Adsorption Mechanism Of Nitrogen-heterocyclic Compounds In Wastewater On Porous Bamboo Charcoal

N-Heterocyclic compounds (NHCs) are widely presented in effluent of cokingwastewater. They are hazardous in nature with a long-term persistence in the environmentbecause of poor biodegradability. Many NHCs have been listed as priority pollutants byU.S. Environmental Protection Agency. Adsorption technique is simple and efficient forremoving NHCs in aqueous phase. Traditional activated carbon (AC) has been commonlyused as adsorbent for NHCs removal. However, AC is mostly made from thenon-renewable source of coal having the problem of high cost and difficult regeneration,which impedes the application in the future. Other carbonaceous materials such as carbonnanotubes (CNTs) have attracted increasing interests in recent years, but the cost is highand potential toxicity exists. Therefore, it is important to develop new environmentallyfriendly and cost-effective adsorbents for NHCs removal.Bamboo charcoal (BC) is a new class of porous carbonaceous materials. It isproduced from the incomplete combustion of moso bamboo at high temperature and undernitrogen atmosphere. In the present study we investigate the adsorption of three NHCs,pyridine, indole and quinoline, on a new renewable porous bioresource of BC. Two typesof BC produced in lab are modified by HNO3, NaOH and microwave (MW) radiation tocreate different physiochemical properties. The performance of BC in NHC adsorption isevaluated, and an adsorbent-adsorbate derived model is develop to measure the relativecontribution of each interaction. The key process controlling the rate of NHC adsorptionon BC is identified, and the spontaneity and thermodynamic feasibility of the adsorptionprocess is analyzed. The feasibility and mechanism of regeneration of spent BC by MWradiation is elucidated. The main conclusions can be summed up as follows:(1) The structure of BC is mainly composed of graphite sheets rimmed with functionalgroups (i.e. C–C, C–O, C=O, COO–, π–π*, C=N) on the edge forming connectedmesoporous network. HNO3treatment decreases BET surface area and micropore volumeas well as hydrophobicity, whereas NaOH treatment increases BET surface area andmicropore volume. MW treatment dramatically increases hydrophobicity and the fraction ofaromatic structure. In comparison with AC and CNTs, BC has relative lower graphite structure and specific surface area but larger O-containing groups and porous structure.These features make BC a promising adsorbent for adsorbing bulky organics such as NHCs.(2) Adsorption isotherms of NHCs are nonlinear and better fitted by Freundlichmodel compared with Langmuir model and Polanyi–Manes model (PMM). BCs exhibitesstrong adsorption affinity to NHCs. The maximum adsorption capacities for pyridine,indole and quinoline reach42.9,93.2and91.7mg/g, respectively. NHC adsorption onBCs is mainly regulated by surface area, hydrophobic interaction, electrostatic interactionand π-π EDA interaction. A model relating NHC adsorption (log K) andadsorbent-adsorbate physicochemical properties is developed. The relative contributiondominating NHC adsorption on BCs is proven to be surface area> hydrophobicinteraction> electrostatic interaction> π-π EDA interaction. Direct evidence is providedfor the relative importance of multiple mechanisms in NHC adsorption by BCs.(3) Adsorption kinetics is analyzed using Lagergren pseudo-first-order and Lagergrenpseudo-second-order as well as Weber-Morris model. The results show that NHC adsorptionon BC is predominantly regulated by surface diffusion in initial1h followed by intraparticlediffusion in later stage. Thermodynamic analysis indicates that the adsorption isspontaneous and endothermic. Adsorption site energy analysis illustrates a distribution ofadsorption energy, which indicates the heterogeneous sites on BC for NHC adsorption.(4) The spent BC with NHC adsorption can be effectively regenerated by MWradiation. The adsorption capacity becomes even higher than that of virgin BC after fivetimes of adsorption-regeneration cycles. The mechanism for MW regeneration can beexplained from the following aspects:(i) MW radiation increases the external surface area,average pore diameter and total pore volume, even though the total surface area isdecreased in some cases;(ii) the hydrophobicity of regenerated BC is increased;(iii) MWradiation increases graphitic content and π–π*content on BC’s surface, thereby increasingthe adsorption via π–π interaction. The fixed-bed column results prove that BC is apromising adsorbent for NHC removal in outflow of coking wastewater.