Preparation of nZVI
The nZVI was synthesized with dispersing agent, polyethylene glycol (PEG-4000). The basic principle of the synthesis was that ferrous ion was rapidly reduced to nZVI by borohydride solution according to the following reaction (Eq. 1) (He et al. 2012):
Fe2+(aq) + 2BH4−(aq) + 6H2O→Fe0(s) + 2B(OH)3(aq) + 7H2↑ (1)
In this method; 1.7868 g FeSO4•7H2O was dissolved in 90 mL 4/1 (v/v) ethanol/deionized water mixture in a 500-mL bottle and then 0.3 g PEG-4000 was added into the above solution. The temperature was kept at 20°C. The solution was stirred at 220 rpm for 30 min to ensure that PEG-4000 was completely dissolved. Before reductant addition, the solution pH was adjusted to about 6.5 with 1 M NaOH. Then, 50 mL of 1.3883 g KBH4 aqueous solution was added dropwise into the mixture at 220 rpm. The solution was stirred for another 30 min after the addition of all KBH4. Then the resulting black solid particles were washed with deoxygenated water three times and deoxygenated absolute ethanol twice and were collected by magnetic separation. Finally, the nZVI particles were dried at 70°C and stored in a under N2 gas to prevent nZVI oxidation from atmospheric oxygen. In our previous work, synthesized nZVI characterization was carried out using XRD, SEM, and EDX techniques (Gocer et al. 2019).
Landfill Leachate
Raw LFL was collected from a municipal sanitary landfill located in Kahramanmaras, Turkey. The total amounts of LFL deposited daily were 815—830tons. LFL was collected from the equilibration tank and stored at 4°C until used. The characteristics of raw LFL is summarized in Table 1.
Table 1 Landfill Leachate Characterization
Parameters
|
Concentration
|
Parameters
|
Concentration
|
Total Organic Carbon (DOC)
|
7058±400(mg/L)
|
NO2-
|
320±20(mg/L)
|
COD
|
16000±1500(mg/L)
|
NO3-
|
670±40(mg/L)
|
BOD
|
1500±300(mg/L)
|
Pt-Co (Color unit)
|
6380±300
|
NH4+-N
|
2120±200(mg/L)
|
PO4-3-P
|
78±10(mg/L)
|
Adsorption experiments and capacity
The effect of various parameters including pH (3–8), contact time (15-330min), nZVI concentration (50–500 mgFe0/L) was tested on NН4+, NO3−, DOC, and COD removal in batch adsorption experiments, for the evaluation of the optimum process conditions. (Table 2).
Table 2
Nano Zero Valent Iron (nZVI) | Adsorbent Concentration (mgFe0/L) | pH | Mixing Rate (rpm) | Contact Time (min) | Temperature (oC) |
50 | 3 | 4 | 5 | 6 | 7 | 8 | 200 | 15–330 | Room Temperature (25oC) |
100 | 3 | 4 | 5 | 6 | 7 | 8 |
200 | 3 | 4 | 5 | 6 | 7 | 8 |
300 | 3 | 4 | 5 | 6 | 7 | 8 |
400 | 3 | 4 | 5 | 6 | 7 | 8 |
500 | 3 | 4 | 5 | 6 | 7 | 8 |
The adsorption capacity, qe, (mg g− 1), and pollutants removal efficiency (%) of the tested nZVI were calculated by Eqs. (2) and (3), respectively:
In Eqs. (2) and (3), V is the volume of the leachate (L), W is the amount of adsorbent (g), Co and Ce are the initial and equilibrium concentrations of pollutants (mg L− 1) in the leachate, respectively.
Analyses
All samples were centrifuged at 4000 rpm for 5 min (Eppendorf Centrifuge 5415R, Hamburg, Germany) and then, were filtered using a sterile syringe 0.45μm filter (Sartorius AG, Gottingen, Germany). DOC and TN concentrations were analyzed using a TOC instrument coupled with TN (Shimadzu TOC-VCPN, Kyoto, Japan). The pH was measured by a pH meter (Thermo, Orion 4 Star, Indonesia). Ionic composition of influent and effluent samples (ammonium, nitrate) was measured by ion chromatography (Dionex ICS-3000, Sunnyvale, CA, USA). The COD measurements were carried out according to the dichromate-closed reflux Colorimetric Method described in Standard Methods (Standard Methods, 5220 D). The color was analyzed as Pt-Co units. Pt-Co color measurements were performed spectrophotometrically at 465 nm during lab-scale studies.
Adsorption Isotherms and Kinetics
The Langmuir, Freundlich isotherms and pseudo 1st order, pseudo 2nd order kinetics models were selected to simulate the isotherm adsorption of nZVI in this work.
Adsorption kinetics
The amount of nZVI absorbed on the pollutants was calculated using the following equation (Eq. 4):
$${q}_{e}=\left(Co-Ct\right)*\frac{V}{m}$$
4
Where qe expresses the adsorption capacity (mg g− 1); C0 and Ct are pollutant concentrations (such as DOC, COD, NH4+, and NO3−) (mg L− 1) at time 0 and t, respectively. V indicates the volume of solution (mL) and m is the mass of nZVI (g). Adsorption kinetic parameters are divided into two; these are expressed as pseudo-first-order equations (Ho, 2004) and pseudo-second-order equations (Azizian 2004; Ho 2006).
Pseudo-first-order kinetics
The so-called first-order equation (Lagergren's equation) is expressed as the adsorption of solid-liquid solutions based on the adsorption capacity of solids (Ho, 2004). The linear form of the so-called first-order model can be expressed by the following equation (Eq. 5):
$$Log\left(qe-qt\right)=Logqe-\left(\frac{k1}{2.303}\right)t$$
5
where qe and qt (mg g− 1) are defined as the adsorption capacities at time t(h) at equilibrium, respectively.
Pseudo-second order kinetics
The pseudo-second-order kinetics are used to define chemical adsorption from liquid solutions (Azizian 2004; Ho 2006). The linear expression of this kinetics is shown in Eq. 6 below (Eqs. 6):
where k2 is the rate constant for pseudo-second-order adsorption (g mg− 1 h− 1) and k2qe2 is the initial adsorption rate (mg g− 1 h− 1).
Adsorption isotherm models
Freundlich isotherm
Freundlich isotherm models are valid for both monolayer (chemisorption) and multilayer adsorption processes. This isotherm is known adsorbing to the heterogeneous surface of an adsorbent. The linear form of the Freundlich equation is expressed as (Eq. 7):
$$Lnqe=LnKF+\frac{1}{n}LnCe$$
7
where KF and n are Freundlich isotherm constants related to adsorption capacity and adsorption intensity, respectively and Ce is the equilibrium concentration (mg L− 1) (Tan 2009).
Langmuir isotherm
The Langmuir isotherm assumes monolayer adsorption on a single surface with a certain number of adsorption sites. After the adsorption zone is filled, no more tendency takes place. In this way, it will reach a saturation point where maximum adsorption of the surface will be achieved. The linear form of the Langmuir isotherm model is expressed as (Eq. 8):
$$\frac{Ce}{qe}=\frac{Ce}{qmax}+\frac{1}{KLqmax}$$
8
where KL is the Langmuir constant related to the energy of adsorption and qm is the maximum adsorption capacity (mg g− 1) (Barkat et al. 2009; Chingombe et al. 2006).