Author’s Accepted Manuscript Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for hexavalent chromium removal from aqueous solutions Chi-Chuan Kan, Mario Jose R. Sumalinog, Kim Katrina P. Rivera, Renato O. Arazo, Mark Daniel G. de Luna www.elsevier.com/locate/gsd
PII: DOI: Reference:
S2352-801X(16)30073-X http://dx.doi.org/10.1016/j.gsd.2017.07.004 GSD65
To appear in: Groundwater for Sustainable Development Received date: 21 December 2016 Accepted date: 26 July 2017 Cite this article as: Chi-Chuan Kan, Mario Jose R. Sumalinog, Kim Katrina P. Rivera, Renato O. Arazo and Mark Daniel G. de Luna, Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for hexavalent chromium removal from aqueous solutions, Groundwater for Sustainable Development, http://dx.doi.org/10.1016/j.gsd.2017.07.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrasound-assisted synthesis of adsorbents from groundwater treatment residuals for
hexavalent chromium removal from aqueous solutions
Chi-Chuan Kana1, Mario Jose R. Sumalinog IIb2, Kim Katrina P. Riverac3, Renato O. Arazob,d4,
Mark Daniel G. de Lunac,*
6 7 8 9 10 11 12 13 14 15 16
Institute of Hot Spring Industry, Chia-Nan University of Pharmacy and Science, Tainan 71710, Taiwan b Environmental Engineering Program, National Graduate School of Engineering, University of the Philippines, Diliman, Quezon City 1101, Philippines c Department of Chemical Engineering, University of the Philippines, Diliman, Quezon City 1101, Philippines d College of Engineering and Technology, University of Science and Technology of Southern Philippines, Claveria 9004, Philippines *
Corresponding author. Ph.D, Tel.: +632-981-8500 local 3114; Fax: +632-929-6640
Groundwater treatment residuals are rich in nano-sized metal oxides which have the potential to remove various water contaminants. In this study, hexavalent chromium adsorbents
Tel: +886-6-366-2006; Fax: +886-6-366-2668 Tel: +632-981-8500 local 3114 3 Tel: +632-981-8500 local 3114 4 Tel: +63-917-827-5540 2
were synthesized from the residuals of a 30,000 m3 d-1 groundwater treatment plant in Taiwan.
The effects of acid type and ratio, acid concentration, ultrasonication time, and heating duration
on adsorption capacity were examined. Adsorbents synthesized using dual-acid solutions yielded
lower adsorption capacities compared to those which used pure acid solutions. Adsorption
capacity decreased with higher ultrasonication time and heating duration. Chromium removal is
improved with higher ionic strength and lower pH of the solution. Results showed high
correlation between experimental data and the Freundlich isotherm (R2 = 0.9607) and the
pseudo-second order kinetic model (R2 = 0.9740) suggesting that the dominant mechanism in the
adsorption process is chemisorption. The involvement of Fe and Mn species during chromium
adsorption was confirmed by the results of energy dispersive X-ray spectroscopy and Fourier
transform infrared (FTIR) spectroscopy analyses.
Groundwater treatment residuals; Ultrasound irradiation; Hexavalent chromium; Adsorption;
Groundwater is a valuable source of drinking water in places where the supply of surface
water is inadequate (Kan et al., 2012). Significant concentrations of soluble iron and manganese
abound in groundwater, but between the two species, iron occurs in greater amounts (El Araby et
al., 2009). In most aquifers, the redox conditions usually promote the formation of divalent
manganese whether as hydroxide, sulfate or carbonate (Dashtban Kenari and Barbeau, 2014; Kan
et al., 2013). During groundwater treatment, these metal species (Fe and Mn) are converted into
toxic solid residuals (Sotero-Santos et al., 2007). The global implementation of stringent
environmental policies to address problems related to the management of heavy metal-laded
solids makes recycling as the most viable option for groundwater treatment residuals.
The extraction of useful metal oxides from groundwater treatment residuals is a promising
research area. Acid treatment and ultrasound irradiation have been used to dissolve organic
components in residuals and concentrate the reusable heavy metals (Yang et al., 2014). Oxides of
iron and manganese in groundwater treatment residuals have the potential to remove various
wastewater contaminants including heavy metals (Chiang et al., 2012; Nelson and Lion, 2003).
The oxides of iron have OH functional groups capable of reacting with other metals while oxides
or hydroxides of other metals such as Al and Mn can react with dissolved contaminants to form
ion pairs through surface complexation (Han et al., 2007).
Ordinarily, metal oxide-containing residuals are not readily usable as adsorbents for
environmental contaminants because, in their powdered form, they are difficult to separate from
the treated solution. Also, poor fluid movement in saturated portions limits the application of
powdered metal oxide adsorbents in continuous column operations (Kundu and Gupta, 2006).
However, metal species extracted from residuals can be coated onto sand media to facilitate
solid-liquid separation in metal adsorption applications (Babel and Kurniawan, 2003).
Hexavalent chromium is among the widely used heavy metals in the metal plating, tanning,
and electroplating industries (Sepehr et al., 2014). Waste streams from electroplating units may
contain up to 2,500 mg L-1 Cr(VI) (Dermentzis et al., 2011) and these chromium-containing
effluents eventually find their way into recipient rivers, lakes, and other water bodies. Chromium
exists in two oxidation states: hexavalent and trivalent (Hawley et al., 2004). Between the two,
hexavalent chromium is considered more hazardous to public health due to its carcinogenic and
mutagenic properties (Costa, 2003; Hamdan and El-Naas, 2014). At acidic solution pH, Cr(VI)
exists in its anionic form HCrO4- (Gode and Pehlivan, 2005). Due to repulsive electrostatic
interactions, Cr(VI) anions are poorly adsorbed by the negatively-charged soil particles in the
environment, allowing these molecules to move freely in the aqueous environments (Silva et al.,
2009). The removal of chromium from wastewater is achieved mainly through various chemical
processes. Adsorption is the technology that is commonly applied for heavy metal removal due
to its low cost and high removal efficiency (Jung et al., 2013).
In this study, hexavalent chromium adsorbents were synthesized from groundwater
treatment residuals by varying acid type and ratio, acid concentration, ultrasonication time, and
heating duration. The well adopted Langmuir (Langmuir, 1918) and Freundlich (Freundlich,
1906) isotherm equations and the pseudo-first order and pseudo-second order kinetic models
were applied to experimental data to understand the mechanism of Cr(VI) removal. The results
of the adsorption studies were validated by various adsorbent characterization techniques
including surface morphology, elemental composition, pH at point-of-zero-charge (pHPZC) and
relevant functional groups.
2. Materials and Methods
2.1 Chemicals, materials and simulated Cr(VI) wastewater
All glassware were detergent-washed and then acid-washed for 8 h before use. Residuals
were obtained from the Chang-Hua Water Treatment Plant in central Taiwan which processes
groundwater (with iron and manganese concentrations ranging from 0.2 to 0.6 mg L-1) in a series
of unit operations: aeration, chlorination by sodium hypochlorite and manganese green sand
filtration (Kan et al., 2012). Groundwater treatment residuals were acid treated using analytical
grade nitric acid (HNO3, 69%, Merck), sulfuric acid (H2SO4, 96%, Merck), and hydrochloric
acid (HCl, 37%, Sigma-Aldrich). Synthetic Cr(VI) solutions were prepared by dissolving
potassium dichromate (K2Cr2O7, 99%, Merck) with deionized water (18.2 MΩ resistivity)
generated by Purelab deionizers.
2.2 Adsorbent synthesis
Silica sand (0.71–1.2 mm) was soaked in 10% HNO3 for 2 h to remove impurities, rinsed
with deionized water until pH 7 was obtained, and then dried at 105 oC in a precision oven (DV
453, Channel) before use. Silica sand was coated with groundwater treatment residuals based on
the procedure reported elsewhere (Kan et al., 2013). The residuals (0.5–0.7 mm) were treated
with 7 mL acid (HNO3, H2SO4 or HCl) in an Erlenmeyer flask for 30 min. The acid-treated
residuals were then mixed with 7.0 g sand. A ratio of sand to residual of 10:1 was used. Sodium
hydroxide (1.0 N, NaOH, 99%, Merck) was introduced to the slurry until its pH reached 7–8.
The slurry was agitated in a reciprocal shaker (BT-350, Yidher) for 24 h at 100 rpm to ensure
complete mixing. The slurry was then heated at 105 °C in a precision oven (DV 453, Channel)
for predetermined durations (8, 24, 36, and 48 h). Groundwater treatment residuals that did not
coat the sand particles were separated through screening. The adsorbents were stored in sealed
plastic containers. For ultrasound-assisted acid treatment of groundwater treatment residuals,
ultrasonic bath (8510, Branson) set at 40 kHz was used instead of a reciprocal shaker. The
durations of ultrasound irradiation were 5, 30, 60, and 90 min.
2.3 Analytical methods
Total chromium was analyzed using inductively coupled plasma – optical emission
spectroscopy (ICP-OES 2000 DV, Optima) while Cr(VI) concentrations were measured using a
UV-Vis spectrophotometer (DR 3900, Hach) with a pre-set wavelength and automatic calibration
for Cr(VI) determination. US EPA Method 8023 (1,5 diphenyl carbohydrazide method) was used
to directly measure Cr(VI) concentration against blank samples (H2O) set as zero concentration.
Samples (10 mL) containing Cr(VI) were mixed with ChromaVer 3 reagent powder for 5 min
before Cr(VI) measurements. The Eq. 1 was used to calculate the adsorption capacity, qad (μg g-
The determination of the pHPZC of residual coated sand adsorbents was based on the
method of Noh and Schwarz (Reymond and Kolenda, 1989). RCS dosages of 0.15, 0.3, 1.5, 3.0,
6.0, and 9.0 g were mixed with deionized water in sealed 30 mL glass bottles for 24 h at 100 rpm
and 25 °C. The pH of each sample was recorded after 24 h of mixing. Adsorbent morphology
and chemical composition were analyzed by a scanning electron microscope (SEM, S-3400N,
Hitachi) and energy dispersive X-ray (EDX), respectively. The functional groups involved in the
adsorption were analyzed using Fourier transform infrared (FTIR) equipment (6700 Nicolet,
2.4 Batch adsorption experiments
A known amount of adsorbents (0.2 g) was added to an Erlenmeyer flask containing a 30
mL Cr(VI) solution at pH 2 and having an initial Cr(VI) concentration of 20 mg L-1. The
resulting suspension was agitated at 100 rpm in a reciprocal water bath shaker (BT 350, YIH-
DER) for 24 h at 25 °C. Each sample was pre-filtered by a 0.45 µm syringe filter and stored in
air tight plastic bottles before Cr concentration determination.
Kinetic studies were conducted following the procedure described for typical adsorption
experiments. The adsorption capacities at predetermined contact times were fitted to pseudo-first
and pseudo-second order kinetic models. Isotherm studies were carried out using a similar
procedure but with varying initial Cr(VI) concentrations (2, 10, 12, 15, and 20 mg L-1) and fixed
contact time of 24 h. Experimental data was fitted to Langmuir and Freundlich isotherm models.
The effect of ionic strength was determined by dissolving known amounts of K2Cr2O7 in
dilute electrolyte solutions of varying KNO3 (99%, Aldrich-Sigma) concentrations (0.0, 0.05 and
0.1 M). The pH of the solution was adjusted using NaOH and HCl. A fixed amount of adsorbents
was added into each electrolyte solution, and the adsorption capacity was measured after 24 h.
3. Results and Discussion
3.1 Effect of ultrasound-assisted acid treatment
Figure 1 shows that adsorbents which had ultrasound-assisted acid pretreatment of
residuals had higher adsorption capacities (i.e. 147.75 μg g-1 at 1% w/w acid) compared to those
produced from residuals pretreated with acid alone (i.e. 61.50 μg g-1 at 1% w/w acid). Increasing
the acid concentration for pretreatment improved adsorption capacities such that at 10% acid
treatment, the adsorption capacity for adsorbents which had ultrasound-assisted acid
pretreatment of residuals reached 870.45 mg Cr(VI) per kilogram adsorbent. Ultrasound
enhances the reduction in particle size of organics, metal oxides, and other residual components.
With ultrasound-assisted acid treatment, more metals specifically Fe and Mn ions were extracted
and liberated from the residuals and coated onto the sand. Deng et al. (Deng et al., 2009)
reported the synergistic effect of ultrasonication and acid treatment to liberate Cu, Zn, and Pb
ions from flocs.
In this study, the breakdown of organics and other residual components was enhanced by
ultrasonication. Sompech et al. (Sompech et al., 2012) reported that prolonged duration of
ultrasound irradiation increased the surface area of metal oxides (from 20 m2 g-1 to 37 m2 g-1).
Overall, the increase in residual coated sand adsorption capacity can be attributed to the
exposure and liberation of more Fe and Mn oxides brought about by the prolonged ultrasound
and acid treatment of groundwater treatment residuals.
On the other hand, long exposures (more than 30 min) of residuals to ultrasonication-aided
acid treatment decreased the adsorption capacity of the adsorbents. After applying 60 min of
ultrasonication-aided acid treatment, the adsorption capacity decreased from 541.95 to 136.95 μg
g-1 (Figure 2a). The adsorption capacity decreased further to 10.35 μg g-1 at 90 min of
ultrasonication time. Prolonged ultrasound-assisted acid treatment (t >30 min) effectively
reduced the particle size of various residual components (Pilli et al., 2011). As a result, the
ensuing smaller organic molecules formed Fe-C and Mn-C bonds through the halogen-metal
exchange, ligand exchange, insertion, haptotropic migration, transmetallation, oxidative
addition, reductive elimination, and other reactions (Bauer and Knölker, 2008). This increase in
Fe- and Mn-organic complexes hindered Fe and Mn oxide precipitation, thereby lowering the
available Cr(VI) adsorption sites.
Acid concentration during acid treatment also affects the performance of residual coated
sand. At 30 min ultrasound-assisted acid treatment, increasing acid concentration from 0.1% to
0.5% improved adsorption capacity by 519.60 μg g-1 (2,324.83%). However, at 60 min of
treatment, the improvement caused by the increase in acid concentration was only 120.90 μg g-1
(753.27%). At 90 min of treatment, adsorption capacity increased almost negligibly by 0.90 μg
g-1 (9.52%). The effect of ultrasound irradiation on adsorption capacity is far greater than that of
In Figure 2b, the highest adsorption capacity (641.2 μg g-1) was obtained when 0.5% w/w
H2SO4 was used during ultrasound-assisted acid treatment of residuals. The value is close to the
adsorption capacity of residual coated sand synthesized with residuals treated with 7% w/w
HNO3 in the absence of ultrasound irradiation (646.50 μg g-1). Other adsorption capacities
obtained were 340.7 μg g-1 (HCl) and 533.8 μg g-1 (HNO3). H2SO4-treated residuals produced
adsorbents with the highest adsorption capacity due to the very high acid strength. The pKa
values, which are inversely proportional to acid strength, of H2SO4, HCl, and HNO3 are -7, -5.2,
and -1.4 respectively (Daffalla et al., 2012). Also, the dibasic state of H2SO4 caused it to release
one H+ more than the other acids, making it the strongest among the three acids investigated.
H2SO4 caused the highest organic floc decomposition, thereby exposing more Fe and Mn oxides
The best among the acid combinations was HCl-H2SO4 giving 28.3 and 13.6 μg g-1 for 1:1
and 1:2 acid molar ratios, respectively. The extent of floc dissolution was lower when two acids
are combined compared to a single acid treatment. The hydrogen ions produced by the stronger
acid tend to suppress the dissociation of the weaker one, and both will tend to suppress the
dissociation of water, thus reducing the source of H+ (Merrill and Logan, 2009) and reducing the
The highest adsorption capacity is achieved at 24 h heating duration with 541.95 μg g-1
(Figure 2c). The heating process caused the decrease of the hydroxide component making the
anhydrous Mn-O-Mn the dominant Mn species in the oxide (Chang et al., 2004). In a separate
study of Laurent et al. (Laurent et al., 2011), it was reported that preheating of residuals before
adsorption increased its affinity to metal ions. Thermal treatment can cause the formation of
amorphous Fe/Mn oxide phases, which are noted to have higher surface areas.
Increasing the heating duration from 24 to 48 h, however, decreased the adsorption
capacity to 241.20 μg g-1. The heat treatment can bring about phase changes in Fe and Mn
oxides. According to studies conducted by Benjamen et al. (Benjamin et al., 1996) and
Schwetmann et al. (Bowles, 2000), prolonged thermal treatment converts amorphous Fe/Mn
oxide phases into crystalline Fe/Mn oxide phases. Also, maghemite transforms into crystalline
hematite during thermal treatment (Bora et al., 2012). The Mn oxide alpha-MnO2 crystal
(surface area 170 m2 g-1) transforms into alpha-Mn2O3 and then to alpha-Mn3O4 (surface area 30
m2 g-1) when thermally heated. Further heating would cause crystallization, removing oxygen
and producing Mn2O3 and Mn3O4 (Mi et al., 2011).
The formation of 3D crystallites requires surface energy, elastic strain, and surface
diffusion kinetics (Gong et al., 2005). Hence, the heterogeneous surface of the residual coated
sand is under constant strain during heat treatment because of metal oxide phase transformations.
The strain, if not homogeneously relieved during the crystal growth, will grow as grooves or
pits. Also, the formation of Fe and Mn crystalline oxide phases require stronger bonds rendering
Fe-O-Si bonds between the coating and the sand weaker. As a result, the residuals easily get
detached from the sand surface.
The larger flocs and cracks formed at 30 min ultrasonication resulted from the uneven
distribution of metal oxides and larger organic molecules (Figure 3b). During heating, the
growth of unevenly distributed metal oxides and organics led to high strain sites (Gong et al.,
2005), thereby creating large cracks and furrows. At 90 min of ultrasonication treatment, the
majority of residual particles were reduced in size, forming a uniform coating and fewer cracks
(Figure 3c). Higher concentrations of Fe and Mn were detected when 30 min ultrasonication was
applied, compared to the 90 min treatment. The smaller organic molecules released at 90 min
ultrasound irradiation readily coated Fe and Mn oxides on the adsorbent surface resulted in
lower Cr(VI) uptake.
Figures 4a, and 4b show the changes in surface morphology of the adsorbent as heating
duration was increased to 36 and 48 h, respectively. More exposed surfaces of the sand can be
seen when 48 h heating period was applied compared to 36 and 24 h. The almost negligible
Cr(VI) uptake of sand and its less rough surface caused lower adsorption capacity (Tansel and
239 240 241 242
3.2 Kinetic studies The linearized equation used for pseudo-first order kinetic model is given in Eq. 2, (2)
here qe is pollutant concentration in solid at equilibrium (mg g-1), where qt is pollutant
concentration in solid at given time (mg g-1), and k1 is the rate constant (L min-1). Eq. 3 shows
the linearized equation for pseudo-second order kinetic model (3)
where qe is pollutant concentration in solid at equilibrium (mg g-1), where qt is pollutant
concentration in solid at given time (mg g-1), h = k2qe2, and k2 is the rate constant (g mg-1-min-1).
The statistical difference between the model and experimental data, can be computed using the
Chi-squared equation given by Eq. 4 (Boparai et al., 2011). ∑
where qe,exp is the experimental equilibrium adsorption capacity (μg g-1) and qe,cal is the
calculated equilibrium adsorption capacity from the model (μg g-1). A smaller
as it shows how much the experimental and model data differs.
value is desired
Equilibrium was achieved in 3 h and at this point, the adsorption capacity of residual
coated sand is 530.10 μg g-1. Figure 5 shows the linearized plots of the pseudo-first order and
pseudo-second order kinetic models. In Table 1, the computed R2 values for both pseudo-first
order (R2 = 0.982) and pseudo-second order (R2 = 0.974) were high. However, the computed
for the pseudo-second order model was lower compared to the pseudo-first order model,
indicating fewer errors. Hence, Cr(VI) adsorption onto residual coated sand follows the pseudo-
second order kinetic model. This model validates the chemisorption phenomenon such as ion
exchange during the adsorption of Cr(VI) onto Fe and Mn oxides on the adsorbent surface.
261 262 263 264
3.3 Isotherm studies The Langmuir isotherm assumes monolayer attachment of molecules onto a surface
containing a finite number of active sites, which are uniform for adsorption (Duranoĝlu et al.,
2012). The linearized Langmuir isotherm is shown in Eq. 5 (5)
where b is the Langmuir adsorption constant (L mg-1). A plot of Ce/qe versus Ce gave a straight
line with a slope 1/qe and an intercept of 1/qob.
The Freundlich isotherm assumes heterogeneous surface energies in which the energy term
in Langmuir varies as a function of the surface coverage (Bello et al., 2011). The linearized
Freundlich isotherm is given by Eq. 6
where qe is pollutant concentration in solid at equilibrium (mg g-1), Ce is pollutant concentration
in solution at equilibrium (mg L-1), kf is a measure of adsorption capacity (mg g-1), and n is
As shown in Table 2, the data fitted to Langmuir and Freundlich isotherms revealed that
Cr(VI) adsorption by residual coated sand follows the Freundlich isotherm (R2 = 0.9605). This
model validates the heterogeneity of the surface adsorption due to various Fe and Mn oxide
phases. The computed value for 1/n (0.149) is within the acceptable range (0.1 to 0.5) for
favorable adsorption (Bello et al., 2011). The Kf value of 464.5 μg g-1 was approximately close to
the adsorption capacities obtained.
281 282 283 284
3.4 Effect of ionic strength Heavy metal ion adsorption onto Fe oxide surfaces is a combination of ion exchange and
inner-sphere complexation with OH groups where metal ions replace the OH. Since heavy metals
exist as anions at acidic pH, the adsorption of these anions on metal oxides follows a ligand
exchange reaction (Eq. 7) where the surface OH group of a metal cation (denoted as M) is
exchanged by the anion ligand (denoted as L) (Su and Suarez, 1997). (7)
These ion-exchange processes can occur either through the formation of outer or inner
sphere complexes. According to Goldberg (Goldberg, 2005), the mechanism of ligand exchange
with surface hydroxyl is defined by the specific adsorption of anions onto the mineral surfaces
forming inner-sphere complexes. When this happens, increasing ionic strength increases
adsorption. This result can be explained by the principle of mass action. Increased adsorption is
caused by the high solution activity of the counter ion of the background electrolyte available to
compensate surface charge generated specific ion adsorption (McBride, 1997).
In Figure 6, increasing the ionic strength from 0.0 to 0.1 M KNO3 improved the adsorption
capacities of residual coated sand (67.82 to 166.96 μg g-1). Goldberg and Johnson (Goldberg and
Johnston, 2001) reported similar findings on the relationship of ionic strength and adsorption
capacities. In the same study, it was postulated that there was a formation of inner-sphere surface
complexes during the adsorption process. The phenomenon of inner sphere complexation by
metal oxides during adsorption has also been discussed in separate studies by Shuman (Shuman,
1977) and Al-Sewailem et al. (Al-Sewailem et al., 1999).
The increased adsorption at higher ionic strength can be explained by the principle of mass
action. At higher KNO3 concentration, the solution activity of the counter ion of the background
electrolyte increases. It compensates for the surface charge generated by the specific ion
adsorption (McBride, 1997). Therefore, Cr(VI) anions were adsorbed more due to the charges
induced at higher KNO3 concentration. Since inner-sphere complexation was formed, the Cr(VI)
anions were directly adsorbed on the surface of Fe and Mn oxides of the adsorbents. The NO3-
ions did not compete with the adsorption onto the Fe and Mn oxides. This inner-sphere complex
mechanism confirms the ligand exchange of OH groups from the Fe and Mn oxides which is a
chemisorption process removing the Cr(VI) from the solution.
3.5 Adsorbent characterization
The FTIR spectra of residual coated sand before and after Cr(VI) adsorption are shown in
Figure 7. Similar peaks were detected for synthetic oxide adsorbents containing Fe and Mn (Lǚ
et al., 2013). The characteristic peaks at wavelength 460 cm-1 (Figure 7c) and 1070 cm-1 (Figure
7b) indicates Mn-O stretching vibration (Malankar et al., 2010) and OH stretching vibration
(Laurent et al., 2009). After Cr(VI) adsorption, a broadening of these peaks showed the
involvement of Mn-O and OH in Cr(VI) removal. The peak near 1500 cm-1 indicates Fe oxides
(Boujelben et al., 2009). The disappearance of this peak after Cr(VI) adsorption implies the
involvement of Fe oxide - Cr(VI) interaction. Overall, the high Cr(VI) adsorption capacities can
be attributed to Cr(VI) interactions with Fe and Mn oxides.
The pHPZC (Figure 8) of RCS was also indicative of Fe and Mn oxides. The measured
pHPZC of the residual coated sand was 6.7, which was close to the pHPZC (7.4) of synthetic Fe-
Mn coated sand (Chang et al., 2012). The pHPZC was also close to the following pHPZC values of
Fe and Mn containing minerals: hematite (Fe2O3) = 7 (Chang et al., 1983), maghemite (γ-
Fe2O3) = 5.2 (Plaza et al., 2002), geothite (α -FeO(OH)) = 7.1 ± 0.4 (Kosmulski, 2009),
hausmannite (Mn3O4) = 5.7 (Kosmulski, 2009), manganite (MnOOH) = 7.4 (Kosmulski, 2009).
The measured pHPZC indicates that at solution pH below 6.7, the residual coated sand surface
becomes positively charged (Lǚ et al., 2013). This result agrees with optimum pH (<6.7)
determined by Chang et al. (Chang et al., 2012), allowing the protonated residual coated sand to
attract more HCrO4- anions. A similar observation was reported by Lǚ et al. (Lǚ et al., 2013),
wherein more anions were adsorbed by a similar adsorbent at pH lower than pHPZC.
333 334 335
4. Conclusion In this study, adsorbents, synthesized from groundwater treatment residuals by ultrasound-
assisted acid treatment, were used to remove Cr(VI) from aqueous solutions. Ultrasound
irradiation of residuals until 30 min improved the adsorption capacity of the ensuing residual
coated sand. Surface morphology and EDX elemental concentrations show that lesser Fe and Mn
oxides were present on the RCS due to prolonged ultrasonication-aided acid treatment.
Acid treatment with 0.5% w/w H2SO4 yielded the highest RCS adsorption capacity. The
exceptionally high acid strength of H2SO4 rendered it capable of extracting the greatest amount
of Fe and Mn for oxide precipitation. DWT residuals treated by mixed acid solutions yielded
lower RCS adsorption capacities due to reduced acid strength.
RCS adsorption capacity is improved with heating duration until 24 h. Further increase in
heating duration decreased adsorption capacity of RCS due to the change in Fe and Mn oxide
phases, causing the loss of adhered Fe and Mn oxides on the Si sand. Surface morphology and
EDX elemental concentration indicating smoother surface and lesser Fe and Mn concentrations
on the RCS also confirmed the effect of prolonged heating duration.
The Cr(VI) adsorption onto RCS follows the Freundlich isotherm (R2 = 0.9605). This
finding is consistent with the non-uniform surface of RCS, arising from the different Fe and Mn
oxide phases and various compounds. The kinetic data fits the pseudo-second order model
indicating chemisorption, which confirms the ion exchange between Cr ligands and Fe-Mn
oxides. The Cr(VI) removal increased with solution ionic strength, indicating inner-sphere
complexation of Cr(VI) with the edge OH-groups. The measured pHPZC of the RCS adsorbent
was 6.7, suggesting favorable adsorption at low pH systems. Based on FTIR spectra peaks, Fe
and Mn oxides comprised the functional groups primarily responsible for Cr(VI) adsorption on
The authors would like to thank the Ministry of Science and Technology, Taiwan (Contract No.
MOST-102-2221-E-041-005) and the Department of Science and Technology, Philippines for
providing financial support for this research undertaking.
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1000 Without ultrasonication
qt (μg g-1)
518 519 520 521
HNO3 concentration (% w/w) Figure 1. Adsorption capacity of residual coated sand at varying amounts of acid.
0.1% w/w 0.5% w/w
qt (μg g-1)
500 400 300 200 100 0 30
Ultrasonication time (min)
0.1 % w/w 0.5 % w/w
qt (μg g-1)
500 400 300 200 100 0 HCl
HCl-H2SO4 HNO3HCl-H2SO4 (1:1) H2SO4 (1:1) (1:2)
(c) 0.1% w/w
qt (μg g-1)
400 300 200 100 0 24
Heating duration (h) 522
Figure 2. Adsorption capacities at varying acid concentration and varying (a) ultrasonication
time, (b) acid type and (c) heating duration.
Element Na K Mg K Si K PK Ca K Cr K Mn K Fe K
Weight % 4.67 12.35 55.52 24.92 2.39 0.04 0.05 0.06
Atomic % 5.72 14.29 55.61 22.63 1.68 0.02 0.03 0.03
Element Na K Mg K Al K Si K PK KK Ca K Ti K Mn K Fe K
Weight % 6.45 4.79 3.94 4.06 4.07 6.17 7.36 11.82 23.27 28.08
Atomic % 11.62 8.17 6.05 5.99 5.44 6.53 7.61 10.22 17.55 20.83
Element Al K Si K KK Ti K Mn K Fe K
Weight % 14.79 77.66 2.59 4.83 0.06 0.07
Figure 3. SEM images and EDX atomic composition of (a) uncoated sand; residual coated sand
at heating duration = 24 h and ultrasonication time of (b) 30 min, and (c) 90 min.
528 529 530 531 (a)
Element Na K
Weight % 8.96
Atomic % 11.46
Element Na K
Weight % 40.22
Atomic % 46.40
Atomic % 15.74 79.40 1.90 2.90 0.03 0.04
27 Mg K Si K Ca K Ti K Mn K Fe K
10.57 54.92 22.19 3.17 0.09 0.11
12.77 57.46 16.27 1.94 0.05 0.06
Si K KK Ti K Cr K Mn K Fe K
49.18 10.40 0.01 0.05 0.06 0.08
46.45 7.05 0.01 0.03 0.03 0.04
Figure 4. SEM images and EDX atomic composition of residual coated sand at ultrasonication
time = 30 min and heating duration of (a) 36 h, and (b) 48 h.
535 536 537 2.73375
(a) log (qe-qt)
2.73370 2.73365 2.73360 2.73355 0
t (min) 400
300 200 100
(b) 0 0
t (min) 538 539 540 541
Figure 5. Linearized plots of (a) pseudo-first order and (b) pseudo-second order kinetics.
No ions added
0.05 M KNO3
0.10 M KNO3
qt (μg g-1)
160 120 80 40 0 3
542 543 544
pH Figure 6. Adsorption capacity at varying initial solution pH and ionic strength.
before Cr(VI) adsorption
after Cr(VI) adsorption
Figure 7. FTIR spectra of residual coated sand indicating (a) FeOH, (b) OH, and (c) MnOH
6 4 2 0 0
Table 1. Kinetic constants of residual coated sand adsorbents.
Value 540.75 1.00 x 10-6 0.982 0.3694
528.80 8.78 x 10-2 0.974 0.3212
Table 2. Isotherm constants of residual coated sand adsorbents. Parameter Langmuir qo (μg g-1) b (L mg-1) R2 Freundlich Kf (μg g-1) 1/n R2
Figure 8. Point of zero charge approximation for residual coated sand.
Parameter pseudo-first order qe (μg g-1) k1 (L min-1) R2 χ2 pseudo-second order qe (μg g-1) K1 (g mg-1-min1 ) R2 χ2 552 553
Solids content (wt %)
Value 1.631 x 103 1.732 x 10-1 0.6704 4.645 x 102 1.490 x 10-1 0.9605
556 557 558
Groundwater treatment residuals underwent ultrasound irradiation and acid treatment
Cr(VI) adsorbents were synthesized by coating pretreated residuals on sand
Cr(VI) removal improved with higher solution ionic strength and lower solution pH
Freundlich isotherm and pseudo second order kinetics define the adsorption process
563 564 565 566 567 568 569 570 571 572 573 574 575 576 577
Statement of Novelty This work takes advantage of the residuals, rich with nano-sized metal oxides, from ground water treatment plant in the synthesis of adsorbent. The adsorbent is uniquely synthesized through ultrasound-assisted irradiation and acid treatment. The synthesized adsorbent was tested for the removal of hexavalent chromium from aqueous solution. The result showed that the synthesized adsorbent ably removed the hexavalent chromium which gives huge implication both in the ground water treatment operation and the removal of heavy metals from wastewater like chromium produced in various industries such as metal plating, tanning, electroplating.