecting the ecological environment. CBM mainly consists of CH4, CO2 and a small amount of H2S. In order to improve the comprehensive energy utilization efficiency of CBM, it is necessary to capture both CO2 and H2S from CBM. In this paper, a comprehensive 2D model was established by COMSOL Multiphysics for simultaneously removing CO2 and H2S in CBM. The K2CO3/potassium lysinate (PL) mixed solution was employed as the absorbent in a membrane contactor. The impacts of gas and liquid velocities, liquid concentration and temperature on the absorbent performance were studied. The results obtained by using the simulated membrane contactor were consistent with the experimental data in the literature. The results indicate that the gas velocity was the most significant influential factor to the gases removal efficiency. Furthermore, it was concluded that the K2CO3/PL mixed solution had a better decarburization performance than the single K2CO3 solution. The optimal operating conditions for simultaneous removal process by using K2CO3/PL mixed solution were obtained by response surface method. More importantly, the blends of amino acid salts and carbonate solution have great prospects in the membrane absorption of acid gases from CBM.

Optimization of a chemical scrubbing process based on a Fe-EDTA-carbonate based solvent for the simultaneous removal of CO2 and H2S from biogas

The potential of a novel Fe/EDTA/carbonate-based scrubbing process for the simultaneous removal of H2S and CO2 from biogas was studied by evaluating the influence of Fe/EDTA molarity (M), carbonate concentration (IC), biogas (B), air (A) and liquid (L) flow rates on biogas upgrading performance using a Taguchi L16(45) experimental design. The ANOVA demonstrated that molarity of the Fe/EDTA solution was a significant factor influencing H2S concentration (0.035 % at 0.00 M to 0.000 % at 0.05 M). IC impacted on the concentrations of CO2 (13.1 and 4.5 % at 4000 and 10,000 mg L−1, respectively), N2 and CH4 (85.9 and 94.5 % at 4000 and 10,000 mgIC L−1, respectively). The biogas flow rate affected the concentrations of CO2 (2.5 to 13.8% at 10 and 40 mL min−1, respectively), O2, N2 and CH4 (95.9 to 85.4% at 10 and 40 mL min−1, respectively). Likewise, the recycling liquid flow rate affected CO2 (8.3 and 5.9 % at 5 and 30 mL min−1, respectively), O2, N2 and CH4 (90.5 and 93.3 % at 5 and 40 mL min−1, respectively) concentrations. Finally, the air flow rate impacted on CO2 (10.8 and 6.7 % at 800 and 1000 mL min−1, respectively), H2S, N2 and CH4 (87.9 and 92.2 % at 800 and 1000 mL min−1, respectively) concentrations. Process optimization provided the optimal conditions for each control factor. Continuous biogas upgrading operation at M2-IC1-B2-A4-L4 (0.05 M, 10,000 mgIC L−1, 10 mL min−1, 1000 mL min−1 and 30 mL min−1, respectively) provided CH4, CO2, O2, N2 and H2S concentration in the upgrading biogas of 97.4, 1.4, 0.29, 0.97 and 0%, respectively, which complied with biomethane regulations.

Cryptogamic Packed Biofilter as Potential Adsorbent for CO2, NH3, and H2S Impurities from Biogas

The presence of elevated concentrations of carbon dioxide, hydrogen sulfide, ammonia, and trace impurities in biogas affect its caloric value as well as causes corrosion and is extremely toxic. There are various methods in existence for removal of these impurities, but most are chemically based and expensive and are limited in use. In our work, cryptogams (moss) integrated with soil and biochar packed in a filter have been employed for simultaneous removal of CO2, H2S, and NH3, from biogas. Different soil types rich in metallic oxides at different masses of 100 g, 150 g, and 200 g with a fixed mass of moss and biochar were tested in an on-site experiment to determine the removal efficiency (RE) and sorption capacity (SC). The adsorption dynamics of the filters were investigated at two flow rates, 80 ml/min and 100 ml/min, by determining removal efficiency. For the contribution of each substrate, sorption capacity and breakthrough time were determined by considering 5 g of each substrate that made up the filter. The soils with a high content of extractable cations showed excellent adsorption capacity for H2S by about 20 g S/100 g, which was higher than other adsorbents tested. It was found that integrated biofilter made up of bed arrangement of the soil, biochar, and moss plant improved the quality of biogas with SC of 11 g S and RE of 93% for H2S, 72% for NH3, and 68% for CO2.

Multilevel screening of ionic liquid absorbents for simultaneous removal of CO2 and H2S from natural gas

For the selection of practically attractive ionic liquids (ILs) as absorbents for simultaneous CO2 and H2S removal from natural gas (NG), a multilevel screening method combining Henry′s law constant (H) and vapor–liquid equilibria (VLE) based prediction of thermodynamic properties, physical properties estimation, and process simulation is presented. To begin with, an H-based Absorption-Selectivity-Desorption index (ASDI) is employed to prescreen potential ILs that have promising target properties at infinite dilution condition. Following this, their simultaneous CO2 and H2S removal performances are further evaluated from the VLE of {IL + NG} systems at the specific composition of interest. After the thermodynamic screening, key physical properties of the obtained ILs are estimated by group contribution methods and the IL stability is assessed empirically to find out solvents suitable for practical application. Finally, continuous acidic gas removal processes based on the remaining ILs are simulated and compared by Aspen Plus, thereby identifying the best ILs from the process point of view.

Simultaneous Removal of CO2 and H2S from Biogas by Blending Amine Absorbents: A Performance Comparison Study

Greenhouse gas treatment is urgently needed because of the impact of climate change caused by greenhouse gas emissions after global economic growth. In this study, postcombustion capture was carrie...

Production and upgrading of biogas through controlled hydrogen injection for renewable energy storage

To augment renewable energy production and utilize surplus wind energy during low-demand intervals, hydrogen produced through electrolysis may be injected into anaerobic digesters in order to increase the energetic content of biogas through promotion of hydrogenotrophic methanogenesis (4H2 + CO2 → CH4 + 2H2O). In this work bubble-free hydrogen injection was investigated in at pilot scale using various membranes. The reactor was then inoculated with anaerobic sludge from a municipal digester which had been pre-adapted to elevated H2 levels; due to selective methanogenesis and simultaneous CO2 removal, gas phase methane concentrations initially increased by a factor of 36% over the theoretical maximum from the anaerobic degradation of glucose during continuous tests. However, a shift in the acid spectrum toward propionate and a deficit in acetate led to intermittent periods during which diminished acetoclastic methanogenesis reduced overall biogas production. Maximum H2 dosing rates were derived for anaerobic digestion of glucose.

Impact of CO2 concentration and ambient conditions on microalgal growth and nutrient removal from wastewater by a photobioreactor

The increase in atmospheric CO2 concentration and the release of nutrients from wastewater treatment plants (WWTPs) are environmental issues linked to several impacts on ecosystems. Numerous technologies have been employed to resolves these issues, nonetheless, the cost and sustainability are still a concern. Recently, the use of microalgae appears as a cost-effective and sustainable solution because they can effectively uptake CO2 and nutrients resulting in biomass production that can be processed into valuable products. In this study single (Spirulina platensis (SP.PL) and mixed indigenous microalgae (MIMA) strains were employed, over a 20-month period, for simultaneous removal of CO2 from flue gases and nutrient from wastewater under ambient conditions of solar irradiation and temperature. The study was performed at a pilot scale photo-bioreactor and the effect of feed CO2 gas concentration in the range (2.5–20%) on microalgae growth and biomass production, carbon dioxide bio-fixation rate, and the removal of nutrients and organic matters from wastewater was assessed. The MIMA culture performed significantly better than the monoculture, especially with respect to growth and CO2 bio-fixation, during the mild season; against this, the performance was comparable during the hot season. Optimum performance was observed at 10% CO2 feed gas concentration, though MIMA was more temperature and CO2 concentration sensitive. MIMA also provided greater removal of COD and nutrients (~83% and >99%) than SP.PL under all conditions studied. The high biomass productivities and carbon bio-fixation rates (0.796–0.950 gdw·L−1·d−1 and 0.542–1.075 gC·L−1·d−1 contribute to the economic sustainability of microalgae as CO2 removal process. Consideration of operational energy revealed that there is a significant energy benefit from cooling to sustain the highest productivities on the basis of operating energy alone, particularly if the indigenous culture is used.

Synthesized Goethite and Natural Iron Oxide as Effective Absorbents for Simultaneous Removal of Co(II) and Ni(II) Ions from Water

This study reports on the adsorption efficiency of a natural iron oxide from Mballam-Cameroon in comparison with synthesized goethite to simulta-neously remove cobalt and nickel ions from aqueous solutions. Chemical analysis on the natural iron oxide sample revealed iron as the main element and hematite (58.52%) goethite (19.42%), kaolinite (12.69%) and quartz (7.79%) as the component phases in the iron oxide sample. The iron oxide was found to be microporous (BET surface area 43.27 m2/g) with fairly spherical polydisperse particles. Results show maximum absorption for Co(II) and Ni(II) ions for both adsorbents occurred at an equilibrium contact time of 80 mins, dose rate of 0.1 g/L, and pH = 7. Goethite was slightly more efficient at removing target metal ions with maximal adsorbed quantities at 117.8 mg/g of Co(II) and 100.6 mg/g of Ni(II), and 103.9 mg/g of Co(II) and 85.2 mg/g of Ni(II) ions for natural iron oxide. Equilibrium modelling presented the Freundlich isotherm as the best fit model for both adsorbents and metal ions, indicating heterogeneity of the surface binding sites during adsorption. The pseudo-second order kinetic model was the best-fit model, indicating chemical adsorption between the adsorbent surface and metal ions, hence a good correlation between equilibrium and kinetics. The findings indicate that the efficacy of the natural iron oxide from Mballam is almost equivalent to that of synthetic goethite, validating its applicability for the simultaneous removal of cobalt and nickel ions from aqueous solution.