Zahid Amjad

Influence of Humic Compounds on the Crystal Growth of Hydroxyapatite

The influence of several natural and synthetic additives containing hydroxy and/or carboxyl groups on the kinetics of crystal growth of hydroxyapatite (HAP) at sustained supersaturation has been investigated using the constant composition method. Addition of low levels (0.25 to 5 parts per million) of fulvic acid, tannic acid, benzene hexacarboxylic acid, and poly(acrylic acid) to supersaturated calcium phosphate solutions has an inhibitory influence upon the rate of crystal growth of HAP. Salicyclic acid, under similar experimental conditions, is an ineffective HAP growth inhibitor. Kinetic analysis suggests Langmuir-type adsorption of added ions on the HAP surfaces with a relatively high affinity for the substrate in the concentration range investigated.

Investigations on the evaluation of polymeric calcium sulfate dihydrate (gypsum) scale inhibitors in the presence of phosphonates

Abstract The effects of polymers with different architecture, phosphonates, and polymer/phosphonate blends on precipitation of calcium sulfate dihydrate (gypsum) precipitation are reported in this paper. It has been found that gypsum inhibition by polymers strongly depends on polymer architecture. Among the various phosphonates (i.e., aminotris(methylene phosphonic acid), AMP; 1-hydroxyethylidine 1,1-diphosphonic acid, HEDP; 2-hydroxyphosphono acetic acid, HPA; and 2-phosphonobutane 1,2,4-tricarboxylic acid, PBTC); tested AMP shows the best inhibition for gypsum precipitation. It has also been observed that presence of PBTC exhibits synergistic effect on the performance of polymers containing carboxyl group. Under the experimental conditions used, no synergistic effects were observed with polymer/phosphonate (AMP, HEDP, HPA) blends.

Kinetic Inhibition of Calcium Carbonate Crystal Growth in the Presence of Natural and Synthetic Organic Inhibitors

Addition of carboxylate-containing polymeric materials to a metastable supersaturated calcium carbonate solution greatly reduced calcite crystal growth rates at constant supersaturation and pH = 8.5. Calcite crystallization rates were decreased to half their value in pure solutions by a tannic acid concentration of about 0.3 ppm (parts per million); a fulvic acid concentration of about 0.2 ppm; and a poly(acrylic acid) concentration of about 0.0175 ppm. An equation relating the calcite crystallization rate and the additive concentration follows an expression based on a Langmuir adsorption model. However, the Langmuir isotherm plot has two linear segments suggesting that these polyelectrolyte inhibitors may selectively adsorb initially at the fastest growing crystal faces. This relation between polyelectrolyte concentration and calcite growth rates implies inhibition by carboxylate-containing polymeric materials involves blockage of crystal growth sites on the calcite surface.

The inhibition of calcium carbonate formation in aqueous supersaturated solutions. Spontaneous precipitation and seeded crystal growth.

The influence of the presence of humic, fulvic, and polyacrylic acid on the nucleation and crystal growth of calcium carbonate in aqueous supersaturated solutions was investigated in batch reactors at 25°C and pH = 8.50. The nucleation of calcium carbonate was investigated by free drift methods, and the crystal growth was investigated with seeded crystal growth experiments at constant supersaturation. In all cases calcite was found to form exclusively, and the presence of all tested compounds at concentrations between 0.1 – 1.0 ppm prolonged the induction time preceding the spontaneous formation of calcite. Humic acid at concentration up to 0.5 ppm inhibited the growth of calcite seeds up to 95% and polyacrylic acid at concentrations up to 0.1 ppm gave the same degree of inhibition. Polyacrylic acid was found to be stronger inhibitor. A concentration of 0.25 ppm of polyacrylic acid completely stopped crystal growth of calcite. Humic acid at concentration 1.0 ppm completely stopped crystal growth of calcite seed crystals. The retardation was explained by the adsorption of the polyelectrolytes onto the active growth sites of the crystals. Application of a Langmuir-type adsorption model on the kinetics data obtained in the presence of the inhibitors tested yielded a higher affinity constant of polyacrylic acid for the calcite seed crystals.

INFLUENCE OF NATURAL AND SYNTHETIC POLYELECTROLYTES ON THE PRECIPITATION OF AMORPHOUS CALCIUM PHOSPHATE

The effect of polyelectrolytes such as fulvic acid,tannic acid, poly(acrylic acid),and acrylic acid-based copolymers containing different functional groups on the precipitation of amorphous calcium phosphate (ACP) at pH 7.40, 37•Ž was studied using the pH-stat method. Of the natural polyelectrolytes investigated, fulvic acid at low concentrations effectively delayed the precipitation of ACP. The results on the influence of synthetic polyelectrolytes as inhibitors for ACP show that polyelectrolyte effectiveness as inhibitor strongly depends upon the type and number of monomers present in the polyelectrolyte. Based on the inhibition data the ranking in terms of decreasing effectiveness of the polyelectrolytes tested is: terpolymer > copolymer > homopolymer > fulvic acid > tannic acid.

Calcium Carbonate and Calcium Phosphate Scale Formation and Inhibition at Elevated Temperature

In the present work, the effect of acrylic acid copolymers in the formation of calcium carbonate and calcium phosphate scale deposits on heated surfaces was studied. The experiments were done in a small heat exchanger and the effectiveness of acrylic acid copolymers was tested for 50 hours. In the calcium carbonate formation experiments, the presence of acrylic acid copolymers was found to stabilize thermodynamically unstable polymorphs of calcium carbonate and did not cause significant changes in morphology of the crystals formed. Ten ppm of polyacrylic acid, molecular weight ca. 2000, was found to be the most effective inhibitor for calcium carbonate yielding 95% inhibition efficiency. Polyacrylic acid effectiveness was found to decrease with increasing pH. The acrylic acid-sulfonic acid copolymer and acrylic acid-sulfonic acid-styrene sodium sulfonate tertpolymer were found to be less effective in comparison with polyacrylic acid at the same concentration (43 and 34% inhibition efficiency respectively) but their effectiveness was found to increase with increasing pH. In the calcium phosphate formation experiments hydroxyapatite, the thermodynamically most stable phase of calcium phosphate, was formed both in the presence and in the absence of acrylic acid copolymers. Polyacrylic acid at a concentration of 10 ppm was found to be the most effective inhibitor for calcium phosphate with inhibition efficiency of 60%. The other copolymers of acrylic acid tested did not affect significantly the formation of calcium phosphate. The inhibition efficiency of PAA was found to decrease with increasing total calcium concentration in the solution.

Scale and Deposit Control Polymers for Industrial Water Treatment

The accumulation of unwanted deposits on equipment surfaces is a phenomenon that occurs in virtually all processes in which untreated water is heated. The deposits commonly encountered may be categorized into the following five groups: (a) mineral scales (e.g. CaCO3, CaSO4 ⋅ 2H2O, BaSO4, Ca3(PO4)2, CaF2, SiO2), (b) suspended matter (e.g. mud or silt), (c) corrosion products (i.e., Fe2O3, Fe3O4, ZnO, CuO), (d) microbiological, and (e) metal-inhibitor salts. The deposition of these materials, especially on heat exchanger surfaces in cooling, boiler, geothermal, and distillation systems, can cause a number of operational problems such as plugging of pipes and pumps, inefficient water treatment chemical usage, increased operation costs, lost production due to system downtime, and ultimately heat exchanger failure. Greater water conservation has been a driver for operating industrial water systems at higher cycles of concentrations, thereby increasing the potential for deposit buildup on heat exchanger surfaces. Operating industrial water systems under stressed conditions demands a better understanding of system (feed and recirculating) water chemistry as well as the development of new and innovative agents for controlling scale/deposit, corrosion, and biofouling. Researchers have proposed several chemical addition options for controlling scale formation including the use of acids, chelants, or inhibitors. The most promising scale control method involves contents

Gypsum scale formation on heated metal surfaces: The influence of polymer type and polymer stability on gypsum inhibition

Abstract The deposition of gypsum (i.e. calcium sulfate dihydrate) on brass heat exchanger surfaces in the presence of homopolymers and copolymers containing different functional groups from aqueous solution has been studied. The amount of gypsum deposited on the heat exchanger surface (in the absence of bulk or spontaneous precipitation) is strongly dependent on the polymer architecture and the polymer concentration. Scanning electron microscopic investigations on the deposit show morphological changes to the crystals due to interactive effects of some of the polymers. The effectiveness of the polymers as inhibitors was reduced by thermal treatment (150–240°C), likely due to polymer degradation.

Performance of Anionic Polymers as Precipitation Inhibitors for Calcium Phosphonates: The Influence of Cationic Polyelectrolytes

This paper addresses the precipitation of calcium salts of phosphonates (i.e., hydroxyethylidine 1,1 -diphosphonic acid, HEDP; aminotri(methylene phosphonic acid), AMP; 2-phosphono-butane 1,2,4-tricarboxylic acid, PBTC) at pH 8.50 and 50°C. The aim of the study was to investigate the role of cationic polyelectrolytes (i.e., poly(diallyldimethyl ammonium chloride), Poly-E and poly(acrylamide:2-(acrolyloxy)ethyltrimethyl ammonium chloride), Poly-F) on the performance of several anionic calcium phosphonate inhibiting polymers. The anionic polymers studied include: acrylic acid/maleic acid-based homo-, co-, and terpolymers; fulvic acid (FA) and tannic acid (TA). The inhibiting data on several anionic polymers indicate that the performance of polymers depends on the type of the functional groups present in the polymer. It has been found that addition of low concentrations (from 0.25 to 1.0 parts per million) of Poly-E and Poly-F adversely impacts the performance of calcium phosphonate inhibiting polymers. The results also indicate that under similar experimental conditions Poly-F exhibits a stronger antagonistic affect than Poly-E.

Inhibition of Calcium Phosphate Precipitation by Polymers in the Presence of Iron (III). The Influence of Chelating Agents

The formation and adherence of alkaline earth metal phosphates and phosphonates is a problem in many heat exchanger applications where the equipment is fed with waters containing high levels of calcium, phosphate, and phosphonate. Historically, polymeric and non-polymeric additives have been used to prevent the formation and deposition of scale forming salts. In the present study the effect of various polymeric and non-polymeric additives as Fe (III) chelants have been evaluated for their effect on the performance of calcium phosphate inhibiting polymers. It has been observed that polymer composition and polymer dosage have a significant impact on the performance of calcium phosphate inhibiting polymers especially in the presence of low concentration of Fe (III). The data on the effect of various polymeric and non-polymeric additives as chelants for Fe (III) show that additives containing hydroxyl and/or carboxyl groups exhibit excellent chelating ability.