Optimizacija kvaliteta materijala dobijenog hidrotermalnom karbonizacijom i njegova primena u adsorpciji teških metala iz vodenih rastvora

Abstract

Predmet istraživanja ove doktorske disertacije je optimizacija kvaliteta materijala dobijenog hidrotermalnom karbonizacijom (HTC) iskorišćenog supstrata gljiva Agaricus bisporus (eng. Spent Mushroom Supstrate - SMS) i ispitivanje primene dobijenih hidročađi supstrata (HC) kao adsorbenta jona Pb2+ i Cd2+ iz vodenih rastvora. Ispitivan je uticaj procesne temperature na fizičko-hemijske, strukturne i gorivne karakteristike SMS-a i HC dobijenih na 180, 200, 220, 240 i 260 °C (HC-180, HC-200, HC-220, HC-240 i HC- 260). Zaključeno je da je hidrotermalni proces pozitivno uticao na gorivne karakteristike HC, što je vidljivo kroz povećanje vezanog ugljenika, gornje toplotne moći i energetske gustine, kao i kroz smanjenje sadržaja isparljivih materija u odnosu na sirovu biomasu. Strukturne i morfološke karakteristike ispitivanih materijala su određene korišćenjem skenirajućeg elektronskog mikroskopa (SEM) i tehnikom infracrvene spektroskopije sa Furijeovom transformacijom (FTIR). SEM analiza je pokazala formiranje mikrosfera i pukotina na površini HC, dok je FTIR spektroskopijom utvrđeno dominantno prisustvo aromatičnih i kiseoničnih funkcionalnih grupa (KFG). Termičko ponašanje ispitivanih materijala je procenjeno na osnovu termogravimetrijske i diferencijalne termičke (TGA/DTA) analize. Određeni su kinetički i termodinamički parametri korišćenjem model-free metoda: Flynn-Wall-Ozawa (FWO) i Kissinger-Akahira-Sunose (KAS). Metodama termičke analize je potvrđeno da je HTC perspektivan tretman konverzije SMS-a u visoko-energetski ugljenični materijal. Ispitana je mogućnost primene HC kao sorbenta jona Pb2+ i Cd2+ iz vodenih rastvora. Kako bi se izvršio odabir optimalne HC koja će se koristi kao sorbent ispitivanih teških metala, izvršen je preliminarni adsorpcioni test, radi utvrđivanja adsorpcionih kapaciteta korišćenih ugljeničnih materijala. HC-200 je pokazao najbolje performanse za uklanjanje ovih metala, čiji je sorpcioni kapacitet za jone Pb2+ i Cd2+ iznosio 98 mg g–1, odnosno 41 mg g–1. Kako bi se dodatno poboljšao kapacitet adsorpcije HC-200, izvršena je njegova fizičko-hemijska aktivacija. Prvo je uzorak tretiran 20% rastvorom CaCl2·5H2O, a zatim je fizički aktiviran procesom pirolize na 500 °C, pri čemu je sintetisan nov ugljenični materijal nazvan Kalcijum-piro-hidročađ (Ca-PHC). Detaljna karakterizacija odabrane hidročađi pre i posle modifikacije izvršena je SEM analizom, Brunauer-Emmett-Teller (BET) analizom specifične površine i poroznosti, i FTIR spektroskopijom, a zatim su ispitane njihove adsorpcione performanse. Ca-PHC je pokazao znatno bolju sposobnost da uklanja jone Pb2+ i Cd2+ iz vodenog rastvora u poređenju sa HC, pa je stoga dalje korišćen za detaljnije ispitivanje adsorpcionih procesa. Adsorpcioni eksperimenti uklanjanja jona Pb2+ i Cd2+ su urađeni u šaržnom sistemu, kako bi se izvršila optimizacija radnih parametara: pH, mase adsorbenta, početne koncentracije jona metala, vreme kontakta i temperature. Maksimalni sorpcioni kapaciteti Ca-PHC za jone Pb2+ i Cd2+ iznosili su 297 mg g–1, odnosno 131 mg g–1. Pokazano je da se proces sorpcije može opisati Frojndlihovom izotermom i da prati kinetički model pseudo-drugog reda. Termodinamički parametri pokazali su da je vezivanje jona metala za sorbent spontan i endoterman proces. Na osnovu dobijenih rezultata utvrđeno je da se joni metala vezuju za površinu sorbenta mehanizmom jonske izmene, površinskom kompleksacijom i katjon-π interakcijom. Prikazani rezultati sugerišu da HTC kao ekološki prihvatljiva tehnologija može biti dobro rešenje za konverziju vlažne otpadne biomase u visoko-energetske materijale. Pored toga, dobijene HC su pokazale dobre adsorpcione performanse, a kapacitet adsorpcije im je dodatno poboljšan fizičko-hemijskom aktivacijom. Na osnovu predstavljenog, može se zaključiti da je novosintetisani ugljenični materijal, Ca-PHC, iz iskorišćenog supstrata gljiva visoko efikasan biosorbent Pb2+ i Cd2+ jona iz vodenih rastvora, čijom se upotrebom na taj način može doprineti cirkularnoj ekonomiji i minimiziranju otpada u rastućoj industriji pečuraka.

The subject of research of this doctoral dissertation is the optimization of the material quality obtained by hydrothermal carbonization (HTC) of the spent mushroom substrate (SMS) of Agaricus bisporus and the examination of application of the obtained hydrochar (HC) as an adsorbent of Pb2+ and Cd2+ ions from aqueous solutions. The influence of process temperature on the physicochemical, structural and fuel characteristics of the SMS and HC obtained at 180, 200, 220, 240 and 260 °C (HC-180, HC- 200, HC-220, HC-240 and HC-260) was investigated. The hydrothermal process had a positive effect on the fuel characteristics of the HC, which was visible through the increase of the fixed carbon, higher heating value and energy densification, as well as the reduction of volatile matter in relation to the raw biomass. Structural and morphological characteristics of SMS and HC were determined using SEM and FTIR analysis. SEM analysis showed the formation of microspheres and cracks on the surface of the chars, while FTIR spectra confirmed the dominant presence of the aromatics and oxygen-rich functional groups. The thermal behavior of the tested material was assessed using the thermogravimetric and differential thermal (TGA/DTA) analysis. The kinetic and thermodynamic parameters were determined using the model-ree methods: Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS). The methods of thermal analysis have confirmed that HTC is a promising treatment for the conversion of SMS to high-energy carbon material. The possibility of using HC as a sorbent of Pb2+ and Cd2+ ions from aqueous solutions was investigated. In order to select the optimal HC to be used as a sorbent of the invastigated heavy metals, a preliminary adsorption test was performed to determine the adsorption capacities of the carbon materials used. The HC-200 showed the best performance for the removal of these metals, whose sorption capacities for Pb2+ and Cd2+ ions were 98 mg g–1 and 41 mg g–1, respectively. In order to improve the adsorption capacity of the selected chars, the physicochemical activation was performed. The HC-200 was first treated with a 20% solution of CaCl2·5H2O, and then physically activated by pyrolysis at 500 °C. The obtained material was labeled as Calcium-pyro-hydrochar (Ca-PHC). The characterization of selected hydrochar before and after modification by SEM, BET and FTIR analysis was performed, and then their adsorption performance was examined. The Ca-PHC showed a significantly better ability to remove Pb2+ and Cd2+ ions from aqueous solutions, and therefore it was further used to investigate the adsorption process. Adsorption experiments of Pb2+ and Cd2+ ions were performed in the batch system, to optimize the following operating parameters: pH, adsorbent mass, initial metal ion concentration, contact time and temperature. The maximum sorption capacities of Ca-PHC for Pb2+ and Cd2+ ions were 297 mg g–1 and 131 mg g–1, respectively. Based on the obtained results, it was shown that the sorption process can be described by the Freundlich isotherm and followed a pseudo- other kinetic model. Thermodynamic parameters showed that the binding of metal ions to the sorbent was a spontaneous and endothermic process. The results obtained by examining the kinetics of the process and the mechanism of ion exchange, and the characterization of Ca-PHC found that metal ions bind to the sorbent surface by ion-exchange mechanism, surface complexation, mineral deposition and cation-π interaction. The presented results suggest that HTC as an environmentally friendly technology may be the good solution for the conversion of wet biomass into high-energy material. In addition, the obtained HCs showed satisfying adsorption performance and and its adsorption capacity was further improved by physicochemical activation. Based on the above, it can be concluced that the newly synthesized carbon material, Ca-PHC, from the SMS is a highly efficient biosorbent of Pb2+ and Cd2+ ions from aqueous solutions, whose use may contribute the circular economy and waste minimization in the growing mushroom industry.