University of Technology, Sydney. Faculty of Engineering and Information Technology.Water scarcity due to persistent drought is forcing the countries around the world to
explore alternative fresh water resources. Groundwater is one of the natural freshwater
resources that can be used for human and agricultural use. But, contamination due to
improper disposal of untreated human and industrial wastes affects the groundwater
quality and renders it unsuitable for human and agricultural purpose unless the water is
treated for contamination removal. Some of the common contaminants that contaminate
groundwater are: landfill leachate from domestic landfills, Persistent Organic Pollutants
(POPs) such as pesticides (Metsulfuron methyl-MeS), Pharmaceutically Active
Substances (PhAcS-Trimethoprim (TMP) and pentachlorophenol (PCP). These
contaminants cannot be removed effectively by conventional treatment processes such
as coagulation, adsorption and are not easily bio-degradable. Therefore, Advanced
Oxidation Processes (AOPs) are preferably being used to remove these contaminants of
concern because of their effectiveness against bio-refractory contaminants, faster
degradation kinetics and economic viability. Common AOPs include: Photocatalysis;
Fenton's oxidation; Ozonation and their combinations. Our study used Adsorption/biosorption
(conventional treatment process) and Photocatalysis and Fenton's oxidation
(AOPs) for the degradation of above mentioned contaminants.
Synthetic Landfill Leachate (SLL)
The adsorption/bio-degradation of diluted Landfill Leachate representing contaminated
ground water on granular activated carbon was investigated both in batch and column
(fixed bed studies) modes. The total organic carbon (TOC) (64 mg/1) removed due to
adsorption by 20, 40, 60 g/l GAC was 44, 48 and 63%, whereas bio-degradation
removed 85, 92 and 97% TOC respectively. The biodegradation of TOC was supported
by consistent increase in microbial count on GAC particles. The Langmuir and Sips
adsorption isotherms were found to fit well with the batch equilibrium. A mathematical
model was developed to simulate the organics removal efficiency of the GAC bio-filtration
system. In the combined process, a pre-treatment of SLL by Fenton's
oxidation followed by bio-filtration led to an organic removal of 75% (an improvement
of 15% over only biodegradation) even with a small oxidant (H2O2) dose of as low as
200-800 milli mole/L and Fe2+ of 15 milli moles. Photocatalysis with TiO2 as catalyst
degraded SLL by only 30%
Metsulfuron methyl (MeS) - an Herbicide
The GAC adsorption removed more than 90% of MeS for an initial MeS concentration
[MeS]o of 50 mg/L. The adsorption and kinetics of MeS on GAC were a function of the
solution pH. The linear driving force approximation (LDFA) kinetic equation with
Langmuir and Freundlich adsorption isotherm models were successfully applied to
predict the batch adsorption kinetics data in various concentrations of MeS. The Bohart-
Adams and Thomas models were found to best simulate the fixed bed adsorption of
MeS.
The Fenton's process was very effective in the degradation of MeS. The MeS was
degraded by more than 99% at a reaction time of 2 h and more at the optimum Fenton's
reagent concentration. The results suggested that as long as a minimum threshold level
of H202 (i.e., 60 mg/L) is applied, the long term (more than 1 hour) removal of MeS is
primarily affected by the initial Fe2+ and MeS concentrations. The Fenton's process was
successfully modeled using an 8-reaction, 2nd order kinetic model.
The removal of MeS by photocatalysis with TiO2 was not effective as Fenton's
oxidation. This study also investigated the toxicity of degradation by-products due to
Fenton's oxidation of MeS. The herbicide toxicity of the parent and degradation by-products
of MeS after Fenton's oxidation was determined by toxicological bioassay.
The plant selected for this bioassay was the small aquatic flowering plant Lemna
disperma, commonly known as duckweed, which is sensitive to MeS. The measured
toxicity to Lemna in these treated samples was comparable to the concentrations of MeS
measured by chemical method (HPLC/UV) detection.
Pentachlorphenol (PCP)
The removal of PCP from contaminated aqueous solution was investigated by GAC
adsorption, photocatalysis and Fenton's oxidation processes. Adsorption by GAC was
very successful in removing PCP from aqueous solution even with very small quantities
of GAC. The adsorption efficiency was highest at lower pH. The adsorption of PCP on
GAC occurred in two phases; a faster and a slower phase. This was modeled.
The combination of UV/ Ti02 photocatalysis removed PCP completely within 30
minutes of reaction. Significant degradation of PCP was achieved even with a very low
dose of Ti02 of 0.05g.L-1 (for [PCP]0 range of 10-40 mg.L-1) and 0.1 g.L-1 (for [PCP]0
of 60-80 mg.L-1). The first order and Sips kinetics were successfully used to predict the
degradation rate of organic contaminants. The chemical analysis of C1- and PCP and
calculation from chemical formula showed that only 44.8o/o PCP was completely
mineralized although all 100% pure PCP underwent degradation to lower chlorinated
phenol and other compounds.
Fenton's process was very effective in PCP degradation. The PCP degradation by
Fenton oxidation was a function of initial concentration of FR and their ratio (H202 and
Fe2+), PCP (organic loading) and initial solution pH. The Sips Kinetic equation gave the
best fit with the experimental data among different kinetic models tried.
Trimethoprim (TMP)
The removal of TMP by GAC adsorption was investigated at alkaline and acidic
conditions. The percent TMP removed by 500 mg/L GAC at pH 3, 7 and 10 was 62.5,
82.5 and 99% respectively. Sips isotherm and dual first order kinetics explained the
equilibrium and kinetic adsorption results. The removal of TMP in a GAC column
(fixed bed) was also studied using 3 different shallow GAC bed heights of 2.5, 5 and 10
cm. Overall, the fit of the Thomas model was the best for fixed bed adsorption of TMP
as indicated by the higher r2 values.
In Fenton's oxidation the percent TMP removed was a function of initial FR dose. The
[Fe2+]0 concentration for maximum TMP removal of 60% at an opti1num [H202]0
concentrations of 1.2 giL was 100 mg/L. The effect of catalyst concentration on the
removal of TMP was more pronounced than the oxidant concentration.
Photocatalysis (Ti02/UV) decomposed 80% of TMP concentration within 180 minutes
of irradiation. The optimum Ti02 dose was 0.5 g/L which degraded TMP by 82% for an
initial TMP concentration of 10 mg/L. In the continuous system, the feed flow rate
through the photoreactor or detention time was an important factor in enhancing the
TMP removal. A detention time of 50 minutes achieved 55% TMP removal.
In-situ Fenton's oxidation (ISFO) of MeS and PCP
In-situ contaminant reduction was investigated in sand columns (fixed beds). Both
adsorption and Fenton's oxidation mechanisms were taken into account in ISFO to
calculate the MeS removed. The In-situ Fenton's oxidation of MeS showed that for the
transport and degradation of MeS in the column the residence time was the primary
factor in determining the amount of MeS removal. The transport and degradation of
MeS was modeled using the advection diffusion equation with reactions and rate limited
sorption. The steady state adsorption of PCP in the sand filter was higher compared to
that observed for MeS. The PCP removed by in-situ Fenton's oxidation was in the range
of 80-90%. The in-situ Fenton's oxidation of PCP also showed that the residence time
was the primary factor responsible in determining the amount of contaminant removal.
Adsorption and Fenton's oxidation of PCP in the sand column was satisfactorily
modeled in the same manner as MeS
