Introduction
The wastewater chain is an important source of priority substances
into the surface water system and is, therefore, one of the
bottlenecks in achieving the European Water Framework Directive
objectives (Directive 2001/83/EC, amended by Directive
2004/27/EC (for human pharmaceuticals), and Directive 2001/82/EC,
amended by 2004/28/EC (for veterinary pharmaceuticals). At various
locations in the Netherlands the standards for priority substances are
exceeded. The current concern in detecting these micropollutants in
receiving waters may call for new approaches in wastewater treatment
(Mohd Amin, 2015). The introduction of an economical and effective
method in removing these compounds from wastewater is therefore
crucial (Virkutyte et al., 2010).
The availability of advanced treatment methods has improved the
removal of these micropollutants from wastewater and existing
conventional wastewater treatments can be upgraded (Ternes, 1998;
Stumpf et al., 1999; Heberer, 2002). However, such treatment is often
costly. A potential low-cost solution is the use of natural substances
for micropollutant removal (Radian and Mishael, 2012), such as clay in
combination with biodegradable polymeric flocculants. The application
of biodegradable polymers such as cationic starch (CS) to flocculate
particles during treatment has the additional advantage of being shear
resistant (Singh et al., 2000).
In the last few decades, interest in the adsorption of
polyelectrolytes on clay surfaces for enhanced removal of pollutants
has grown (Ternes, 1998; Singh et al., 2000). For example, Churchman
(2002) demonstrated the removal of toluene by
polystyrene–montmorillonite (MMT) composites. Radian and Mishael
(2012) showed that, at high loadings of PDADMAC on MMT, the composite
is positively charged, promoting the binding of anionic herbicides. In
a recent study, the advantages of composites of
poly-4-vinylpyridine-co-styrene (PVPcoS) and MMT for the removal of
atrazine from water, even in the presence of dissolved organic matter
(DOM), were reported (Zadaka et al., 2009).
The combination of clays and polymers can be viewed as a symbiosis
interaction, because organic micro-pollutant absorbing clays need to
be settled to avoid being flushed out. For that, the dosage of
a cationic polymer can be beneficial due to its ability in particle
removal (Van Nieuwenhuijzen, 2002). Cationic polymers have also been
proven to be efficient in e.g. atrazine reduction from water matrixes,
but it requires attachment to a negatively charged surface (Mohd Amin
et al., 2014a). The combination of cationic polymers with clays is
thus hypothesised to enhance the micro-pollutant reduction by creating
a diffuse zone (Eslinger and Pevear, 1988; Mohd Amin et al., 2014c).
Clay such as smectite has a high cationic exchange capacity
(80–150 meq(100g)-1) and can also act as
a coagulant aid in wastewater treatment for destabilisation of the
other particles in water before floc formation with polymeric
flocculants occurs (Sawhney and Singh, 1997; Mohd Amin
et al., 2014b). This paper reports on the direct interaction between
clays and a CS with atrazine as a model compound in demineralised
water. Four different types of clay were compared and the best
performer was selected for optimisation. The flocculation aspect of
the clays with CS was also examined in order to find the combined
dosage for optimal performance.
Material and methods
Experimental studies were carried out with four different clays, which
were selected based on their different properties. The attapulgite
(ATT), smectite (SME), and sepiolite (SEP) were supplied by Tolsa
Group (Spain) through Keyser & Mackay (the Netherlands) while
Na-Bentonite (BEN) was purchased from Sigma-Aldrich. All the clays
consist of a ∼ 1–4 nm thick surface layer and are
around ∼ <1–5 µm-sized. The Nalco cationic
starch EX10704 was used and obtained from Nalco Netherlands
BV. Atrazine (PESTANAL®, analytical
standard) and analytical grade methyl tertiary butyl ether for gas
chromatography measurements were purchased from
Sigma-Aldrich. Properties of the clays and the polymer are listed in
Tables 1 and 2. Demineralised water was obtained from tap water that was
treated with reverse osmosis and ion exchange. Whatman Spartan 30/0.45
RC syringe filters (0.45 µm, Whatman UK) were used to
filtrate the samples as pre-treatment.
Clay selection and optimisation
Clay selection experiments were prepared by adding a range of clay
dosages (0–50 gL-1) to a 200 mL solution with
a concentration of 6±2 µgL-1 atrazine in
a 500 mL Duran glass bottle. The solutions with the clay were
stirred at 40 rpm for 24 h before settling took place for
2 h. After the experiment, the solution was filtered with the syringe
filter (0.45 µm). The collected samples were, after
filtration, pre-treated before being analysed for atrazine residues.
The best clay (i.e. SME) from the first experiment was optimised for
low dosages to adsorb an initial atrazine concentration of 2±0.2 µgL-1 (normally found in wastewater). The
solutions with the clay were stirred at 40 rpm for 8 h before
settling took place for 1 h. The rest of the procedure was similar as
aforementioned.
Clay flocculation with cationic starch
The clay (SME, ATT) combination with CS was studied to determine the
effect on atrazine reduction. The experiments were carried out in
a pyrolysed 2000 mL Duran glass bottle. Both clays in
concentrations in the range 0–120 mgL-1 (based on
previous optimisation experiments) were first dosed to adsorb the
atrazine (initial concentration of 2±0.2 µgL-1)
for 8 h. For ATT the adsorption time applied was 24 h, to ensure
that equilibrium between atrazine and the clays was reached before
flocculation. Then the CS with a concentration of
20 mgL-1 was added and mixed slowly at 40 rpm for 2 h
before settling for 1 h. Extended agitation was applied to produce
smaller flocs resulting in a higher surface availability especially
for atrazine removal by ATT. Samples were taken, filtered with the
syringe filter (0.45 µm) and analysed.
ATT in combination with CS was further tested with different starch
concentrations (10, 20, 40, 60 mgL-1) to prove that the
atrazine reduction is enhanced by the starch concentration. The
experimental procedure was similar as aforementioned.
SME-CS: atrazine removal, flocculant dosage and turbidity relation
The objective of this experiment was to further optimise the
flocculation of the clays with CS. The flocculation experiments of the
SME using CS (no atrazine added) were carried out in jar test
equipment filled with demineralised water (1.8 L). All the
jars contain four baffles to increase the energy gradient during
stirring. Figure 1 illustrates the specific relationship between the
energy gradient (G value) and the rpm of the stirrers for this jar
tester. The clay dosage used in this experiment was in the range of
0–100 mgL-1 and the CS concentration was fixed at
20 mgL-1. Dosing of the CS was conducted at a stirring
velocity of 300 rpm. After a mixing time of 30 s, the stirring
velocity was reduced to 40 rpm for 10 min. During this period, floc
formation took place. All the jars were observed visually, and after
a settling time of 20 min, samples were taken for analysis. Turbidity
was measured using a Hach DR5000 spectrophotometer according to APHA
(1999) standard methods.
The settled and suspended clay is measured based on the turbidity
level of SME from 0–40 mgL-1 dosage based on Fig. 2. The
turbidity measurement will be taken after each of the settling
periods. The turbidity reading will then be compared to the reading in
Fig. 2 in order to determine the SME dosage. This measured SME dosage
represents the suspended SME in the water matrix. The rest of the SME
in the water matrices will have settled.
The atrazine concentration is determined by calculating the removal
from the blank sample (quadruplicate) of SME (40 mgL-1)
and atrazine without any polymer addition. On average, 82 % of
atrazine is removed and the final turbidity after settling is
approximately 5.5 NTU. The turbidity value was used for the
determination of SME concentration based on Fig. 2. Based on this
turbidity reading and Fig. 2, it can be deduced that
35 mgL-1 is suspended in the water matrixes. The rest of
the 5 mgL-1 of SME is expected to settle. With this
information, the amount of atrazine attached to the settle clay can be
calculated based on Eq. (1).
(A/B)⋅82%=C(%)Where:A:the settled clay concentrationB:total concentration of clayC:percentage of atrazine removed by settled clay
The rest of the sample and replicate were measured and calculated
based on this equation. The atrazine removal percentage in the
solution is the remainder that is removable by clay.
Analytical methods
The atrazine concentrations (2±0.2 µgL-1) were
analysed by gas chromatography (Agilent's 7890A) based on the
U.S. Environmental Protection Agency 551.1 (1995) method. Atrazine in
the sample was extracted using liquid–liquid micro-extraction with
MTBE as a solvent. The injection sample was 1 mL of the
extracted sample. A volume of 2 µL-1 was injected in
splitless mode and the injector temperature was 200 ∘C. The
carrier gas was helium (linear velocity was
33 cms-1). The injector temperature was
260 ∘C. The oven temperature was held at 35 ∘C for
9 min, and then raised at 15 ∘Cmin-1 intervals
to 225 ∘C. The temperature of 225 ∘C was held for
10 min before being raised at 20 ∘Cmin-1
intervals to 260 ∘C. The recovery of atrazine was in the
range of 90–110 %.
Results and discussion
Clay selection and maximising the adsorption of atrazine on clay
The reduction of atrazine, with initial concentrations of 6±0.2 µgL-1, for four different clays dosed in the
range of 0–50 gL-1 is shown in Fig. 3. Atrazine
reduction from the water was in the range of 10–99 % for three
out of the four selected clays. There was no reduction achieved with
BEN. BEN has a hydrophilic surface, which attracts water molecules for
attachment due to the presence of sodium ions thus limiting the
interaction with atrazine. For ATT, atrazine reduction was increased
only at dosages larger than 5 gL-1 and reached a maximum
of 40 % at 50 gL-1. It is reported that the surface
charge of ATT is approximately negative to neutral (White and Hem,
1983), which led to a low atrazine affinity at dosages lower than
5 gL-1. This is supported by the low cation exchange
capacity (CEC) value of ATT being around
20–30 meq(100g)-1. Similar observations were
also reported by Haden (1961). SEP showed a constant increase in
removal, with 10 % atrazine reduction at a clay dosage of
0.05 gL-1 to >99 % at a dosage of
25 gL-1. The ability of SEP in adsorbing organic
compounds has previously been reported by Rytwo (2012). SME showed the
highest affinity to atrazine removal with reductions >99 % at
dosages lower than 1 gL-1. SME has the highest specific
surface area (200–800 m2g-1) and the highest CEC
(80–150 meq(100g)-1) value compared to the
other three clays. It was also reported that SME can adsorb other
compounds, such as carbamazepine (Zhang et al., 2010) and naphthalene
and phenanthrene (Lee et al., 2004). SME was thus viewed as the most
suitable clay for further investigation.
The SME as the best clay and ATT as the low performer reference clays
were further tested for optimal dosages in atrazine (initial 2±0.2 µgL-1) adsorption. In this experiment, more than
86 % atrazine was removed using 60 mgL-1 SME and this
reached a maximum of >99 % at dosages up to
100 mgL-1. Therefore, we limited the dosage in the
following experiment to clay dosages below 100 mgL-1.
Clay flocculation with cationic starch
Atrazine reduction from water with SME was 45–99 % before the
addition of CS. After the CS addition, the reduction increased at low
clay dosages (Fig. 4). The largest effect of the CS addition in
enhancing the atrazine reduction was observed at SME dosages of
10–40 mgL-1. The high reduction is expected to be due to
the attachment of CS that covers the entire clay surface during floc
formation, thus creating a diffuse zone that is able to trap atrazine
(Mohd Amin et al., 2014a). At an SME dosage higher than
40 mgL-1, the clay's own ability in adsorbing the
atrazine predominates, thus resulting in a higher removal than without
the addition of CS. The ATT alone did not have much ability in
reducing the atrazine concentration at low dosages as shown in
Fig. 3. However, after the addition of CS, the atrazine reduction
was considerably increased, specifically for the 10 mgL-1
clay dosage with an atrazine reduction of 45 %. The reduction
percentage decreased with the clay dosage increments, to 7.5 % at
a dosage of 120 mgL-1. At a lower clay dosage
(10 mgL-1), the polymer was expected to cover the entire
clay surface. With the clay dosage increment, more flocs were formed,
and fewer polymer surfaces were available for atrazine attachment. At
higher clay dosages, even more and larger flocs were formed, and less
free surface was available which resulted into a lower removal
percentage.
From Fig. 4, it is observed that the clay dosages limit the atrazine
reduction by ATT. This limitation was further studied at CS
concentrations from 10–60 mgL-1 (Fig. 5). Best results
were obtained at a clay dosage of 10 mgL-1 and a CS
dosage >20 mgL-1. The higher CS dosages did not have
a large influence in increasing the atrazine reduction. A further
increase in the ATT dosage resulted in a similar pattern. At a limited
ATT concentration of 10 mgL-1, there was high competition
for CS attachment. The extent of CS attachment to the ATT layer is
limited by the CS concentration, which translated into different
atrazine reduction levels with different CS concentrations. It can
also be that, when the ATT dosage increases, a higher surface
availability results in less CS multilayer formation and thus in less
atrazine diffusion, which leads to less atrazine reduction.
SME-CS: atrazine reduction, flocculation dosage and turbidity relation
In this experiment, the relation between CS dosage and clay is further
studied to find a relation between the atrazine reduction, CS dosage
and SME turbidity. Atrazine in the water matrix can be divided into
three different phases after settling, suspended (adsorbed to SME but
does not settle) and remaining in the solution (not adsorbing).
In Fig. 6, it can clearly be seen that the SME had a high ability in
atrazine adsorption, which is reflected in the “suspended” phase. At
40 mgL-1 without any CS, around 62 % atrazine is
adsorbed from the solution but suspended and not removed from the
wastewater matrix. By increasing the CS dosage, the amount of settled
SME that contains atrazine increased from 10 % without any CS
dosage to a maximum of 77 % with 30 mgL-1 dosage. The
amount of suspended SME (approximately 2.5 mgL-1) was at
the lowest at 40 mgL-1 CS, which accounted for 5 %
atrazine removal.
Figure 6 also shows that a further increase in CS dosage did not
improve the atrazine reduction although a higher settled clay
percentage was observed. The maximum atrazine reduction was 82 %
at 30 mgL-1 CS and was slightly reduced (80 %) at
higher dosages. The turbidity removal reached a maximum at
40 mgL-1 CS with around 92 % removal. However, the
amount of SME settled was slightly less compared to the settled SME at
30 mgL-1 CS dosage.
Conclusions
The present study was designed to determine the ability of different
clays in combination with a CS to reduce atrazine in water. SME, as
the best performing clay, was further optimised to lower the dosage
based on atrazine concentrations regularly found in the
environment. The effective SME dosage was around 20 mgL-1
to reduce about 60 % atrazine from demineralised water, and
a maximum of >90 % atrazine reduction was reached with a clay
dosage >80 gL-1. A combined dosage of SME and CS was
performed to study the effect of polymers on the clay's ability in the
reduction of atrazine.
Efficient flocculation by addition of CS increases the settled SME and
simultaneously increases atrazine reduction from the water. However,
here 20 mgL-1 CS dosage was viewed as sufficient for
atrazine reduction through the settling of clay (72 %) with around
82 % of total turbidity removal. Application of higher CS dosages,
although improving the turbidity removal and settling the clay, did
not significantly improve the total atrazine reduction. It should be
noted that the application of inorganic matter such as clay in some
countries like the Netherlands is undesirable due to increased waste
production, which will increase the disposal cost. However, in certain
circumstances, the high sludge productions are also preferable for the
production of biogas and can even be used as fertilizer in some
countries. An extended study of the applications of the clay-polymer
combination in terms of its performance, reuseability, impact and
economic value, is required for more accurate information.