One-pot removal of pharmaceuticals and toxic heavy metals from water using xerogel-immobilized quartz/banana peels-activated carbon

ABSTRACT Here, we report the efficient one-pot removal of heavy metal ions and pharmaceuticals using xerogel, activated carbon and gravel. The simplest xerogel from tetramethyl orthosilicate showed ∼ twice the adsorption capacity of activated carbon derived from banana peels at 600 °C and far greater (x 35) than quartz pellets of diameter ∼300 μm. Functional groups present in both xerogel and activated carbon greatly enhanced the adsorption process. The combination of the three adsorbents resulted in the removal of > 95% of heavy metal ions and pharmaceuticals from water. The data fitted well in linear equations of Langmuir and Freundlich adsorption isotherms, and pseudo-first-order and pseudo-second-order kinetics, an indication for physisorption, chemisorption, adsorption and desorption processes during pollutant capture by the adsorbents. GRAPHICAL ABSTRACT


Introduction
Clean and safe water for human and animal consumption is still a global challenge in the 21 st century with only ∼ 1% of available water drinkable as per international standards (1).Activities such as oil processing and use, mining, farming, constructions, poor synthetic methodologies and disposal of fine chemicals, pharmaceuticals and agrochemicals release a wide range of pollutants into the environment.
Heavy metals naturally exist in the earth's crust and some such as cobalt, copper, iron, manganese, vanadium, and zinc are essential at trace levels for the normal growth and functioning of organs but can be fatal when you surpass the threshold.
On the contra, non-essential metals pose immediate detrimental effects.Reports showed a cancer risk of ∼76% and ∼15.7% by cadmium and arsenic, respectively from consumption of paddy rice grown on contaminated soil (2).
Most hazardous non-essential heavy metals and metalloids, for example, arsenic, lead, cadmium, and mercury have been reported in food crops and are deleterious in various respects (3,4).They have been included in the top 20 list of dangerous substances by the United States Environmental Protection Agency and the Agency for Toxic Substances and Disease Registry.The toxic heavy metals enter the body in various ways, such as skin absorption, inhalation, and consumption of contaminated water (5) after which the metal ions react with biomolecules in the body to form toxic compounds which are difficult to isolate (6).Their accumulation in vital body organs such as the liver, heart, kidney, and brain disrupts normal biological functioning and can cause organ damage and failure, headache, vomiting, nausea and circulatory system diseases.
On the other hand, it is worth mentioning the alarming danger of pharmaceuticals and their metabolites in the aquatic environment (7).Pharmaceuticals are continuously introduced into the aquatic environment, and they can cause toxic effects on living organisms, even at concentrations on the ng L −1 level and their concentration is on the rise due to the slow decay rates, constant emissions, and inefficient wastewater treatment techniques (8,9).Detection of ng/L up to a µg/L of >150 active pharmaceutical ingredients has been reported in industrial wastewater, receiving waters and fish (10).Therapeutic groups most commonly detected in water include anti-inflammatories, analgesics, antidepressants, antiepileptics, lipid-lowering drugs (fibrates), β-blockers, antiulcer drugs, antihistamines and antibiotics which may be consumed by non-targeted organisms causing acute and chronic adverse effects on ecosystems and humans (11).In Uganda, significant levels of ibuprofen, sulfamethoxazole, ciprofloxacin, tetracycline, caffeine, metronidazole, acetaminophen, and diclofenac were detected in showers of Lake Victoria, a freshwater supply to millions of people and animals in Uganda (12).
Water purification methods that employ natural adsorbents mostly use sand (silica), gravel and charcoal/activated carbon are not efficient in removing all organic and inorganic wastes.In addition, activated carbon varies greatly depending on the source of raw material to be burnt, which necessitates knowing the composition to fully anticipate and understand the adsorption mechanism and range.In this work banana peels of a variety of Musa species commonly consumed in Central and Western Uganda were burnt to produce the activated carbon.Further, sand and gravel may contain a variety of mineral compositions where leaching of inorganic residues during water filtration may occur causing secondary pollution.Xerogel is a dry silica with (SiO 2 ) n framework similar to the sand (SiO 2 ) replaced sand.In 3-dimension, the xerogel structure has the [SiO 4 ] 4-tetrahedra linked by an oxygen atom adopting cages with varying pore sizes which in addition to the high negative charge can act as trap for metal ions and other small molecules.For example, they have been used to immobilize enzymes while retaining biological activity (13).Xerogels are synthesized by the sol-gel method, a quick, cheap, and environmentally process that involves acid-catalyzed hydrolysis of silane to form a sol followed by condensation to form a gel (14).Tetramethyl orthosilicate (TMOS) is the simplest silane used in sol-gel formation (Scheme 1).The negative charge of the oxygen atoms in the xerogel's (SiO 2 ) n structure and the free hydroxyl groups not condensed during sol formation allow electrostatic attractions, hydrogen bonding and chemical coordination with metals ions and pharmaceuticals.In addition, the porous nature of xerogel can trap particles inside enhancing the adsorption.The proposed adsorption action of the xerogel is shown in Figure 1.
It is against this background that a characterized adsorbent system made of xerogel, activated carbon and gravel was envisaged to remove selected  pharmaceuticals and toxic heavy metals due to the possible physical and chemical interaction by the xerogel (Figure 1) and the micropores and residual function groups in activated carbon, a widely studied area well documented in the literature.The incorporation of xerogel to replace sand not only serves as an adsorbent but also acts as a filter that immobilizes activated carbon particles and quartz fragments that may end up in the water after filtration.This helps to overcome secondary pollution faced by conventional filtration processes that use sand, charcoal and gravel in batch or continuous flow filtration where tiny fragments of the adsorbents leach into the filtered water.
The overall objective was to synthesize activated carbon with retained function groups and incorporate it with quartz and xerogel for the adsorption of heavy metal ions and pharmaceuticals from water.

Results
The initial stages of the study involved the synthesis and characterization of activated carbon.The thermal degradation stability of the dry banana peels powder was determined by thermogravimetric analysis (TGA) using a TGA-DSC analyzer (Jupiter STA449 F3 NTZSCH GmbH) at continuous temperature rise from 27 to 1000 °C.A slight drop in weight was observed between 100-175 °C which then dropped sharply with temperature rise until 500 °C (Figure 2).The two-stage drops are in agreement with the literature where the first and second weight loss is attributed to loss of moisture and decomposition of organic matter/surface function groups, respectively.Temperatures 600 and 700 °C were used for carbonization since a fairly constant mass was recorded > 600 °C (Figure 2).
The surface morphology of the activated carbon produced at 600 °C (AC 600 ) and 700 °C (AC 700 ) was determined using Scanning electron microscopy (SEM) using Gemini-SEM 500M/s Carl ZEISS-EDAX Z2 Analyzer AMETEK to compare the pore sizes that act as particle traps during adsorption (Figure 3).The material contained activated carbon that created voids in the materials hence the creation of different pore volumes due to the specific surface area of the carbon material used.These dark spots in the material are exhibited as shadows beneath the material flakes, as shown in Figure 3 where AC 600 (Figure 3(a)) has larger pore than AC 700 (Figure 3(b)).
Relatively similar pore sizes were observed as indicated by dark spots in the SEM images (Figure 3), which leaves function groups after carbonization as the enhancement factor for adsorption.The presence of functional groups was observed using Shimadzu IRTracer-100 spectrometer in Fourier transformation infrared (FTIR) spectra of activated carbon produced at 600 °C than at 700 °C (Figure 4).More function groups were observed in AC 600 than AC 700 which gave a slight edge to the former since interactions like hydrogen bonding can occur between the hydrogen and oxygen containing-functional groups in adsorbent and the pollutants such as pharmaceuticals with functional groups such as the hydroxyl group and electronegative  heteroatoms that facilitate the interaction.In addition, the electron-rich function groups in the adsorbent interact with electron-deficient species such as the metal ions and electrophilic groups in the pharmaceutical residues which enhances adsorption.
AC 600 was used in the adsorption studies after the final test using nitrogen (N 2 ) adsorption-desorption by Brunauer-Emmet-Teller (BET) technique using 11-2370 Gemini Micromeritics (Figure 5).The relatively high adsorption by AC 600 is attributed to the large surface area of 253.50 m 2 /g and total pore volume of 0.1452 cm 3 /g compared to 104.22 m 2 /g and 0.0740 cm 3 /g for AC 700 from BET analysis.
Therefore, carbonization temperature >700 °C was avoided due to the anticipated loss of valuable functional groups (Figure 4) that contribute to the adsorption properties of the synthesized activated carbon.The AC 600 was used in the adsorption studies because of high N 2 adsorption (Figure 5) and larger pore size and volume compared to AC 700 from BET studies.
It is worth saying that xerogel chemistry is known and reported in the literature mostly in the immobilization of enzymes and other biomolecules but its adsorptive potential to capture metal ions and pharmaceuticals has not been attempted.We, therefore, proceeded to investigate the adsorption capacity and mechanism of AC 600, xerogel (with only the (SiO 2 ) n framework from TMOS precursor) and their combination in capturing selected heavy metal ions and pharmaceuticals.
The adsorption capacity (Equation 1, Experimental section) and mechanism of adsorbents were then investigated.Selected pharmaceuticals including ciprofloxacin, tetracycline, metronidazole and caffeine, and heavy metals ions; lead, copper and zinc were used in this study.Pharmaceuticals and heavy metal ions were analyzed by ultraviolet-visible (UV/Vis) spectroscopy using PerkinElmer Lambda 365 UV/VIS Spectrophotometer and atomic absorption spectroscopy (AAS) using AA500 from PG Instrument.Quantification was reached by comparing pharmaceuticals and metal ion levels in spiked water before and after filtering with calibration curves generated using the respective standard solutions.Gravel (quartz pellets, 300 μm) was included in the study for comparison with natural water filters.The xerogel had ∼twice the adsorption capacity compared to AC 600 whereas gravel showed the least adsorption for both ciprofloxacin and caffeine for the same mass of the adsorbent (Table 1, entries 1-3).
Higher adsorption by the xerogel is likely due to intense hydrogen bonding between the pharmaceuticals and oxygens of (SiO 2 ) n or the outer hydroxyl groups of the xerogel not subjected to condensation.Hydrogen bonding by the functional groups of AC 600 (Figure 4) makes it superior to gravel.The role of each adsorbent is further manifested by the reduced adsorption in the combination of gravel with either xerogel or AC 600 (Figure 6).The peaks at 276 nm correspond to the adsorption maximum (λ max ) of ciprofloxacin.The absorbance peak at around 225 nm is associated with both the carboxylic and aromatic chromophores which are by-products of the oxidation of organic matter which concurs with previous observations reported by Holc et al. (15).Xerogel's superiority to AC 600 and gravel was also manifested in the adsorption of lead ions from water (Figure 7).This is attributed to the net negative charge and surface hydroxyl groups of the xerogel which enable both physical and chemical interactions with metal ions, A concoction of copper, lead and zinc ions in spiked water containing 10 ppm of each was passed through a capillary tube packed (top-bottom) with AC 600, quartz pellets (300 μm) and xerogel.The combination of the three adsorbents captured >97% of the metal ions (Table 2, entries 1-3).
For large-scale filtering, the adsorbents were able to remove > 95% of the pharmaceuticals and heavy metal ions from 12 L of spiked water after a contact time of 2 h.
The mode of adsorption of the adsorbates was investigated using linear equations (Equations 2 and 3, Experimental section) for Langmuir and Freundlich adsorption isotherms (16,17).The strength and relationship between variables were assessed using Pearson's Rvalue.The data fit well in the Langmuir model with Pearson's R values > 0.99 for both metal ions (lead, copper and zinc) and pharmaceuticals (tetracycline, caffeine, metronidazole and ciprofloxacin) compared to R > 0.95 for the metal ions and pharmaceuticals in Freundlich isotherm (Tables 3 and 4).Representative graphs are shown for copper in Figure 8.The relatively higher R values of Langmuir isotherms suggested monolayer adsorption attributed to physical interaction and also satisfies the assumption of adsorption on a homogeneous surface with a fixed number of adsorption sites which implies adsorption likely occurred on the xerogel surface since it meets the surface requirement with the interlinked SiO n groups.Multi-layer adsorption of adsorbate also occurred evidently from R > 0.95 from Freundlich linear isotherms suggesting chemisorption possibly due to interactions of the adsorbates with functional groups in AC 600 and the xerogel.The presence of heterogeneous surfaces due to the different surface morphologies of activated carbon and quartz used also allowed multilayer adsorption.
Pseudo-first-order and pseudo-second-order kinetics were used to determine the effect of adsorbate concentration and the adsorption/ desorption process, respectively on the adsorption capacity as previously reported (18,19).Equations 4 and 5 in the Experimental section were applied to evaluate the kinetics of adsorption of the metal ions and pharmaceuticals.The adsorption process conformed more to pseudo-second-order than pseudo-first-order rate mechanisms indicated by the high Pearson's R values of the latter calculated from the linear plots (Equations 4 and 5, Experimental section)    (Tables 5 and 6).Representative graphs are shown for copper in Figure 9.The relatively high Pearson's R values order (> 0.94) of pseudo-first kinetic models suggest that the rate of adsorption depends on the adsorbate concentration (Table 5, entry 2).The relatively high R values of pseudo-second-order kinetics (> 0.98) suggest adsorption and desorption of both metal ions and pharmaceuticals on the adsorbent's surface.

General
Tetracycline, caffeine, ciprofloxacin, metronidazole, and metal ion standard solutions were used as purchased from Sigma-Aldrich but with appropriate dilutions in de-ionized water.
The adsorption capacity of the adsorbents was calculated according to Equation 1: where q is the adsorption capacity(mg/g), C o is the initial concentration of adsorbate (mg/L), C i is the concentration at equilibrium (mg/L), V is the volume of the adsorbate solution (L) and M is the mass of the adsorbent (g).
Langmuir adsorption isotherm plots of C e q e against C e were according to linear Equation 2: Where C e is the equilibrium concentration of adsorbate (mg/L), q e is the equilibrium adsorption capacity (mg/ g), k L is the Langmuir constant (L/mg) which signifies affinity toward binding sites on the adsorbent surface and q max is the maximum adsorption capacity (mg/g).
Freundlich adsorption isotherm plots of ln q e against ln C e were according to linear Equation 3: Where C e is the equilibrium concentration of adsorbate (mg/L), q e is the equilibrium adsorption capacity (mg/ g), k F is the Freundlich constant related to adsorption capacity and n is the adsorption intensity.First-order kinetics was based on the assumption rate of adsorbate was proportional to the difference between the concentration of adsorbate adsorbed and present in the water.Conformation to pseudo-1 st -order kinetics was checked by fitting in linear Equation 4 (18), whereas pseudo-2 nd -order kinetics (Equation 5) (19) was used to check whether the rate of direct adsorption/ desorption process controls the overall sorption kinetics.ln (q e − q t ) = ln q e + K 1 t 2.303 (4) Where q e is the amount adsorbed per unit mass of adsorbent at equilibrium, q t is the amount adsorbed per unit mass of adsorbent at a time (t), K 1 and K 2 are rate constants.

Preparation of activated carbon from banana peels
The activated carbon was prepared from banana peels as described by Kigozi et al. (20) but with slight modifications.The peels were washed thoroughly and cut into small pieces of length ∼3 cm, then dried in an oven at 110 °C for 48 h.The dry peels were grounded and passed through a 1.0 mm sieve.The powder was then functionalized by chemical treatment using sulphuric acid (14% w/v) to eliminate any carbonates present, prevent ash formation and increase micropores.It was left to stand for 48 h in a fume hood, then washed with deionized water to pH 6.5.The sample was then dried in an oven at 120°C for 18 h.The dried sample was divided into two portions.The portions were heated at 600 °C and 700 °C in a furnace under a nitrogen atmosphere (300 mL/min) with a ramped temperature of 3 °C /min and a holding time of 2 h to obtain AC 600 and AC 700 .For TGA, a small portion (1 g) of  Unless stated, reactions were performed by allowing 1>, 10-, 30-, 60-and 90-min contact time between spiked water containing pharmaceuticals (2 ppm, 100 mL) and all three adsorbents packed in a column.
Figure 9. Pseudo-first-order and pseudo-second-order graphs of adsorption of copper with at 0-90 min contact time with adsorbents.
banana peel powder was put in a sample holder and placed in an autosampler, and the temperature program was set from 28 °C to 1000 °C with a 300 mL/ min flow rate with a ramped temperature of 3 °C/min.Plots of Mass (%) loss against temperature were acquired.

Preparation of xerogel
The xerogel was prepared according to Karume et al. (21) but on a bigger scale.Tetramethoxysilane (2035 μL, 1 mol eq.) water (470 μL, 2 mol eq.), and HCl (40 μL, 0.04 M) were vigorously shaken to form a sol and left to stand for 24 h in a closed Eppendorf tube to allow gel ageing.The tube was then opened and left to dry in the open air for 24 h forming the xerogel.

Selection and preparation of quartz
Quartz was collected from sedimentary rocks in Eastern Uganda.The stones were crushed, thoroughly washed with distilled water and then passed through a 300 μm sieve to obtain the pellets used in the adsorption studies.

Determination of adsorption capacity of individual adsorbents for metal ions and pharmaceuticals
Deionized water (100 mL) spiked with tetracycline, caffeine, ciprofloxacin and metronidazole (2 ppm), and lead, copper and zinc ions (10 ppm) were added to separate beakers containing a known mass of adsorbent (Quartz pellets passed through 300 μm sieve, xerogel and AC 600 ).The mixture was left to stand for 10 min.
The water was then analyzed by UV/Vis and AAS spectroscopy.
The adsorption capacity of individual adsorbents was investigated using one adsorbent at a time.
For combined adsorbents, the three adsorbents were packed in a column packed with xerogel, AC 600 and gravel arranged from bottom to top.
Concentrations of 5-50 ppm of metal ions and 2-25 ppm of pharmaceuticals were used during the studies on adsorption mechanisms.Kinetics studies were conducted at different contact time intervals between 0 -90 min.

Conclusions
The combination of xerogel with activated carbon and gravel efficiently adsorbs heavy metal ions and pharmaceuticals in water.The sorption mode was by both physical and chemical interaction between the adsorbates and the adsorbents with the concentration of the adsorbate in water greatly influencing the sorption/desorption modes.The incorporation of xerogel in the adsorbent system is a significant improvement over approaches that rely on sand, charcoal and gravel.The adsorbent system removed >95% of heavy metal ions and pharmaceuticals from water.The use of quartz, sand and xerogel with a silica framework poses no harm to the environment.Our approach should be readily extendible to natural filters that use natural adsorbents since the xerogel is inert and robust and can be produced on a commercial scale using readily available sand as an adsorbent and as a starting material for the synthesis of the TMOS precursor.

Figure 1 .
Figure 1.Proposed physical/chemical interactions of xerogel with solid particles, metal ions and pharmaceuticals.

Figure 2 .
Figure 2. The TGA analysis profile of banana peel powder.

Figure 3 .
Figure 3. (a) SEM image of AC 600 and (b) SEM image of AC 700 .

Figure 4 .
Figure 4.The FTIR spectra show functional groups in AC 600 and AC 700 .

Figure 6 .
Figure 6.UV/Vis spectra of ciprofloxacin adsorption in filtered water.

Table 1 .
The adsorption capacity of adsorbents for pharmaceuticals.
Unless stated, spiked water samples (100 mL) each containing ciprofloxacin (2 ppm) and caffeine (2 ppm) were added to separate beakers containing Gravel (quartz pellets, diameter ∼300 μm), AC 600 and xerogel.The mixture was left to stand for 10 minutes before analysis.

Table 2 .
Adsorption efficiency of the adsorbents for metal ions.Unless stated, the process was performed by passing spiked water containing copper, lead and zinc metal ions (10 ppm, 100 mL) through a column packed with xerogel, AC 600 and gravel (quartz pellets, diameter ∼300 μm).The filtrate was analyzed collected and analyzed by AAS.

Table 3 .
Adsorption isotherm parameters for linear Langmuir and Freundlich isotherms of selected metal ions.
Unless stated, reactions were performed by running spiked water containing metal ions (5 -50 ppm, 100 mL) through an open column packed with all three adsorbents.The filtrate was collected and analyzed by AAS.

Table 4 .
Adsorption isotherm parameters for linear Langmuir and Freundlich isotherms of selected pharmaceuticals.

Table 5 .
Pseudo first and second order kinetics parameters of adsorption of selected metal ions.

Table 6 .
Pseudo-first-order and pseudo-second-order kinetics parameters of adsorption of selected pharmaceuticals