Eleftheria Mavredaki

University of Crete, Réthymnon, Kriti, Greece

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Publications (8)1.62 Total impact

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    Konstantinos D. Demadis, Maria Somara, Eleftheria Mavredaki
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    ABSTRACT: The effect of various fluorine-containing chemical additives on the dissolution of colloidal silica is systematically studied. These silica scale dissolvers are ammonium bifluoride (NH4·HF2), ammonium fluoride (NH4F), sodium tetrafluoroborate (NaBF4), and disodium fluorophosphate (Na2PO3F). The most effective dissolver was NH4·HF2, which was extensively studied at the pH range 2–7. The highest dissolution efficiency was demonstrated in the pH range 2–4. The dissolution capability of Na2PO3F was monitored not by the silicomolybdate method, but on the basis of a weight-loss approach. It showed substantial dissolution ability at pH’s 7 and 9.
    Industrial & Engineering Chemistry Research. 02/2012; 51(7).
  • Konstantinos D. Demadis, Eleftheria Mavredaki, Maria Somara
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    ABSTRACT: The effect of various environmentally friendly chemical additives and natural products on the dissolution of amorphous silica (Aerosil 200 and laboratory-synthesized, SSD) is studied. The silica scale dissolvers tested include the following: ascorbic acid (vitamin C, ASC), citric acid (CITR), carboxymethyl inulin (CMI), 3,4-dihydroxybenzoic acid (catechuic acid, DHBA), 3,4,5-trihydroxybenzoic acid (gallic acid, GA), dopamine hydrochloride (DOPA), iminodiacetic acid (IDA), histidine (HIST), phenylalanine (PHALA), and malic acid (MAL). The chemical structures of these chemical additives contain potentially dissolution-active moieties, such as 1,2-dihydroxyethylene (ASC), α-hydroxycarboxylate (MAL and CITR), catecholate (DHBA, GA, and DOPA), α-aminocarboxylate (HIST and PHALA, both aminoacids), and finally carboxy-modified fructofuranose units (CMI). It was found that all studied molecules showed variable dissolution efficiency, with MAL, CMI, HIST, and PHALA being the slowest/least effective dissolvers, and the catechol-containing DHBA, GA, and DOPA being the most effective ones. IDA and CITR have intermediate efficiency.
    Industrial & Engineering Chemistry Research. 11/2011; 50(24):13866–13876.
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    Konstantinos D. Demadis, Eleftheria Mavredaki, Maria Somara
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    ABSTRACT: The effect of various chemical additives (small molecules and polymers) on the dissolution of two kinds of colloidal silica (Aerosil 200 and laboratory-synthesized, SSD) is systematically studied at pH 10. The silica scale dissolvers tested are 5-carboxybenzotriazole (CBZT), amino-tris(methylene phosphonic acid) (AMP), a phosphino-polycarboxylic acid (PPCA), diethylenetriamine pentacarboxylic acid (DETPA), a proprietary polymer (Genesol 40), poly(acrylic acid) (PAA), ethylenediamine-tetrakis(methylenephosphonic acid) (EDTMP), phosphonobutane-1,2,4-tricarboxylic acid (PBTC), sodium metaborate, and N-phosphonomethylimino-diacetic acid (PMIDA). Of the polymeric additives only Genesol 40 shows some dissolution activity, dissolving 280 ppm silica at 10 000 ppm dosage after 72 h. PBTC and DETPA are the best-performing additives of all those tested. PBTC is effective even at the 2500 ppm dosage, as it solubilizes 290 ppm silica after 72 h. Its efficiency is dosage-dependent. DETPA is also an effective silica dissolver. Its behavior is similar to that of PBTC. Its best dosage is 7500 ppm, which yields dissolution of 322 ppm silica (after 24 h), 340 ppm (after 48 h), and 333 ppm (after 72 h). SSD silica is a very recalcitrant deposit showing resistance to dissolution even by the most effective additives, PBTC and DETPA.
    Industrial & Engineering Chemistry Research. 10/2011; 50(22).
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    ABSTRACT: Supersaturated process waters high in silicates frequently result in deposition of colloidal silica or metal silicate salts. Silica cannot be inhibited by conventional phosphonate mineral scale inhibitors. Chemical cleaning poses hazards and requires operational shut-downs. This paper is focused on a dual approach for silica scale control, inhibition of colloidal silica formation and colloidal silica dissolution in water technology applications by use of designed chemical approaches. The additives used for silica inhibition were polyaminoamide dendrimers (PAMAM) and polyethyleneimine (PEI), in combination with carboxymethyl inulin (CMI) and polyacrylate (PAA) polymers. In principle, silica inhibition is a function of time and inhibitor dosage. Amine-terminated PAMAM-1 and 2 dendrimers as well as PEI combined with anionic polymers, such as CMI and PAA, seem to have a significant inhibitory effect on silica formation, most likely at its earlier stages where the reaction products are oligomeric silicates. CMI and PAA assist the inhibitory action of PAMAM-1 and 2 and PEI by alleviating formation of insoluble SiO2-PAMAM precipitates. This most likely occurs by partial neutralization of the positive charge that exists in –NH+3 surface groups. Increase of anionic polymer dosage above a certain threshold has a detrimental effect on the activity of the cationic inhibitors. In that case the polymer’s negative charge “overwhelms” the cationic charge of the inhibitor and poisons its inhibition ability. For silica dissolution, acetic, oxalic, citric acids, histidine and phenylalanine were used as potential replacements of ammonium bifluoride (NH4F·HF). Silica dissolution is dependent in a rather unpredictable fashion on the structure of the dissolver, time and dosage. This paper continues our research efforts in the discovery, design and application of antiscalant additives that have mild environmental impact. These chemicals are also known as “green additives”.
    Desalination. 01/2007;
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    ABSTRACT: Mineral scale deposits such as calcium carbonate and phosphate, calcium oxalate, barium and strontium sulfate, magnesium silicate and others and colloidal inorganic species such as silica present important challenges for process water applications. When silica is left uncontrolled it forms hard and tenacious deposits that are difficult and hazardous to remove. Conventional phosphonate mineral scale inhibitors do not inhibit silica formation and deposition. Chemical cleaning is not free from hazards and requires operational shut-downs. Another challenge is corrosion of critical metal surfaces of industrial equipment. Last but not least, biofouling due to the development of microorganisms. This paper is focused on the presentation of the general scope of these problems and their solutions. More specifically, it concentrates on (a) inhibition of colloidal silica formation, (b) colloidal silica dissolution, and (c) metallic corrosion, in water applications by use of designed chemical approaches. The additives used for silica inhibition were polyaminoamide dendrimers, polyethyloxazoline and polyethyleneimine polymers. For silica dissolution the dissolvers tested were carboxymethyl inulin (CMI), Genesol 40 (a proprietary blend of additives), polyacrylic acid. In principle, silica inhibition is a function of time and inhibitor dosage. Silica dissolution is dependent in a rather unpredictable fashion on the structure of the dissolver, time and dosage. Mild steel corrosion inhibition has been achieved by synergistic use of zinc ions and polyphosphonate anions that create protective films.
    Desalination. 01/2007;
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    ABSTRACT: Colloidal silica (SiO 2) and other sparingly soluble salts such as CaCO 3 present a challenge for desalination systems used for brackish or seawater desalting. When SiO 2 is left uncontrolled, it forms hard and tenacious deposits that are difficult and hazardous to remove. Conventional phosphonate mineral scale inhibitors do not inhibit SiO 2 formation and deposition. Chemical cleaning is not free from hazards and requires operational shut-downs. CaCO 3 is also troublesome. This paper focused on silica and CaCO 3 formation, deposition and inhibition with designed chemical approaches in water applications that require the use of membranes. It also describes SiO 2 scale removal by dissolution approaches with environmentally friendly and non-hazardous chemical additives. The general scope of silica formation and inhibition in waters relevant to desalination systems is also discussed. This paper continues our research efforts in the discovery, design and application of anti-scalant additives that have a mild environmental impact. These chemicals are also known as "green additives". In light of increasing environmental concerns for discharge of saline water coming from desalination systems, this research is of significant interest.
    Desalination. 01/2005; 179:281-295.
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    ABSTRACT: This paper is focused on a dual approach for silica scale control, inhibition and dissolution by use of designed chemical approaches. Inhibitors that are tested include the polyaminoamide STARBURST dendrimers (PAMAM) of generations 0.5, 1.0, 1.5, 2.0, and 2.5. Of these, only the NH 2 -terminated ones (PAMAM-1.0 and 2.0) show significant inhibitory activity, in contrast to COOH-terminated ones (PAMAM-0.5, 1.5, and 2.5), which show virtually no inhibition perfor-mance. The synergism between the above dendrimers and an anionic polyelectrolyte (poly(acry-lamide-co-acrylate) copolymer) is also described. Addition of poly(acrylamide-co-acrylate) copolymer in silica supersaturated solutions containing PAMAM-1 or 2 alleviates the appearance of silica-PAMAM insoluble precipitates, resulting in stable colloids. The paper also describes silica dissolution approaches, as an alternative to inhibition, by using nonhazardous additives based on polycarboxylates with one to five -COOH groups (acetate, oxalate, citrate, diethylen-etriaminepentaacetate, and others), mixed polycarboxylates/phosphonates (2-phosphonobutane-1,2,4-tricarboxylate), and amino acids (L-histidine and L-phenylalanine). Their reactivity is linked to their chemical structure in this structure/function study. The presence of additional chemical groups (e.g., -PO 3 H 2 , -NH 2 , or -OH) in the dissolver molecule augments the dissolution process.
    Industrial & Engineering Chemistry Research - IND ENG CHEM RES. 01/2005; 44(17).
  • Source
    Konstantinos D. Demadis, Eleftheria Mavredaki
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    ABSTRACT: The effect of various environmentally friendly chemical additives on the dissolution of colloidal silica is systematically studied. These silica scale dissolvers are principally polycarboxylates with one to five –COOH groups, mixed polycarboxylates/phosphonates and aminoacids. Based on these results, an effort is made to link their dissolution performance to structural features in this structure/function study. Presence of additional groups (eg. –PO3H2, –NH2, or –OH) in the dissolver molecule augments the dissolution process.
    Environmental Chemistry Letters 01/2005; 3(3):127-131. · 1.62 Impact Factor