Selective Enrichment Of Precious Metal Ag(?) And Mechanism Using Chemically Modified Biosobents

Compared with traditional pyro-and hydro-metallurgy methods,biosorption is well recognized as a promising technology to recover precious metals.Developing new biosorbents and revealing biosorption mechanism will still remain the main research direction in the future.Currently,the research on biosorption of precious metals,however,mainly focused on gold and platinum metals,it is relatively lack in the biosorption of silver,which is the most widely used element among precious metals.Silk sericin,persimmon tannin and apple pomace are resourceful biomass yet they are lack of appropriate application in practice.In this context,this doctoral thesis focused on the fabrication of four kinds of biosorbents using above metioned waste biomass by chemically modification in the guidance of structure-performance relationships.The property of the resulting biosorbents were evaluated by selective enrichment and recovery of silver from electronic waste leachates and industrial effluents.Using epichlorohydrin as cross-linking agent,N,N-dimethylformamide(DMF)as solvent,and dithiodiurea(BTU)and 2,5-dimercapto-1,3,4-thiadiazole(DMTD)as modifiers,the new biosorbents BTU-SS,DMTD-PT and DMTD-AP were synthesized under basic medium by grafting BTU on the matrix of sericin(SS),and DMTD to the active hydroxyl groups of persimmon tannin(PT) and apple pomace(AP).The AP-based magnetic biochar,M600APB,was prepared by pyrolyzing apple pomace at 600°C in N₂ atmosphere,followed by Fe(?)/Fe(?I)impregnation and aging in an aqueous solution.The composition,structure and surface properties of the biosorbents were analyzed and characterized by means of Fourier transform infrared spectroscopy(FTIR),elemental analysis,TG-DTG thermal analysis,end group titration,TOC analysis,XRD,CP/MAS¹³C-NMR,X-ray photoelectron spectroscopy(XPS),zeta potential analysis,SEM-EDS,and BET specific surface area measurement.The effects of pH,temperature,dose,time and metal ion concentration were studied by batch adsorption in multi-component solution to evaluate the adsorption behavior of biosorbents towards Ag(?)before and after modification.Adsorption thermodynamics,kinetics and mechanisms were also investigated.BTU-SS,DMTD-PT,DMTD-AP and M600APB were respectively used as fillers in the fixed bed adsorption experiment to determine the breakthrough curves of each ion during the adsorption process,upon which the possibility of biosorbents for enrichment and separation of Ag(?)were evaluated.The main findings of the thesis are as follows:1.Batch adsorption revealed that adsorption rates of BTU-SS and SS towards Ag(?) were 93.9%and 32.0%respectively in acid leachate containing Ag(?)of 113.6mg?L^-1with a dosage of 3.5 g?L^-1at room temperature at pH 5.0,and less than 5%of coexisting metal ions were adsorbed by BTU-SS,indicating selectivity of BTU-SS towards Ag(?).The adsorption of BTU-SS towards Ag(?)was consistent with the monolayer Langmuir isothermal model and the maximum adsorption capacity reached48.56 mg?g^-1.As further demonstrated by column experiments,BTU-SS allowed the separation of Ag(?) from an electronic waste leachate.Thermodynamic studies showed that adsorption of BTU-SS towards Ag(?)was exothermic and spontaneous,while adsorption kinetics revealed that chemisorption dominated the adsorption with activation energy of 47.67 k J?mol^-1.Adsorption mechanism of BTU-SS to Ag(?) mainly involved electrostatic,ion exchanges and coordination interactions.2.As revealed by the adsorption experiments,both DMTD-PT and crude PT showed poor affinity towards coexisting base metal ions,however,DMTD-PT and PT adsorbed 98.7%and 20.7%,respectively,of Ag(?) in an e-waste leachate with an initial Ag(?) concentration of 117.3 mg?L^-1with a dosage of 5.0 g?L^-1each at pH 3.0at ambient temperature,indicating the selectivity of DMTD-PT for Ag(?) was superior to that of PT.The adsorption of DMTD-PT towards Ag(?) well fitted the typical monolayer type of Langmuir isotherm model,with the maximum adsorption capacity of 52.1 mg?g^-1.The column experiments further proved DMTD-PT could separate Ag(?) from e-waste leachate.Thermodynamic study showed DMTD-PT adsorption to Ag(?) was spontaneous with endothermic nature,while kinetic study indicated the adsorption was dominated by chemisorption with activation energy of 103.0 k J?mol^-1.The plausible adsorption mechanism of DMTD-PT to Ag(?) was mainly attributable to the complexation via N and S atoms on diazole ring.Also,DMTD-PT exhibited better reusability without distinct decline of Ag(?) adsorbed even after 5 repeated cycles.3.As revealed by the adsorption experiments,DMTD-AP showed poor affinity to coexisting base metal ions,however,DMTD-AP adsorbed 99.3% whereas AP adsorbed less than 30%,of Ag(?) in a solution with an initial Ag(?) concentration of218.1 mg?L^-1with a dosage of 5.67 g?L^-1each at p H 3.0 at ambient temperature,indicating the selectivity of DMTD-AP for Ag(?) was superior to that of AP.The adsorption of DMTD-AP towards Ag(?) exhibited the BET-like adsorption feature,with the maximum adsorption capacity of 196.89 mg?g^-1.Thermodynamic study indicated that DMTD-AP adsorption towards Ag(?) was spontaneous with endothermic nature,while kinetic study showed adsorption was dominated by chemisorption with activation energy of 50.78 k J?mol^-1.The column adsorption experiment further revealed the possibility of separation and enrichment of Ag(?) from Ag(?)-Ni(?)-Zn(?)-Cu(?)-Pb(?)system.The mechanism of DMTD-AP adsorption to Ag(?) mainly relies on complexation between Ag(?) and ligand atoms of N and S.4.The saturation magnetization value and the Brunauer-Emmett-Teller(BET)specific surface area of magnetic biochar,M600APB,were 9.52 emu?g^-1and 102.18m²?g^-1,respectively.M600APB showed poor affinity to coexisting base metal ions,however,it adsorbed 99.5%of Ag(?)in a solution with an initial Ag(?)concentration of 295 mg?L^-1with a dosage of 4.67 g?L^-1at pH 4.0 at ambient temperature with the maximum adsorption capacity of 818.4 mg?g^-1in an Ag(?)-Pb(?)-Cu(?)-Ni(?)-Zn(?)aqueous system.The column adsorption experiments showed M600APB could effectively enrich and separate Ag(?) from the same aqueous mixture and had good reusability.The presumable mechanism of Ag(?) adsorption on M600APB involves intra-particle diffusion,coordination,ion exchange and reduction.In short,this work provides a reference for development of waste biomass-based biosorbent for enrichment of Ag(?) and other previous metals from effluents,and is also significant for resource recycling as well as sustainable development.