Removal of toxic elements from soil and sewage sludge

Morales Arteaga, Juan Francisco (author), Leštan, Domen (mentor)

Potentially toxic elements (PTEs) are present in soil and sewage sludge (SS), posing health and environmental issues. A washing process, known as ReSoil, has been developed at the Biotechnical Faculty of the University of Ljubljana. The process uses ethylenediaminetetraacetic acid (EDTA) as the main chelator to extract PTEs from the soil. After the remediation, the process solutions are recycled and reused in a series of batches where no wastewater is generated, only solid waste. The aim of the doctoral dissertation was to optimize the ReSoil process to extract the PTEs and the metalloid As from the soil and to adapt the process to extract PTEs from SS. With the modified ReSoil process, soil historically contaminated with Pb, Zn, and Cd, and artificially contaminated with As, was washed with EDTA, oxalic acid, and dithionite on a pilot-scale. After a series of batches, the PTEs and the metalloid As were successfully extracted from the soil, and the process solutions were recycled in a pH gradient of 12.5–2, which was imposed by the addition of quicklime (CaO) and H2SO4. The washing process generated only solid waste. The use of oxalic acid in combination with EDTA activated the chelator by capturing Ca and forming insoluble Ca-oxalate, reducing the washing time to only 1 hour. After the washing process, the soil quality was not irreversibly impaired. In SS, the use of EDTA in a closed-loop washing process proved to be feasible for removing PTEs. However, the efficiency of PTEs removal varied depending on the origin of the SS samples. Other treatments, such as Hydrodynamical Cavitation (HC) and acid hydrolysis (AH), were found to be compatible with the EDTA washing process and enhanced PTEs removal efficiency. The adapted ReSoil washing process for SS was performed in a series of batches, did not generate wastewater, and allowed for the recycling of the chelator. EDTA washing coupled with AH proved to be the most efficient method for PTEs removal, except for Cu and Cr. However, it was found that this method led to a significant depletion of nutrients compared to only EDTA washing and EDTA washing coupled with HC. In a pot experiment, it was observed that the use of washed SS slightly improved the growth of Brassica juncea, although the improvement was not statistically significant.

• Language: English
• Keywords: Potentially toxic elements, sewage sludge, EDTA washing, soil, phosphorus, Hydrodynamical cavitation, acid hydrolysis
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: BF - Biotechnical Faculty
• Year: 2023
• PID: 20.500.12556/RUL-151640
• COBISS.SI-ID: 168612611
• Publication date in RUL: 14.10.2023

Secondary language

• Language: Slovenian
• Title: Odstranjevanje strupenih elementov iz tal in blata čistilnih naprav

Abstract

Potencialno strupeni elementi (PTEs) so prisotni v tleh in blatu iz čistilnih naprav (SS), kar predstavlja zdravstveno in okoljsko problematiko. Na Biotehniški fakulteti Univerze v Ljubljani je bil razvit postopek pranja, znan kot ReSoil. Postopek uporablja etilenediamintetraocetno kislino (EDTA) kot glavni kelator za ekstrakcijo PTEs iz tal. Po remediaciji se procesne raztopine reciklirajo in ponovno uporabijo v več serijah, pri čemer ne nastaja odpadna voda, temveč le trdni odpadki. Cilj doktorske disertacije je bil optimizirati postopek ReSoil za ekstrakcijo PTE in metaloida As iz tal ter prilagoditi postopek za ekstrakcijo PTE iz SS. S prilagojenim postopkom ReSoil so bila tla, ki so bila v preteklosti onesnažena s Pb, Zn in Cd ter umetno onesnažena z As, v pilotnem obsegu izprana z EDTA, oksalno kislino in ditionitom. Po več serijah so bili PTE in kovinski As uspešno ekstrahirani iz tal, procesne raztopine pa so bile reciklirane v gradientu pH 12,5-2, ki je bil določen z dodajanjem gašenega apna (CaO) in H2SO4. Pri pranju so nastali le trdni odpadki. Uporaba oksalne kisline v kombinaciji z EDTA je aktivirala kelator tako, da je zajel Ca in tvoril netopen Ca-oksalat, kar je skrajšalo čas pranja na samo 1 uro. Po postopku pranja se kakovost tal ni nepopravljivo poslabšala. V SS se je izkazalo, da je uporaba EDTA v postopku pranja v zaprti zanki izvedljiva za odstranjevanje PTEs. Vendar se je učinkovitost odstranjevanja PTEs razlikovala glede na izvor vzorcev SS. Druge obdelave, kot sta hidrodinamična kavitacija (HC) in kislinska hidroliza (AH), so bile združljive s postopkom pranja z EDTA in so povečale učinkovitost odstranjevanja PTEs. Prilagojeni postopek pranja ReSoil za SS je bil izveden v več serijah in je omogočal recikliranje kelatorja brez nastanka odpadne vode. Pranje z EDTA v kombinaciji z AH se je izkazalo za najučinkovitejšo metodo za odstranjevanje PTEs, razen Cu in Cr. Vendar je bilo ugotovljeno, da je ta metoda povzročila znatno zmanjšanje hranil v primerjavi s samo pranjem z EDTA in pranjem z EDTA v kombinaciji s HC. V lončnem poskusu je bilo ugotovljeno, da je uporaba opranega SS nekoliko izboljšala rast Brassica juncea, čeprav izboljšanje ni bilo statistično značilno.

• Keywords: Toksični kovinski, komunalna blata, EDTA pranje, zemlja, fosfor, hidrodinamična kavitacija, kislinska hidroliza