PhD in Physics (# of pos: 2)

- Tutor: S. Sennato, F. Bordi, Physics Department & Institute for Complex Systems, Sapienza University of Rome, Italy

- PhD School Physics, XVIII cycle

Details about the procedure on

https://phd.uniroma1.it/web/concorso37pon.aspx?s=&i=3503&m=&l=EN&p=&a=

deadline: 14.00 (local time) 27 October 2021

- Company (6 months-stage): ACEA Ato 2, Roma

- Partner for mandatory 6-month stage: Dott. D. Truzzolillo, University of Montpellier & CNRS, France

Abstract

Natural waters contain pollutants that are present is colloidal form. The suspended particles vary considerably in source, composition charge, particle size, shape, density and nature. In many cases the colloidal suspension may contain clays, silica, microbial cells or algae, and/or as dissolved organic substances, a complex mixture of soluted macromolecules and various organic materials known as Natural Organic Matter (NOM). The presence of NOM causes many problems in drinking water and drinking water treatment processes, since NOM interferes with the removal of other contaminants and is responsible for fouling of membranes, contributes to corrosion and is a substrate for bacterial growth in the distribution system. For these reasons and due to the increasingly stricter regulations for drinking water treatment, there is a strong need for more efficient and still economical methods for the removal of suspended material and NOM.

NOM can be removed from drinking water by several treatment options, of which the most common and economically feasible processes are considered to be coagulation and flocculation followed by sedimentation/flotation and sand ultrafiltration [1]. Due to its micrometric and sub-micrometric size, this NOM colloidal component is the most difficult to eliminate. Actually, because of the variable composition of NOM, different removal mechanisms have demonstrated more or less effective in different circumstances. The complex nature of these substances is related to the variability of molecular structure, molecular weight, acidity and electrical properties. In general, at pH values higher than 4, NOM behave in general as negatively charged colloids or anionic polyelectrolytes. For this reason, the electric charge-driven mechanisms of coagulation demonstrated to be highly effective when exploited for the removal of suspended material and NOM.

In water purification process synthetic or natural polyelectrolytes are widely employed as both primary coagulants or coagulant aids. Their use presents several advantages compared to multivalent inorganic salts, mainly connected to the smaller volumes of sludge obtained by employing comparatively lower doses, which, in turn, also reduces the ionic load of the treated water, as well as the overall treatment costs. Despite their widespread use, contradictory, unexpected and sometimes disappointing results keep turning up, making often difficult the decision-making for those in charge of water purification plants. Up to now there is not a great deal of published information on the relationship between polyelectrolyte structure (molecular weight, charge density, hydrophobicity, chain flexibility, etc.), process characteristic (dosage, mixing conditions, temperature, pH, ionic strength, presence of polyvalent ions) and water treatment performance (precipitation and sedimentation rates, solids content of the final sludge) [2].

During the last decade, our research group widely investigated both experimentally and theoretically the aggregation processes and the properties of different polyelectrolyte-colloid and polyelectrolyte-polyelectrolyte complexes [3]. Recently, considering the strong need for rationalizing the complex phenomenology observed in polyelectrolyte-based water treatment process in terms of a comprehensive model of the interactions between all the players, we have started to transfer this advanced knowledge to the process of polyelectrolyte-induced coagulation of suspended impurities in water purification to the company ACEA Ato2 [4] .

On the basis of the encouraging results already obtained, we consider that a deep understanding of the interaction between colloidal NOM and polyelectrolytes will be valuable in driving the optimization of the coagulation and flocculation processes. To this aim, we plan to study experimentally the phase diagram of different coagulant agents as multivalent ions and cationic polyelectrolytes among those that are currently more frequently used in these processes, i.e., PolyAluminium Chloride (PAC), Poly(diallyldimethyl ammonium chloride (PDADMAC), cationic polyacrylamides and natural biocompatible polymers as chitosan, using clay aqueous suspensions as model impurities, with variable amounts of high molecular weight humic acids and low MW fulvic acids, and at varying pH, ionic strength and temperature of the solution. Then, we will examine the effect of the polyelectrolyte topology, structural and electrical properties, by comparing polymer with different structures (linear, branched or star), charge density and hydrophilic/hydrophobic properties.

In a second phase, building up on the results of these systematic investigations, we plan to specify the details of optimal water treatments, particularly in terms of sludge volume, by designing practical protocols that will be tested with samples from natural sources in different conditions. This will be the key objective of the project to which we will devote the strongest effort of both academic and industrial partner. In fact, guided by the theoretical predictions and by the results of the experiments performed in controlled and known conditions, we will learn how to tune the treatment (mainly in terms of polyelectrolyte concentration and pH) to respond to the requirements of different conditions (composition and concentration of the impurity load, pH, ionic strength). It will be possible to design effective procedures, with the required degree of flexibility and scalability, to be optimized for the different practical conditions which will be a sound basis to guide the design of practical treatments for a more efficient removal of NOM. In this way, strong indications to help the decision making process, not at a generic level, but within an appropriate treatment, could be provided to the management of water treatment plan. Within the collaboration with the company, these new procedure could be tested preliminary on jar tests and then on the pilot plants in selected purifiers.

Our investigations will have a positive impact on the improvement on water purification processes by increasing their environmental sustainability. Understanding the interaction between these complex systems will allow to the rational control of the process by choose the best polyelectrolyte and operating conditions, thus allowing to decreasing its dose, which in turn, will result in decreased sludge volumes and production of harmful polymer disinfection by-products and decreased levels of complexed heavy metals and adsorbed organic pollutants. Considering the huge volumes of purified water in Lazio and the purification costs, a polyelectrolyte dose reduction will allow a consistent money saving, which can be re-invested to bring innovation in this ambit.

More, since cationic polyelectrolytes are considered to be toxic to aquatic organisms and a few countries have restricted their use in water purification, as an important part of this project, we will explore the use of negatively charged thermoresponsive PNiPam microgels to reduce the residual quantities of cationic polyelectrolyte in the product water with the ambitious aim to further improve water treatment with novel protocols based on novel state-of art nanomaterials. For this part we will capitalize on our experience with differently functionalized Poly(N-isopropylacrylamide) (PNiPam) microgels [5], in collaboration with Dr. D. Truzzolillo (France). Microgels will be used as eco-friendly adsorbent flocculants, using their capacity to physically absorb a broad range of suspended waste like polyelectrolytes, as well as nanoparticles and charged living microorganisms.

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