Removal of Arsenic Species through Coagulation-Flocculation Processes
Date
2017-04-27
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Masters
Abstract
Through the traditional water treatment process of coagulation-flocculation, this thesis aims to investigate the effectiveness of the removal of organic and inorganic forms of arsenic using a two-component coagulant-biopolymer flocculant system. Among the metal salts chosen in this study, the optimal dosage required was 30 ppm for alum, and 15 ppm for Fe(II) chloride and Fe(III) chloride metal salts. The dosage of the biopolymers was 2.5 ppm for medium molecular weight chitosan (MMWC) and 10 ppm for high viscosity sodium alginate (HVA). In addition to the optimal dosage parameters, pH effects were evaluated at several values (pH 3, 5, 7 and 9) due to the importance of charge neutralization in coagulation-flocculation processes. The results showed no significant decrease in turbidity removal at variable pH using a Jar test apparatus. The stirring time and speed was optimized at several conditions: 3 minutes (295 rpm) and 20 minutes (25 rpm) and finally, the addition of the polymers was added in a sequential manner rather than as a single step using a premixed polymer system. This last step was chosen because premixed polymers were less effective.
The next stage of research involved the use of an optimized system to study the removal of roxarsone (4-hydroxy-3-nitrobenzene arsonic acid). The use of a coagulant-biopolymer system (Fe(III)-MMWC/HVA) was shown to be effective for the removal of roxarsone at initial concentrations of 30, 40, 50 ppm. Jar test studies reveal that the concentration of free roxarsone decreased when Fe(III) alone was used as a coagulant, in the presence and absence of kaolinite. Addition of biopolymers generally lead to a decrease in roxarsone uptake. Arsenate (V) was studied using a Jar test setup at an initial concentration of 30 ppm. The removal (%) of arsenate (V) was shown to be slightly higher than roxarsone at 42%, as compared to 23% using the metal salt-biopolymer system without kaolinite. Addition of kaolinite to the arsenate (V) caused a 10% increase in removal (%) when using Fe(III) and Fe(III)-HVA system, where no discernible change was found for the Fe(III)-MMWC/HVA system.
The one-pot method was used to study the kinetics during floc formation and settling. The removal efficacy (%) was plotted against time and revealed that Fe(III) species were primarily responsible for roxarsone removal, while alginate hinders the roxarsone uptake. Kinetic studies were carried out using the one-pot method and evaluated by two kinetic models, the pseudo-first order (PFO) and pseudo-second order (PSO) models by plotting “arsenic uptake” (Qt) in mg/g against time (t). The PFO model was more favorable, indicating reversible binding interactions throughout the coagulation-flocculation process of roxarsone and arsenate (V) with Fe(III), MMWC and HVA. Thermodynamic studies were carried out at variable temperatures (20 ºC, 30 ºC and 40 ºC) using only Fe(III) and the Fe(III)–MMWC/HVA systems, in the presence and absence of a model colloid (kaolinite). All “apparent” Ea values were negative for the removal of As(V), except for the removal of arsenate(V) in the presence of kaolinite with Fe(III)-MMWC/HVA. Thermodynamic activation parameters (ΔH‡, ΔS‡ and ΔG‡) were estimated by Eyring theory, where most ΔH‡ values were negative for the coagulation-flocculation process. A dominant and negative value of ΔS‡ was obtained due to floc formation with arsenic and Fe(III), chitosan and alginate, with a resulting positive ΔG‡ for the process.
Description
Keywords
arsenic, coagulation-flocculation
Citation
Degree
Master of Science (M.Sc.)
Department
Chemistry
Program
Chemistry