Biochar Modified By Ultraviolet Irradiation And Its Adsorption Performance And Mechanism On VOCs Elimination

Volatile Organic Compounds?VOCs? are a portion of the amplitude hydrocarbon family, which were defined by US EPA as any compound of taking part in atmospheric photochemical reactions carbon, but does not contain CO, CO₂, carbonic acid, metallic carbides or carbonates, and ammonium carbonate. VOCs are usually released into the atmosphere in gaseous form, which have significant importance on account of their immense effects of changes in human health, climate change and ecology. At present, biofiltration technology and adsorption technology generally are used to treat the emission of VOCs. The activated carbon adsorption technology is the most commonly used technology, because of simple operation and high efficient adsorption. However, the actived carbon is low decisively cost-effective, as the preparation and activation of the actived carbon is not very economically feasible. Biochar has been attracted great attention, due to cheap production cost, particularly abundant agricultural by-products, which are used for biochar produce.A number of studies demonstrated that biochars are capable of sorbing organic substances in water or soil. However, the adsorption capacities of gaseous organic matter on biochars have not been clearly defined. Hence, it is very nesessery to investigate the biochar used in VOCs elimination and it is also favorable in the comprehensive utilization of agricultural wastes and atmospheric environmental protection.In this work, walnut shell and coconut shell from agricultural waste were use to prepare a series of biochars in this work, and an innovative ultraviolet irradiation modification technology was used to modify biochar. In order to investigate the mechanism of ultraviolet irradiation modification, the physicochemical characteristics of boichars were measured by Elemental analysis, Fourier transform infrared spectra, Bohem titration and Specific surface area and porosity analyzer. And the column adsorption experiments were conducted for the assessment of the VOCs adsorption behavior. In addition, the dynamic adsorption,the desorption behavior and the recycling using effect of modified biochar were anaysised. These researchs would provide theoretical basis and technical guidance for industrial application of biochar. This paper's main conclusions are as follows:?1? The adsorption performance of coconut shell biochar was better than walnut shell biochar under the same preparation conditions. The adsorption capacity of biochar was increased with the increasing pyrolytic temperature within the temperature range?400? to 700??. The adsorption process of low pyrolytic temperature biochar?400?? and high pyrolytic temperature biochar?700?? were described with the pseudo-second-order model and the pseudo-first-order model, respectively. At the adsorption temperature of 30?, the isothermal adsorption process could be fitted by Toth model, and the maximal adsorption amounts of benzene and toluene calculated by Toth were 18.98 mg/g and 61.73 mg/g. The surface acidic groups and porous structure of biochar played an important role on adsorption capacity, affecting the surface adsorption and intraparticle diffusion process.?2? The adsorption capacity of biochar could be significantly improved by ultraviolet irradiation with UV lamp of main emission line at 365 nm. The maximum adsorption capacity of UV modified biochar?300? irradiation temperature, 16 h irradiation time? at 303 K was up to 478.33 mg g^-1 benzene and 712.48 mg g^-1 toluene, which was even higher than some activated carbons in literatures. The adsorption isotherms are better fitted with Langmuir model. Elovich kinetic equation provides the best correlation for the adsorption process of adsorbents. The anaysis results of Weber and Morris model suggested that intraparticle diffusion was not the only rate-controlling step, and some degree of boundary layer has effect on the adsorption process. The adsorption quantity analysis of surface adsorption and diffusion showed that more than 90% of adsorption capacity of modified biochar was attained by surface adsorption. The adsorption capacity of modified biochars and the correlation with biochars properties was investigated. The results obtained reveal the biochar surface acidic groups play a dominant role in the adsorption process.?3? Biochar properties showed significant increase of acidic functional groups and external special surface areas. After 16 hour ultraviolet irradiation at 300 ?,the amount acidic functional groups and external special surface areas was increased from 0.28 mmol.g^-1 to 2.01 mmol.g^-1 and from 0.89 m².g^-1 to 34.47 m².g^-1. The irradiation temperature and irradiation time are two important factors. The higher irradiation temperature and the longer irradiation time, the more the formation of biochar surface acidic functional groups and the external surface area. The ultraviolet irradiation time for 4 h was better for formation porous below 1 nm. The carboxylation and demethylation reaction occurred on the biochar surface during ultraviolet irradiation, however, boichar was also a highly condensed ring structures. The formation of oxygen-containing groups is associated with the oxygen covering the surface of biochar.?4? The dynamic adsorption behavior of bezene and toluene on modified biochar showed the adsorbate inlet concentration is higher, the bigger the driving force, and the adsorption breakthrough curve moved towards the left, and the saturated adsorption quantity increased. However, the maximum adsorption capacity decreased with the increase of adsorption temperature. The adsorption breakthrough curve of benzene and toluene on modified biochar were fit well by Yoon-Nelson, Thomas and BDST model, R2>0.98. The results suggested that these three models could be used to predict the adsorption process, which could provid theoretical base and practical references for biochar's application.?5? The desorption behavarior of benzene and toluene on modified biochar was similar and benzene is more easily desorption, suggesting that the interaction between toluenen and modified biochar was stronger. The TG-DTG date under three kinds of heating rates of 10 K.min-1,15 K.min-1 and 20 K.min-1 showed that the desorption activation energy values of benzene and toluene on modified biochar were 59.03 kJ.mol-1 and 63.42 kJ.mol-1, calculated by Tang Wanjun method, and the desorption kinetic mechanism functions of benzene and toluene on M-Y700 were ? ?0.193 G a =[-ln?1- a?] and? ?3G a =[-ln?1- a?], the values of ln A were-3.078 and-3.103. The desorption activation energy values of benzene and toluene on modified biochar were 50.80 kJ.mol-1 and 75.41 kJ.mol-1, calculated by C-R integral method, and the desorption kinetic mechanism functions of benzene and toluene on M-Y700 were ? ?2G a =[-ln?1- a?] and? ?3G a =[-ln?1- a?], the values of lnA were-3.60 and-8.656.The desorption mechanism of benzene and toluene on modified biochar was random nucleation and subsequent growth, S-shaped ? VS t curve.?6? The five time recycling curves of benzene and toluene on modified biochar showed that the adsorption properties of biochar gradually reduced with the increase of cycling times. This may be because some oxygen-containing functional groups on the surface of the biochar gradually disappear in the thermal regeneration process, partial adsorbate was not completely away from the surface of the adsorbent, and some of the adsorbent in the thermal regeneration process lost.