Excessive nitrate in the water impose a danger to human health and contribute to eutrophication. The present continuous fixed bed pilot study was carried out using granular activated carbon made from walnut shell for removal of nitrate from aqueous solution and natural groundwater. The carbon was characterized using SEM, FTIR and BET. The BET specific surface area and average pore size before nitrate adsorption were 1434.6 m2g−1 and 2.08 nm, respectively, and after were 633.28 m2g−1 and 2.04 nm, respectively. Optimum removal of nitrate was achieved at a contact time of 2 min, pH of 6.5 and a nitrate concentration of 200 mg/l. The hydraulic loading rate was calculated to be 10 m3/h.m2 and the maximum adsorption capacity using the Langmuir adsorption isotherm model (R2 = 0.99) was 10 mg NO3/g. These experiments were also carried out using groundwater and the removal of nitrate decreased from 68% to 60% because of competition with other cations and anions.

NOMENCLATURE

  • Amount of nitrate adsorbed at equilibrium (mg.g−1)

  • volume of nitrate solution (L)

  • Initial NO3-N concentration in aqueous solution (mg L−1)

  • Equilibrium NO3-N concentration in aqueous solution (mg L−1)

  • M

    Mass of adsorbent used (mg)

  • Langmuir constant (l.g−1)

  • Langmuir constant (mg.g−1)

  • t

    Time (min)

  • tb

    Breakthrough time(h)

  • 1/n

    Sorption intensity, dimensionless

  • k

    Constants of the Freundlich isotherm

  • R2

    Correlation coefficient

  • χ2

    Chi-square analysis

  • Experimental equilibrium adsorption capacity (mg L−1)

  • Adsorption capacity calculated from models (mg L−1)

  • N

    Number of experimental points

INTRODUCTION

In many parts of the world, groundwater is the major source of drinking water. Nitrates are highly soluble in water and a common groundwater contaminant. Sources of NO3 contamination include agricultural and urban runoff, disposal of untreated sanitary and industrial waste, leakage from septic systems, landfill leachate and animal manure (WHO 2011).

High NO3 concentration in drinking water sources are a potential risk to the environment through eutrophication in water bodies and to public health, such as blue-baby syndrome (methemoglobinemia) in infants and cancer (WHO 2011). Environmental regulatory agencies have established standards for NO3 in drinking water. The US Environmental Protection Agency has set a maximum contaminant level of 10 for NO3 (USEPA 2012). WHO has proposed that the ratio of nitrate and nitrite to its guideline value should not exceed one (WHO 2008) and the Iranian standards set a maximum acceptable concentration of 50 mg/l for NO3 (ISIR 2010).

Removal of nitrate and nitrite can be conducted by ion exchange (WHO 2014), reverse osmosis (Richards et al. 2010), biological processes (Chung et al. 2014), chemical reduction (Öznülüer et al. 2013) and adsorption (Chatterjee et al. 2009). From various methods of water treatment, adsorption has emerged as a promising technique and is preferred for the removal of nitrate because of its high efficiency, ease of handling, availability of adsorbents and cost effectiveness (Bhatnagar & Sillanpll 2011). Activated carbon is a universal adsorbent for the removal of diverse types of aquatic pollutants such as heavy metals (Altun & Pehlivan 2012), pesticides (Salman et al. 2011), natural organic matter (Humbert et al. 2008) and mineral anions (Mahmudov & Huang 2010).

Walnut shell have been shown to have a high capacity for removal of heavy metals from aqueous solutions (Feizi & Jalili 2015). An adsorbent prepared from walnut shell was used to remove Caesium and obtained 91.9% removal efficiency (Ding et al. 2014). Adsorption study of Rhodamine B dye from aqueous solutions was carried out using walnut shells and the efficiency ranged from 1.451 to 2.292 mg.g−1 (Shah et al. 2013).

Nitrate removal by carbon derived from agricultural wastes has been a subject of research. Removal of nitrate-nitrogen from drinking water using bamboo powder charcoal was studied by Mizuta et al. (2004). Activated carbon was prepared from sugar beet bagasse by chemical activation (ZnCl2) and used to remove nitrate from aqueous solutions. After chemical activation, ZnCl2 was used as a chemical agent (Demiral & Gündüzoğlu 2010). In similar studies, activated carbon was treated with ZnCl2 (Namasivavam & Sangeetha 2005; Bhatnagar & Sillanpää 2011).

The present scientific study used the continuous flow approach using groundwater to simulate real conditions. As well, the commercial carbon was activated by a simple method. To verify the feasibility of granular walnut shell for the nitrate removal, nitrate adsorption was evaluated as the effect of initial nitrate concentration, contact time, pH and time of breakthrough. An adsorption isotherm was used to determine the nitrate adsorption mechanism of the granular activated carbon (GAC) made from walnut shell.

MATERIALS AND METHODS

Materials

NaNO3, HCl (assay 37%) and NaOH were obtained from Merck Co., Darmstadt, Germany. Different concentrations of nitrate solution were prepared by dissolving NaNO3 (assay 99.99%) in deionized water

Adsorbate

A stock synthetic solution of NO3 was first prepared (1,000 mg/l) by dissolving NaNO3 in deionized water to obtain the desired concentration of 30 to 200 mg/l. Groundwater was also tested to evaluate the influence of coexisting ions on nitrate removal.

Adsorbent

Commercial grade walnut shell was used to create the GAC in this study. The walnut shell was purchased from Part Chemical Co, in Tūyserkān, Iran. To prepare the modified activated walnut shell carbon, physical carbon was mixed with 5% under constant magnetic stirring for 30 min. The chemo-physical activated carbon was washed with hot ultra-pure water until the pH of the filtrating solution was neutral. Next, the samples were dried overnight at 120 °C and placed in a dry place in plastic containers for further use.

Experiments

Continuous mode adsorption studies

Continuous adsorption experiments were performed by placing 200 g of the GAC into contact with 230 ml of aqueous solution at initial concentrations of 30, 50, 80, 100, 150, and 200 mg/l at a pH of 6 to 8. The experiments were performed at a controlled temperature of ±25 °C. The adsorption capacity was calculated at equilibrium as: 
formula
1
where (mg.g−1) is the amount of nitrate adsorbed at equilibrium, C0 (mg.l−1) and Ce (mg.l−1) are the initial and equilibrium NO3-N concentrations in aqueous solution, respectively, Ve is the volume of the nitrate solution and M (g) is the mass of the adsorbent used.

Column adsorption studies

A Plexiglas column of 50 cm in height with an internal diameter of 4.5 cm are shown in Figure 1. The column was packed with adsorbent between two supporting layers of sand and glass wool. The adsorbent was added from the top of the column and allowed to settle by force of gravity, and then the column mounted vertically. Samples were collected at regular time intervals and stored for analysis.
Figure 1

Experimental setup for column studies.

Figure 1

Experimental setup for column studies.

Analysis

The NO3 remaining in each sample after adsorption at different time intervals was determined by spectrophotometer (model UV; Mini 1240; Shimadzu) at a wave length of 220 nm. The amount of coexisting ions was determined by ion chromatography (IC850; Metrohm). The surface morphology and particle size were determined by scanning electron microscopy (SEM) (TESCAN®8745R). The specific surface area and pore size distribution of the granular walnut shell was determined using a Brunauer–Emmett–Teller (BET) specific surface analysis device (Belsorp mini II). Fourier transform infrared (FTIR) spectroscopy (Tensor 27; Brucker) was used to examine the walnut shell GAC.

RESULTS AND DISCUSSION

Physical and chemical characterization of the adsorbent

BET

The BET specific surface area, total pore volume and average pore size of the walnut shell GAC are shown in Table 1 and indicate that, after adsorption of nitrate, the surface area and pore volume decreased. The average pore diameter was 2 nm and molecular size of the nitrate was 0.25 nm. The very small ionic radius of the nitrate is conducive to their penetration into the inner layer of the adsorbent.

Table 1

BET specific surface area, total pore volume and average pore size of walnut shell GAC

  Before adsorption After adsorption 
Surface area [m2 g−11434.6 633.28 
Total pore volume [cm3 g−10.747 0.323 
Average pore diameter [nm] 2.08 2.04 
  Before adsorption After adsorption 
Surface area [m2 g−11434.6 633.28 
Total pore volume [cm3 g−10.747 0.323 
Average pore diameter [nm] 2.08 2.04 

SEM

The microstructure of the walnut shell GAC was examined by SEM and micrographs of 30000× and 50000× magnification are shown in Figure 2(a) and 2(b), respectively. This figure shows that the adsorbent had a regular surface with tiny holes. This observation is supported by the BET surface area of the activated carbon.
Figure 2

SEM images with 30000× (a) and 50000× (b) magnification.

Figure 2

SEM images with 30000× (a) and 50000× (b) magnification.

Table 2 shows the characteristics of walnut shell, compare with almond and hazelnut shell. As seen, the walnut shells have a high carbon content, which makes them a suitable material from which to obtain activated carbon. Activated carbon from walnut shells had the highest surface area and iodine number. All the GACs had high bulk density and hardness. These properties can be explained by the high lignin and low ash contents of the shells. For all three types of GAC, the carbon content increased and the hydrogen content decreased when compared with their precursors. This occurs from the release of volatiles during pyrolysis that results in the elimination of non-carbon species and enrichment of carbon. Activated carbon from almond shells had the lowest burn-off and the lowest carbon content.

Table 2

Characterization of walnut shell compare with almond and hazelnut shell

  Walnut shell (This study) Almond shell (Aygün et al. 2003Hazelnut shell (Aygün et al. 2003
C% 88.7 50.3 51.4 
H% 6.05 5.95 
Ash% 2.6 0.31 0.49 
Moisture (%) 8.7 7.7 
Iodine number (mg g−1905 638 965 
BET surface area (m2 g−11434.6 736 793 
  Walnut shell (This study) Almond shell (Aygün et al. 2003Hazelnut shell (Aygün et al. 2003
C% 88.7 50.3 51.4 
H% 6.05 5.95 
Ash% 2.6 0.31 0.49 
Moisture (%) 8.7 7.7 
Iodine number (mg g−1905 638 965 
BET surface area (m2 g−11434.6 736 793 

FTIR

Chemical characterization was carried out by FTIR spectroscopy. The hydration of compounds (water crystallization) has a large effect on the spectrum and often adds to its complexity in the form of additional absorption bands and in the structure of existing bands. Example group frequencies for common inorganics are 1,490–1,410 (880–860), 1,130–1,080 (680–610) and 1,380–1,350 (840–815) cm−1 and represent the functional groups of carbonate, sulfate and nitrate ions, respectively. Typically, initial adsorption (in parentheses) is intense and broad and subsequent adsorption is weak to medium in intensity and is narrow. Both often exist as multiple band structures and this can be used to characterize the individual compounds.

The spectra were plotted for the adsorbent walnut shell GAC as shown in Figure 3 (before adsorption of nitrate) and Figure 4 (after adsorption of nitrate). The broad band at 3,433 cm−1 represents bounded O–H in the hydroxyl groups. The C = O or S–O in carboxyl group stretch can be ascribed to the band that appears at 1,184 cm−1. The new peaks just after adsorption at 3394.16, 1101.17 and 991.25 cm−1 can be attributed to –C ≡ C–H, C–N and = C–H, respectively. It can clearly be seen from the curve in Figure 4 that the peaks at 1571.09 and 1184.44 cm−1 disappear after the reaction and transmittance at 2391.34 cm−1 sharply increased due to the occurrence of dipole moment change compared with the original walnut shells, suggesting that an interaction occurred between the nitrate ions and protonated carboxyl groups.
Figure 3

FTIR spectra of GAC of walnut shell before nitrate adsorption within the range of 400–4,000 cm−1 wave number.

Figure 3

FTIR spectra of GAC of walnut shell before nitrate adsorption within the range of 400–4,000 cm−1 wave number.

Figure 4

FTIR spectra of GAC of walnut shell after nitrate adsorption within the range of 400–4,000 cm−1 wave number.

Figure 4

FTIR spectra of GAC of walnut shell after nitrate adsorption within the range of 400–4,000 cm−1 wave number.

Dynamic column adsorption

Effect of contact time

The effect of contact times of 0.5 to 20 min in the GAC column was examined. Figure 5(a) shows the effect of contact time on adsorption with an initial NO3 concentration of 100 mg/l at room temperature. As seen, the removal efficiency of nitrate first increased as the time increased because of increased availability of the surface area for adsorption. After saturation of the surface of the adsorbent, no appreciable increase in adsorption rate occurred. Equilibrium between the solid phase and solution was reached after 2 min.
Figure 5

Effect of contact time (a) and pH (b) on nitrate removal efficiency.

Figure 5

Effect of contact time (a) and pH (b) on nitrate removal efficiency.

Effect of pH on NO3 adsorption

The pH of aqueous solution influences the extent of adsorption. A pH of 6 to 8 was tested because the pH of natural water varies (Figure 5(b)). The pH was increased using NaOH (0.1 mol/l), and decreased using HCl (0.1 mol/l). No significant changes occurred in terms of nitrate removal efficiency at a pH of 6 to 8 and at pH = 6.5 the removal of NO3 reached a maximum level.

Column adsorption capacity

The adsorption capacity of the GAC was determined at initial concentrations of 80 to 200 mg/l. Breakthrough is the point on the S-shaped curve at which no further nitrate removal is observed (usually 5% of influent value) and the point of column exhaustion is the point where the effluent concentration reaches 95% of its influent value. Table 3 shows the effect of initial NO3 concentration on the GAC and variable flow rates. Breakthrough for a bed height of 0.5 m, hydraulic loading rate of 10 (m3/h.m2) and initial concentration 200 mg/l was 2.31 mg/g. Comparison of adsorption capacity from the column experiments and Langmuir isotherm shows a decrease in adsorption by the GAC.

Table 3

Column adsorption capacity at various operating conditions

Initial concentration (mg/lit) Hydraulic loading rate m3/(hm2Breakthrough time (hr) Adsorption column capacity (mg/gr) 
80 10 1.02 
100 10 3.6 1.23 
150 10 1.38 
200 10 2.4 1.53 
200 2.8 1.34 
200 10 2.5 2.31 
200 12 1.8 1.76 
200 14 1.4 1.69 
Initial concentration (mg/lit) Hydraulic loading rate m3/(hm2Breakthrough time (hr) Adsorption column capacity (mg/gr) 
80 10 1.02 
100 10 3.6 1.23 
150 10 1.38 
200 10 2.4 1.53 
200 2.8 1.34 
200 10 2.5 2.31 
200 12 1.8 1.76 
200 14 1.4 1.69 

The effect of initial concentration

The effects of initial nitrate concentration on the pilot column at a flow rate of 10 m3/(h.m2) at initial concentrations of 80 to 200 mg/l were examined. Initial concentration had a significant effect on the breakthrough curve as illustrated in Figure 6. In increase in initial feed concentration decreased the breakthrough time.
Figure 6

Breakthrough curve for different feed concentration at constant hydraulic loading rate of 10 m3/(h m2).

Figure 6

Breakthrough curve for different feed concentration at constant hydraulic loading rate of 10 m3/(h m2).

The data reveals that the amount of adsorbed NO3 increased as the concentration of the solution increased, but the percentage of adsorption decreased. This indicates that the removal of NO3 is highly concentration-dependent. At lower concentrations of NO3, the number of NO3 available in solution is less than the number of sites available on the adsorbent; however, at higher concentrations the available sites for adsorption decrease and the percentage removal of NO3 depends on the initial concentration.

The effect of flow rate

Testing was conducted at a bed height of 0.5 m, at constant feed concentration of 200 mg/l, and a hydraulic loading rate of 8 to 14 m3/(h.m2). Figure 7 shows that breakthrough time decreased from 2.8 to 1.4 h as the hydraulic loading rate increased from 8 to 14 m3/(h.m2). Maximum adsorption capacity was achieved at 10 m3/(h.m2). The adsorption capacity appears to be based on the mass transform zone. An increase in hydraulic loading rate increased the zone speed, decreasing the time required to achieve breakthrough.
Figure 7

Breakthrough curve for different hydraulic loading at constant feed concentration at 200 mg/lit.

Figure 7

Breakthrough curve for different hydraulic loading at constant feed concentration at 200 mg/lit.

Adsorption isotherms

An adsorption isotherm is an invaluable curve that describes the phenomenon governing retention. In this work, the Langmuir isotherm was used to describe the relationship between the amount of NO3 adsorbed and the equilibrium concentration (Figure 8).
Figure 8

Langmuir isotherm (a) and Freundlich isotherm (b) and plot of the experimental data.

Figure 8

Langmuir isotherm (a) and Freundlich isotherm (b) and plot of the experimental data.

Langmuir isotherm

The main assumption of the Langmuir method is that adsorption occurs uniformly on the active part of the surface. When a molecule is adsorbed onto a site, it does not affect the other molecules. The Langmuir equation can be written as: 
formula
2
where qe is the amount of solute adsorbed per unit weight of adsorbent (mg.g−1), Ce the equilibrium concentration of solute in the bulk solution (mg.l−1), Kl and b are the Langmuir constants representing the maximum adsorption capacity for the energy constant and the heat of adsorption, respectively. Figure 8(a) shows that the isotherm data fits the Langmuir equation well (R2=0.99). The values for Kl and b as determined from the figure were found to be 10 mg/g and 0.1 l/mg, respectively. Maximum adsorption capacity was 10 mg of NO3/g of GAC adsorbent.

Freundlich isotherm

The Freundlich equation is a purely empirical for sorption on a heterogeneous surface as: 
formula
3
The constants of the Freundlich isotherm (k, 1/n) can be obtained by plotting log qe versus log Ce, (Figure 8(b)). The isotherm data fit the Freundlich model well (R2=0.97). The values for k and 1/n were 1.12 and 0.627, respectively. In addition to correlation coefficient (R2), chi-square analysis (χ2) and normalized standard deviation (NSD) were employed to reasonably evaluate the validity of the models as (Boulinguiez et al. 2008): 
formula
4
 
formula
5
where (mg.g−1) is the experimental equilibrium adsorption capacity, (mg.g−1) is the adsorption capacity calculated from the models and N is the number of experimental points.

The results of χ2 and NSD were 0.060 and 0.066%, respectively, for the Langmuir isotherm and 0.534 and 0.179%, respectively, for the Freundlich isotherm. The fit is good when χ2 and NSD values are small. The lower χ2 and NSD values of Langmuir isotherm indicate it is the best fitting model for adsorption.

The results of this study were compared with studies on other sorbents. Qili Hu studied nitrate adsorption from an aqueous solution using granular chitosan-Fe3+ complex and achieved a maximum adsorption capacity of 8.35 mg/g based on the Langmuir-Freundlich model (Hu et al. 2015). Jahangiri-Rad tested adsorption of a packed bed column for nitrate removal using PAN-oxime-nano and the data confirmed that the breakthrough curves were dependent on flow rate and bed depth. The adsorption capacities observed under different flow rates (2, 5 and 7 ml/min) were 11.65, 24.38 and 25.89 mg/g. The maximum adsorption capacity increased as the flow rate increased (Jahangiri-Rad et al. 2014). Bao et al. removed bromate and nitrate ions from water using GAC from coconut shell in a pilot test. The maximum adsorption achieved after 20 min at a pH of 7.1 with 2.1 mg/g of adsorbent (Bao et al. 1999). The maximum adsorption capacity of walnut shell GAC (10 mg/g) for removal of nitrates was higher than for their adsorbent.

Removal nitrate from groundwater

Ions coexisting with nitrate in water could exert a negative influence on adsorption process. These anions occupied adsorption sites on the adsorbent surface and increased electrostatic repulsion between the walnut shell GAC and the nitrate ions, decreasing the nitrate removal efficiency. Table 4 and Figure 9 show the results of removal of nitrate in a GAC column using groundwater. As seen, the nitrate removal decreased from 68% to 60% because of competition other cations and anions.
Table 4

Results of GAC column using groundwater

Parameter Unit Feed to column After 2 min 
pH – 
NO3 ppm 20.355 4.405 
PO4 ppm <0.02 1.23 
SO4 ppm 190.9 21.47 
EC μs/cm 1210 1050 
TDS ppm 712.34 633.19 
Parameter Unit Feed to column After 2 min 
pH – 
NO3 ppm 20.355 4.405 
PO4 ppm <0.02 1.23 
SO4 ppm 190.9 21.47 
EC μs/cm 1210 1050 
TDS ppm 712.34 633.19 
Figure 9

Walnut shell GAC performance for nitrate removal from synthetic aqueous solution and groundwater.

Figure 9

Walnut shell GAC performance for nitrate removal from synthetic aqueous solution and groundwater.

CONCLUSIONS

Column performance characteristics were investigated for walnut shell GAC in a packed bed for removal of nitrate from drinking water. The adsorption of nitrate to the GAC followed the Langmuir adsorption isotherm model. Adsorption capacity for the 200 mg/l feed concentration of nitrate at a hydraulic loading rate of 10 m3/(h.m2) and a 0.5 m bed height was 10 mg/g; however the experiment predicted a breakthrough adsorption capacity of 2.31 mg of NO3/g.

SEM analysis indicated that GAC from walnut shells is an amorphous material. The BET specific surface area was 1434.6 m2.g−1, total pore volume was 0.747 cm3.g−1 and the average pore diameter was 2.08 nm.

Selection of a suitable adsorbent media for nitrate removal from water depends on factors such as the range of initial nitrate concentration, presence of competing ions and their concentration in water, optimization of adsorbent, adjustment of pH in the water and proper operation and maintenance. Improvements in nitrate adsorption by activated carbon treated with phosphoric acid appears to result from surface acid (protons) remaining on the carbon surface after H3PO4 treatment. More research is needed on regeneration and for environmentally-safe disposal of nitrate.

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