Hydrothermal carbonization (HTC) is a thermochemical treatment of sewage sludge and leads to the formation of a high carbon content product called hydrochar (HC). It is a product that can have multiple uses such as: (i) fuel, thanks to a higher heating value (HHV) similar to that of vegetable carbon, low ash content and high fixed carbon content; (ii) fertilizer or soil improver since, if applied to the soil, it can improve water retention and the availability of nutrients; (iii) source of phosphorus (P) which, in the HTC process, is mostly retained in the solid phase; (iv) adsorbent, thanks to its porosity, specific pore surface and oxygen functional groups. In this study, HC was used as an adsorbent to remove Zn and Cu, as they are the two most present heavy metals in the sewage sludge. Tests were first carried out on a synthetic solution and, subsequently, on the liquid produced by the acid leaching of bottom ash obtained from the mono-incineration of sludge. To improve the adsorption efficiency, activation procedure with potassium hydroxide (KOH) was tested. The effect of pH was studied and the best value for the adsorption of Zn and Cu was found to be at pH 8. Therefore, setting the HC dosage at 2000 mg/L and a contact time of 24 h, the best removal efficiency for both HC and activated hydrochar (ACT-HC) was greater than 94% for both Zn and Cu. Next, the effect of hydrochar concentration was investigated. Dosages between 5 mg/L and 2000 mg/L (pH 8 and contact time of 24 h for HC and ACT-HC) allowed high removal efficiency equal to over 88% for Zn and over 94% for Cu. Kinetics experiments were then studied by varying the times (2, 6 and 24 h), while keeping constant the dosage of 40 mg/L for HC and ACT-HC at pH 8. With a contact time of 2 h the removal was greater than 94% for Zn and 99% for Cu, demonstrating that 2 h is sufficient to have a high percentage of removal, even without activation, which would entail additional costs without significant efficiency gains. HC adsorption capacity was also tested on the acid leached product obtained from the bottom ash of a sludge incinerator. For the single test conducted, same experimental parameters of the synthetic solution were applied. By dosing 40 mg/L of HC at pH 8 and contact time of 2 h, the removal efficiency of Zn was much lower than that of the synthetic solution, about 7% for Zn and 5.5% for Cu. To investigate the real potential of HC as an adsorbent of Cu and Zn, it will be necessary to perform a greater number of tests on the acid leachate, at different dosages and contact times, as well as investigating the interference and competition factors that may have influenced the adsorption performance.
La carbonizzazione idrotermica (HTC: hydrothermal carbonization) è un trattamento termochimico dei fanghi di depurazione e porta alla formazione di un prodotto ad alto contenuto di carbonio denominato hydrochar (HC). Si tratta di un prodotto che può avere molteplici utilizzi, quali: (i) combustibile, grazie a un potere calorifico superiore (PCS) simile a quello del carbone vegetale, del basso contenuto di ceneri e dell'alto tenore di carbonio fisso; (ii) fertilizzante o ammendante poiché, se applicato al suolo, può migliorare la ritenzione idrica e la disponibilità di nutrienti; (iii) fonte di fosforo (P) che, nel processo HTC, viene trattenuto per la maggior parte nella fase solida; (iv) adsorbente, grazie alla sua porosità, superficie specifica dei pori e gruppi funzionali contenenti ossigeno. In questo studio l’HC è stato utilizzato come adsorbente per rimuovere Zn e Cu, in quanto sono i due metalli pesanti maggiormente presenti nei fanghi di depurazione. Si sono dapprima eseguite prove su una soluzione sintetica e, successivamente, sul liquido prodotto dalla lisciviazione acida delle ceneri ottenute dal mono-incenerimento di fanghi. Per migliorare l’efficienza di adsorbimento, è stata testata una procedura di attivazione con idrossido di potassio (KOH). L'effetto del pH è stato studiato e il miglior valore per l'adsorbimento di Zn e Cu è risultato pari a pH 8. Quindi, fissando il dosaggio di HC a 2000 mg/L e un tempo di contatto di 24 ore, la migliore efficienza di rimozione sia per HC che per hydrochar attivato (ACT-HC) è stata superiore al 94% sia per Zn che per Cu. Successivamente, è stato studiato l’effetto della dose di hydrochar. Dosaggi compresi tra 5 mg/L a 2000 mg/L (pH 8 e tempo di contatto di 24 h per HC e ACT-HC) hanno permesso un’efficienza di rimozione maggiore dell’88% per Zn e oltre il 94% per Cu. È stato studiato successivamente l’effetto della cinetica di rimozione, variando il tempo di contatto (2, 6 e 24 h) e mantenendo costante il dosaggio di 40 mg/L di HC e di ACT-HC a pH 8. Con un tempo di contatto di 2 h la rimozione è stata superiore al 94% per Zn e al 99% per Cu, dimostrando che 2 h di contatto sono sufficienti per avere un'elevata percentuale di rimozione, anche senza attivazione, che comporterebbe costi aggiuntivi sena significativi incrementi di efficienza. La capacità di adsorbimento è stata testata sul lisciviato acido ottenuto dalle ceneri di fango di un inceneritore di fanghi. È stata condotta una prova singola preliminare adottando gli stesi parametri sperimentali utilizzati per la soluzione sintetica. Dosando 40 mg/L di HC a pH 8 e tempo di contatto di 2 h l'efficienza di rimozione di Zn è risultata molto inferiore a quella della soluzione sintetica, pari a circa il 7% per Zn e 5,5% per Cu. Per investigare le reali potenzialità dell’HC come adsorbente sarà necessario eseguire un maggior numero di prove sul lisciviato acido a diversi dosaggi e tempi di contatto, oltre a investigare i fattori di interferenza e competizione che possono aver influenzato le prestazioni di adsorbimento
Removal of heavy metals using hydrochar derived from HTC of sewage sludge
MIREMAD, SEYEDMOEIN
2021/2022
Abstract
Hydrothermal carbonization (HTC) is a thermochemical treatment of sewage sludge and leads to the formation of a high carbon content product called hydrochar (HC). It is a product that can have multiple uses such as: (i) fuel, thanks to a higher heating value (HHV) similar to that of vegetable carbon, low ash content and high fixed carbon content; (ii) fertilizer or soil improver since, if applied to the soil, it can improve water retention and the availability of nutrients; (iii) source of phosphorus (P) which, in the HTC process, is mostly retained in the solid phase; (iv) adsorbent, thanks to its porosity, specific pore surface and oxygen functional groups. In this study, HC was used as an adsorbent to remove Zn and Cu, as they are the two most present heavy metals in the sewage sludge. Tests were first carried out on a synthetic solution and, subsequently, on the liquid produced by the acid leaching of bottom ash obtained from the mono-incineration of sludge. To improve the adsorption efficiency, activation procedure with potassium hydroxide (KOH) was tested. The effect of pH was studied and the best value for the adsorption of Zn and Cu was found to be at pH 8. Therefore, setting the HC dosage at 2000 mg/L and a contact time of 24 h, the best removal efficiency for both HC and activated hydrochar (ACT-HC) was greater than 94% for both Zn and Cu. Next, the effect of hydrochar concentration was investigated. Dosages between 5 mg/L and 2000 mg/L (pH 8 and contact time of 24 h for HC and ACT-HC) allowed high removal efficiency equal to over 88% for Zn and over 94% for Cu. Kinetics experiments were then studied by varying the times (2, 6 and 24 h), while keeping constant the dosage of 40 mg/L for HC and ACT-HC at pH 8. With a contact time of 2 h the removal was greater than 94% for Zn and 99% for Cu, demonstrating that 2 h is sufficient to have a high percentage of removal, even without activation, which would entail additional costs without significant efficiency gains. HC adsorption capacity was also tested on the acid leached product obtained from the bottom ash of a sludge incinerator. For the single test conducted, same experimental parameters of the synthetic solution were applied. By dosing 40 mg/L of HC at pH 8 and contact time of 2 h, the removal efficiency of Zn was much lower than that of the synthetic solution, about 7% for Zn and 5.5% for Cu. To investigate the real potential of HC as an adsorbent of Cu and Zn, it will be necessary to perform a greater number of tests on the acid leachate, at different dosages and contact times, as well as investigating the interference and competition factors that may have influenced the adsorption performance.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/192138