Dissertation committee:
1. Νικόλαος Χριστοδουλάκης, αφυπηρετήσαντας Καθηγητής Ανατομίας Φυτών του Τμήματος Βιολογίας ΕΚΠΑ.
2. Αθηνά Οικονόμου – Αμίλλη, Ομότιμη Καθηγήτρια Οικολογίας Υδάτινων Οικοσυστημάτων του Τμήματος Βιολογίας ΕΚΠΑ.
3. Χρήστος Κατσαρός, Ομότιμος Καθηγητής Δομής και Ανάπτυξης Φυτών – Φυκολογίας του Τμήματος Βιολογίας ΕΚΠΑ.
4. Δημήτριος Χατζηνικολάου, Αναπληρωτής Καθηγητής του Τμήματος Βιολογίας ΕΚΠΑ
5. Ιωάννης – Δημοσθένης Αδαμάκης, Επίκουρος Καθηγητής του Τμήματος Βιολογίας ΕΚΠΑ
6. Σάββας Γενίτσαρης, Επίκουρος Καθηγητής του Τμήματος Βιολογίας ΕΚΠΑ
7. Εμμανουήλ Φλεμετάκης, Καθηγητής του Τμήματος Βιοτεχνολογίας, Γεωπονικό Πανεπιστήμιο Αθηνών
Summary:
In the present study the potential of a biofilter containing a mixture of dried micro-algal/bacterial biomass for removing heavy metals (Cu2+, Cd2+, Zn2+, Ni2+) from dilute single-metal solutions and real Ni2+ electroplating waste, was tested in both batch and fixed bed column experiments.
Heavy metal contamination exists in aqueous waste streams of many industries, such as metal electroplating facilities, mining operations and tanneries. Heavy metals are not biodegradable and tend to accumulate in living organisms causing various diseases and disorders, as well as deleterious ecological effects. Conventional methods for removing metals are either becoming inadequate to address current stringent regulatory effluent limits or are increasing in cost. As a result, alternative, cost effective technologies are in high demand. The use of biological materials, including living and non-living micro-organisms, to remove and recover toxic or precious metals from industrial waste waters has gained popularity over the years due to good performance, availability, and low cost of raw materials.
Optimum biosorption conditions (biomass conditioning, initial pH, contact time, initial metal concentration) for each of the 4 metals were determined in baseline batch experiments using a series of full factorial balanced experimental designs with n = 4, pH and AAS replicate readings per treatment. Micro-algal/bacterial biomass from the secondary treatment stage of the Lavrio wastewater treatment plant (Prefecture of Attica, Greece), consisting mostly of filamentous forms (Oscillatoria sp) was used, following sun-drying and autoclave sterilization, thorough washing with deionized water and drying to constant weight. Maximum percent removal (100%) was observed for Cd2+, followed by Zn2+, Cu2+and Ni2+ at 95-97%, 80% and 60%, respectively, with the deionized-H2O conditioned biomass at initial pH = 4.0, within 5 min of contact time. Cd2+ had the highest uptake (1 mg/g) at an initial metal concentration of 10 mg/L, pH = 4 and 15 mins contact time. At the highest concentration tested (1000 mg/L), peak Cd2+ uptake reached 50 mg /g within 120 mins contact time. Sufficiently good results of % removal (70-80%) and metal uptake were also observed for the untreated and 1M Na2CO3 treated biomass, at pH 5.5 for Zn2+ and Cd2+. Biosorption data were fitted successfully by the Langmuir model and results showed a different affinity of the used biomass for each metal with Cd2+ and Zn2+ having a qmax of 31.3 and 18.8 mg/g, respectively, and Cu2+ and Ni2+ a qmax of 18.3 and 13.2 mg/g, respectively.
Subsequently, metal adsorption/desorption was assessed in a set of fixed bed flow-through columns using another series of full factorial balanced experimental designs with n = 4, pH and AAS replicate readings per treatment. The columns were packed with biomass produced in a small-scale artificial stream inoculated with the post-secondary stage effluent of a municipal wastewater treatment plant as a nutrient source, dominated by a mixed community of unicellular microalgal and bacterial forms, such as Chroococcus spp., Pseudanabaena sp., Leptolyngbya spp., and representatives of Chlorophyceae (e.g. Scenedesmus sp., Tetraedron sp., Chlorella sp., & Chlorococcus sp.), and Bacillariophyceae (e.g. Navicula spp., Nitzschia spp., Cyclotella sp.). The experimental system had the additional benefit of 70–80% phosphorus and nitrogen load decrease within four days of commissioning. Ca-alginate was used to prepare alginate + biomass beads to improve metal solution flow through the packed fixed bed columns. The results showed that metal adsorption depends on flow rate, pH and volume of treated waste passing through the columns. Cumulative adsorption capacity q for Cu2+ was 200 mg/g and 150 mg/g at a flow rate of 44 mL/h, and 136 mL/h, respectively. Based on the used flow through solution volume (1-165 column volumes) Cu2+ adsorbed on the column did not seem to reach a dynamic equilibrium with Cu2+ desorbed. In other words, the column does not appear to have reached its maximum capacity and can still be used to adsorb/remove Cu2+, albeit at reduced efficiency. The four metals were successfully desorbed (up to 100%) and eluted from the columns using relatively limited volumes (10-15 column volumes) of acids (0.2N HCl, 0.1N H2SO4). Biomass immobilization on Ca-alginate beads improved column flow through characteristics. However, bead disintegration due to solubilization and leaching of sodium alginate following treatment with Na2CO3 prohibited the repeated use of the biosorbent (biomass + alginate beads). On the question of whether the technology is effective, the final pH values did not achieve the final recipient effluent specifications for final pH and final [Me2+] (see Tables 2,31 & 37, Charts 3, 49,50 & 59), but the metal solutions could have been further processed, for example passing the treated solution through the packed bed twice, or adding more biomass in the batch experiments. Finally, the procedure exhibited different results when artificial or actual electroplating wastes were used, with lower % removal in the artificially prepared solution of Ni2+.
In conclusion, the present research focused and provided answers to each, and every question asked initially using the standards of data collection and rigorous statistical treatment procedures. Under the present global circumstances of natural resource availability, increased demand for metals, and the slow transition to the circular economy, the research remains current and innovative both domestically and internationally. The host of interesting questions generated by the experiments performed certainly merit further and more detailed research for the development of heavy metal biosorption and recycling technology.
