Biochar For Removing Aqueous Sulfide Via Adsorptive Oxidation:Synthesis,Mechanism And Mass Transfer Processes

The surge in sulfide(S^2-)laden wastewater from industry,agriculture and urban processes is threatening the sustainability of aquatic organisms,human health and structural integrity of water distribution systems and calls for remediation.Adsorptive oxidation is the facile method for removing S^2-from wastewater at a large scale and point of use.Nonetheless,two key concerns hindered its widespread adoption.The first issue is that carbon-based adsorbents produced at high temperatures and pressures from nonrenewable coal/petroleum-based carbon sources and multistep synthesis conditions,which consume more energy and time and pollute the environment,are widely used.The second issue is that the mechanism of S^2-sorption and oxidation mediated by carbon adsorbents critical for designing and scaling an adsorption system is unclear.S^2-transport to carbon adsorbents is thought to include graphitic and porous structures and oxygen functional groups（OFGs）for the oxidation reaction.One hypothesis is that sulfide shuttle electrons through delocalized pi-electron on the graphitized carbon to OFGs as the active sites.The other hypothesis is that S^2-ion diffuse into porous carbon and the active site.However,the dominant mechanism of S^2-transport and specific reactive active moieties（RAMs）in OFGs that oxidized sulfide are still speculated because no previous research has closely examined the influence of porous and conductive structures and redox active OFGs of carbon adsorbents on the adsorptive oxidation of S^2-based on its phenomenological mass transfer processes.The phenomenological mass transfer model provides a theoretical basis for exploring pollutant removal mechanisms,mass transfer limiting steps and strategies for improvement.Biochar can cost less and sequester carbon;however,it is widely overlooked for removing S^2-from water because the rational design of biochar with suitable conductive and porous structure and RAMs remains uncertain.Remarkably,energy of transition from amorphous and non-porous carbon to the conductive and porous carbon structure can be lowered.Redox metal centers and RAMs can be induced at a lower temperature via heterogeneous catalytic graphitization.However,heterogeneous catalysts have thus far been largely limited to precious metals.Although the first-row transition metals such as Fe,Co,or Ni have recently emerged as catalysts for numerous organic transformations,such as dehydrogenation reactions that were traditionally in the realm of precious metal catalysis,their carbothermal catalysis during pyrolysis and the performance of engineered biochar in adsorptive oxidation has not been investigated.Therefore,this thesis used Fe,Co,or Ni ion to transform biochar’s structure and physiochemical proprieties during pyrolysis and proposed the mechanism of S^2-adsorption and oxidation mediated by biochar through extensive experimentations,state-of-the-art characterizations of adsorbents and mathematical modelling of phenomenological mass transfer of S^2-adsorption for the first time.Three major objectives guided this research.In objective 1,BC-CO and BC-Fe engineered from Co²⁺,or Fe³⁺and pristine biochar（PBC）were produced to differ in specific surface area values,conductive and pore structure and redox activity to explore how these properties influence the kinetics and oxidation of sulfide in an aqueous solution.Co²⁺,or Fe³⁺impregnated in corn stover were anchored on the hydroxyl groups in cellulose and hemicellulose and decomposed them into the volatile matter while improving the porous and conductive structure and decorated as elemental cobalt（Co⁰）or magnetite（Fe₃O₄）on engineered biochars（BC-Co or BC-Fe）.BC-Co had higher mesopore volume,specific surface area,and pore size distributions than BC-Fe due to the superior catalytic activity of Co²⁺,during pyrolysis than Fe³⁺.Electrochemical impedance spectra showed BC-Fe was more electrochemically conductive,possibly due to Fe₃O₄ being decorated on the carbon matrix.The conductive structure is a facile electron transport to the active site.However,the porous structure is more critically important and serves as S^2-ions diffusion channel to the active site at the wall and pores of the biochar.BC-Co benefited from superior mesopore volume,pore size distribution and specific surface area to attain811±146 mg-S^2-/g maximum capacity.BC-Fe adsorbed S^2-far less than PBC because Fe₃O₄ on its surface blocked S^2-diffusion channel to the active site.It is suggested that high contact surface area,meso pore volume,and pore size distributions is needed for faster and higher S^2-adsorption.In addition,the specific surface area exposed huge oxygen surface groups as active sites for the oxidation of S^2-to sulfur and sulfate.Objective 2 was designed to explore further the impact of biochar’s porous structure and how operation conditions affect sulfide adsorptive oxidation.It is hypothesized that Ni²⁺impregnated in corn stover would catalyze the formation of mesoporous biochar（BC-Ni）due to its similar carbothermal catalysis with Co²⁺and higher position in the transition metal series.As expected,Ni²⁺catalyzed mesopore biochar（BC-Ni）formation and was compared with PBC.BC-Ni adsorbed S^2-2.72-fold faster than PBC alone and outperformed PBC at different temperatures,contact time,adsorbents dosages,initial sulfide concentration,and solution p H and in the presence of PO₄^3-and NH₄⁺-N as some competing ions.BC-Ni attained 1244±252 mg-sulfide/g maximum adsorption capacity due to markedly increased mesopore volume and specific surface area,while BC attained 583±250 mg-sulfide/g.Experimental results and state-of-the-art characterization of PBC and BC-Ni examined give clues to suggest the mechanism of sulfide adsorption and oxidation and develop the phenomenological mass transfer model of sulfide adsorption.In objective 3,BC-Co and BC-Ni were employed to develop a mathematical model describing the sulfide transport phenomenon during adsorption processes based on adsorption equilibrium and kinetic data due to their similar equilibrium time and explored the mass transfer-limiting step.The error analysis and the model plots interpretation suggest that S^2-adsorption at the active sites was the rate-limiting step of S^2-adsorptive oxidation,possibly because of the strong redox reaction between S^2-with quinone,semiquinone radicals,or hydroxyl radicals at the pore walls.The EMT rate constant of PBC,BC-Co and BC-Ni was 0.012,0.10 and 0.126 L g^-1min^-1.This shows that rate of S^2-diffusion from bulk liquid to the vicinity of BC-Ni pore was faster than on PBC and BC-Co,possibly because the huge SSA resulted in wide distribution of BC-Ni particles in liquid and shortened the film diffusion path than the path connecting to PBC and BC-Co.The IMT rate of S^2-ions sorption onto BC-Co pores was 50 and 0.55 times faster than PBC and BC-Ni,suggesting its superior pore size distributions facilitated the diffusion of S^2-ions from the exterior of biochars into the pores and active sites.Based on the experimental results and phenomenological mass transfer model,the S^2-ions adsorption and oxidation mechanism is proposed and summarized:First,S^2-transports from the bulk liquid to the film connecting the biochar pores via cation bridges.Second,S^2-diffuses from the film zone into the biochar pores via surface and pore diffusion.Lastly,S^2-was adsorbed and oxidized to elemental sulfur by quinone moieties,particularly quinone and semiquinone-PFRs,and sulfate by in situ generated hydroxyl radicals at the wall of the biochar or hydrogen peroxide in the biochar pores.It is concluded that a balanced pore volume and pore size distribution are needed for faster S^2-ions diffusion to the active sites and a higher specific surface area for its higher adsorption capacity.Co,Fe or Ni ions impregnated in the corn stover catalyzed the formation of electroactive and porous biochars.Ni and Co ions had similar catalytic activity in transforming biochar pore volume and specific surface area by dehydrogenating hydroxyl and carboxylic functional groups into the volatile matter to:（i）open new pores;（ii）widen the pre-existing pores that are not fully developed,and（iii）merge the existing pores while breaking down the walls between adjacent pores.Fe ion impregnated in corn stover had less catalytic activity and mainly opened:（i）new pores;（ii）previously underdeveloped pores,increasing the specific surface area and electron conductivity of BC-Fe more than PBC during the pyrolysis.This fundamental information would not only would serve as theoretical guidance for designing biochar and scaling large sulfide adsorptive oxidation units but would promote a circular economy.These novel adsorbents can potentially remove sulfides in anaerobic digesters and other redox-active contaminants in wastewater.