A review on the emerging applications of cellulose, cellulose derivatives and nanocellulose in carbon capture.

Carbon capture can be implemented at a large scale only if the CO2 selective materials are abundantly available at low cost. Since the sustainable requirement also elevated, the low-cost and biodegradable cellulosic materials are developed into CO2 selective adsorbent and membranes recently. The applications of cellulose, cellulosic derivatives and nanocellulose as CO2 selective adsorbents and membranes are reviewed here. The fabrication and modification strategies are discussed besides comparing their CO2 separation performance. Cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) isolated from cellulose possess a big surface area for mechanical enhancement and a great number of hydroxyl groups for modification. Nanocellulose aerogels with the large surface area were chemically modified to improve their selectivity towards CO2. Even with the reduction of surface area, amino-functionalized nanocellulose aerogels exhibited the satisfactory chemisorption of CO2 with a capacity of more than 2 mmol/g was recorded. Inorganic fillers such as silica, zeolite and MOFs were further incorporated into nanocellulose aerogels to enhance the physisorption of CO2 by increasing the surface area. Although CO2 adsorbents developed from cellulose and cellulose derivatives were less reported, their applications as the building blocks of CO2 separation membranes had been long studied. Cellulose acetate membranes were commercialized for CO2 separation, but their separation performance could be further improved with silane or inorganic filler. CNCs and CNFs enhanced the CO2 selectivity and permeance through polyvinyl alcohol coating on membranes, but only CNF membranes incorporated with MOFs were explored so far. Although some of these membranes surpassed the upper-bound of Robeson plot, their stability should be further investigated.

Bioelectrochemical technology for recovery of silver from contaminated aqueous solution: a review.

A large amount of silver-rich wastewater is generated from different industrial processes. This wastewater is not considered a waste, but a valuable source for recovery due to the precious silver (Ag). Previous studies have used traditional methods such as membrane filtration, electrolysis, chemical precipitation, electrochemical, and cementation for Ag recovery. However, many drawbacks have been reported for these techniques such as high cost, hazardous waste generation, and the needed refinement of recovered products. In this study, a bioelectrochemical system (BES) for Ag recovery from aqueous solution is introduced as an effective and innovative method, as compared with other techniques. Different types of Ag(I)-containing solutions that have been investigated in recent BES studies (e.g., Ag+ solution, [Ag(NH3)2]+, [Ag(S2O3)]-, [Ag(S2O3)2]3- complexes) are reported. A BES is an anaerobic system consisting of anode and cathode chambers, which are normally separated by an ion-exchange membrane. The electron flow obtained from the anodic biological oxidation of organic matter is used directly for the cathodic electrochemical reduction of Ag(I) ions. The recovered product is Ag electrodeposits, formed at the cathode surface. Several studies have reported high Ag recovery efficiency by using a BES (i.e., > 90%), with high purity of metallic silver, and simultaneous electricity production. Furthermore, a BES can be employed for a wide range of initial Ag(I) concentrations (e.g., 50-3000 mg/L). The advantages of BES technology for Ag recovery are highlighted in this study for further practical applications.

Electrochemical reduction of different Ag(i)-containing solutions in bioelectrochemical systems for recovery of silver and simultaneous power generation.

In this study, dual-chamber bioelectrochemical reactors (i.e., R1, R2, R3, and R4) were employed to investigate the Ag recovery and electricity production from different Ag(i)-containing artificial wastewaters (i.e., Ag+ solution, [Ag(NH3)2]+ and [Ag(S2O3)2]3- complexes, and mixed metal solution). Results showed that the electrochemical reductions of Ag(i) ions in all reactors were rapid reactions. The reaction rate in R1 was the fastest. At the same initial conditions (i.e. Ag(i) concentration of 1000-1080 mg L-1), the Ag recovery efficiency was 81.8% for R3 operated with the [Ag(S2O3)2]3- complex. Although high Ag removal efficiency (i.e., >99%) was found in other reactors, some diffusion of positively charged Ag(i) ions through the membrane was also observed along with the electrochemical reduction. In all cases, pure silver electrodeposits, mainly as dendrites and crystals in different morphologies, were observed at the cathode surfaces when characterized by SEM, EDX, and XRD. The performance of electricity production was evaluated by the open circuit voltage (OCV) and maximum power density (P max) obtained during the BES operation. Reactor R1 showed better performance (i.e., OCV of 828 mV, P max of 8258 mW m-3), due to its high standard reduction potential. The lower performance in other reactors was due to the complexity of solutions, other co-existing metals (mixed metal solution), and lower standard reduction potential. In general, the existing forms of Ag(i) in solutions affect the Ag(i) reduction rate. This further influences the Ag removal efficiency, morphology of electrodeposits, and power generation.

Factors influencing silver recovery and power generation in bio-electrochemical reactors.

The recovery of silver from Ag+ solution coupled with power generation was investigated in bio-electrochemical system (BES). In this system, chemical energy existing in the organic matter in the anode chamber can be converted biologically to electrical energy which can be used for the reduction of Ag+ ions in the cathode chamber. Results showed that type of substrate influenced the metabolic pathway and affected the cell voltage progression, and columbic efficiency. Silver recovery was not affected by increasing initial pH (2.0 to 7.0) and Ag+ concentration (100 to 1000 mg/L) in the catholyte, whereas power generation was improved. A maximum power density of 8258 mW/m3 and a columbic efficiency of 21.61% could be achieved with 1000 mg/L of Ag+. Ag+ ions were reduced to form metallic deposits as Ag0 crystals on the cathode surface, which were then confirmed by scanning electron microscope (SEM) image and energy dispersive X-ray (EDX) spectrum. The BES reactor had high silver removal (i.e., >96%) after 24 h of operation. When considering the crossover of Ag+ ions through the cation exchange membrane, the removal was in the range of 83.73-92.51%. This crossover was not considerable as compared to the Ag+ initial concentration. At higher initial Ag+ concentration (2000 mg/L), the silver removal decreased to 88.61% and the maximum power density decreased to 5396 mW/m3. This study clearly showed that BES can be employed for silver recovery, wastewater treatment, and also electricity generation.