Utilization of waste humate product (iron humate) for the phosphorus removal from waters

Desalination 265 (2011) 88–92

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Utilization of waste humate product (iron humate) for the phosphorus removal from waters Pavel Janoš ⁎, Andrea Kopecká, Stanislav Hejda Faculty of the Environment, University of Jan Evangelista Purkyně, Králova Výšina 7, 400 96 Ústí nad Labem, Czech Republic

a r t i c l e

i n f o

Article history: Received 25 April 2010 Received in revised form 15 July 2010 Accepted 17 July 2010 Available online 14 August 2010 Keywords: Phosphorus removal Sorption Iron humate Phosphorus extraction Wastewater treatment

a b s t r a c t Iron humate (IH) produced as a waste by-product during an industrial manufacture of humic substances from young brown coals was tested as a new cost-effective sorbent for the removal of inorganic phosphorus from waters. The sorption capacity approaching ca. 10 mg P g− 1 was comparable with that reported for other non-conventional sorbents, and was nearly independent on pH in a slightly acidic to neutral working range of the sorbent. It was found that the phosphate binding to IH is a relatively slow process requiring several days to attain equilibrium. The kinetics of the phosphate sorption was described by a recently introduced modified pseudo-n-order (MPnO) rate equation. Extraction tests showed that a major part of phosphorus in IH is associated strongly with iron-containing compounds or humate matrix of the sorbent and may be hardly liberated into the environment. Only minor fraction of phosphorus is readily mobilizable by leaching with water. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The phosphate release to surface waters and water reservoirs is of environmental concern, because it is an essential nutrient for growth of organisms in ecosystems, and thus it may cause eutrophication and subsequent deterioration of water quality. To minimize phosphorus emissions into surface waters, various techniques such as chemical precipitation, adsorption or biological treatment [1] and membrane techniques [2] have been employed for the removal of phosphates from wastewaters. The utilization of industrial wastes and byproducts for phosphorus removal has been given a great attention, because of their potential cost effectiveness. Various solid sorbents, often based on iron-containing compounds, have been tested for the phosphate removal, namely iron oxide tailings (waste material derived from a mineral processing industry) [3], iron hydroxide [4], biogenic iron oxides [5], binary Fe–Mn oxides [6] or Al–Fe (hydr) oxides [7]. Sorption of phosphorus onto coal ashes [8], paper sludge [9], crystalline MnO2 [10,11] and functionalized silica [12,13] has been also studied. In recent time, several researchers examined a utilization of a water treatment residual (WTR) material as a sorbent for the phosphorus retention [14–17]. This Al- or Fe-based (less often Caor Mg-based) material is able to retain phosphorus and affect its mobility in the aquatic and soil environment. It is also effective in the removal of arsenic from waters [18,19]. In our previous works, a new

⁎ Corresponding author. Tel.: + 420 475 284 148; fax: + 420 475 284 158. E-mail address: [email protected] (P. Janoš). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.07.036

kind of humate-based sorbent has been introduced, originating from an industrial manufacture of alkaline humates. The sorbent is prepared by precipitation of (residual) soluble humates by iron salts during the wastewater treatment, and is available under the name “iron humate” (IH). It was found to be effective in retaining of various inorganic and organic pollutants [20], especially of cationic species, such as basic dyes [21] or heavy metal cations [22]. In our recent work, we have demonstrated its ability to immobilise hexavalent chromium anions by a complex binding mechanism, in which both anionexchange and cation-exchange processes are involved together with the chromium reduction [23]. In the present work, iron humate was tested as a sorbent for the removal inorganic phosphate anions from waters. In a batch mode, basic kinetic and equilibrium characteristics were measured and a sorption capacity was estimated. Using a sequential extraction procedure, a leachability of phosphorus from the spent (loaded) sorbent was also examined. 2. Material and methods 2.1. Sorbent and chemicals Sorbent (iron humate, IH) was obtained from Severoceske Doly (North-Bohemian Mines), Bilina, Czech Republic. IH is produced by a precipitation of wastewaters from alkaline humates manufacturing with iron salts. It is supplied dried and granulated with grain sizes of ca. 0.5– 3 mm. According to the manufacturer's specification, IH should comply with the following criteria: content of humic substancesN 53%, ash contentb 27%, maximum admissible concentrations of heavy metals 10 mg kg− 1 (Pb), 1 mg kg− 1 (Cd), 30 mg kg− 1 (Cr), 20 mg kg− 1 (As)

P. Janoš et al. / Desalination 265 (2011) 88–92

and 2 mg kg− 1 (Hg). Content of Fe was ca. 11%. Point of zero charge (PZC) of the sorbent was ca. 4.1. IH exhibited a very low leachability of toxic elements (Cd, Cu, Hg, Pb, Zn) – their concentrations in the water extract (solid/liquid ratio 1:10) were below 0.1 mg L− 1, pH of the water extract was 4.38 [22]. IH was used as received, without any additional pretreatment, with an exception of a size classification by sieving. The fraction with grain sizesb 2 mm was used for all laboratory experiments. 0.01 M stock solutions of H3PO4 and KH2PO4 were prepared from reagentgrade chemicals (Lachema, Neratovice, Czech Republic). When necessary, the stock solutions were diluted with deionized water before sorption experiments, or mixed together at appropriate ratios to cover a wide range of initial pH values. 2.2. Sorption experiments Batch experiments were carried out by shaking a known amount of the sorbent (typically 0.5–2 g) with 50 mL of the solution containing desired concentrations of phosphorus (typically ranging from 0.1 to 4 mM) in polyethylene bottles. The solution together with the sorbent were agitated in the closed bottles for the predetermined time intervals using a horizontal shaker LT 2 (Kavalier, Sazava, Czech Republic) with an intensity of agitation of 2 rps. Then the solid phase was separated by sedimentation/centrifugation and the concentration of the phosphate ion was determined by ion chromatography. Simultaneously, the equilibrium pH values were measured. To investigate an effect of pH on the phosphate sorption, solutions with various pH values were prepared by mixing of the H3PO4 and KH2PO4 stock solutions and adjusting (when necessary) the initial pH with 1 M KOH. After the sorption experiment, the phosphate concentration and equilibrium pH were determined as described above. In some series of experiments, the ion chromatographic determination of phosphate ion and spectroscopic determination (ICP-OES) of total phosphorus were carried out simultaneously; it was found that both methods give identical results. All sorption experiments were carried out at laboratory temperature 22± 1 °C in an air-conditioned box. In each series of measurements, two kinds of quality-control experiments were carried out: procedure blank experiments with the sorbent and working solutions, without the presence of phosphate, and 2–3 experiments with various concentrations of phosphorus in working solutions, without the presence of sorbent. 2.3. Extraction procedures Leachability of phosphorus from the loaded sorbent was examined with the aid of the sequential extraction test developed for the fractionation of phosphorus in soils and sediments [24]: In the first step, 1.5 g of the loaded sorbent was leached with 100 mL of 1.0 M NH4Cl for 2 h at room temperature and then the supernatant was separated by centrifugation. The procedure was repeated and the extracts were combined and analyzed for the phosphorus content. In the second step, the residue from the first step was extracted with 100 mL of 0.1 M NaOH for 17 h in a similar way as described for the first step. In the third step, the residue from the second step was extracted with 100 mL of 0.5 M HCl for 24 h. Simultaneously, the leachability of phosphorus with water was determined with the aid of the standard leaching test for granular wastes and sediments DIN 38 414/4 [25].

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nations were carried out using a Dionex IonPac AS22 column with mobile phase containing 4.5 mM Na2CO3 and 1.4 mM NaHCO3, and suppressed conductivity detection. MS Excel and DataFit 8.1 (Oakdale Engineering, USA) software were used for calculations and data evaluations. 3. Results and discussion It was shown in our preceding study [23] that the removal of anionic species with the aid of IH requires sufficiently long time in the range of days rather than hours. The time dependencies for the phosphate sorption on IH were measured using 1.0 mM KH2PO4 as sorbate and various doses of the sorbent — see Fig. 1. To evaluate the adsorption kinetic dependencies, the pseudo-first-order (PFO) and pseudo-second-order (PSO) models are used most frequently. It was shown that these models, although empirical by their nature, approximate well more rigorous theoretical approaches based e.g. on a statistical rate theory [26]. The PFO adsorption rate equation may be expressed as: dqt = k1 ðqe −qt Þ dt

ð1Þ

where qt is the amount adsorbed at time t, qe = qt (t → ∞), and k1 is the PFO rate constant. An integral form of Eq. (1) is:
 −k t qt = qe 1−e 1

ð2Þ

The PSO adsorption rate equation is: dqt 2 = k2 ðqe −qt Þ dt

ð3Þ

with a corresponding integral form:

qt =

k2 q2e t 1 + k2 qe t

ð4Þ

k2 is the PSO rate constant. Both PFO and PSO models were used to evaluate the experimental dependencies. In addition, a new empirical rate equation, introduced recently by Azizian and Fallah [27] under

2.4. Analyses and data evaluation The total concentrations of phosphorus in solutions were determined by inductively coupled plasma – optical emission spectrometry (ICP-OES) with the aid of an Optima 3000 spectrometer (Perkin Elmer, Norwalk, USA). Ion chromatography was used to determine phosphate anions in solutions using an ICS-1000 Ion Chromatography System (Dionex, Sunnyvale, USA). The ion chromatographic determi-

Fig. 1. Time dependencies for the phosphate sorption on IH with various doses of the sorbent. Initial concentration of KH2PO4 1 mM, volume 100 mL. c0 is the initial concentration of phosphate in solution, ct is the phosphate concentration at time t.

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Table 1 Parameters of kinetic models for the sorption of phosphorus on iron humate. Weight of the sorbent (g)

1.2 1.6 2.0

PFO model

PSO model

MPnO model

qe (mmol g− 1)

k1 (h− 1)

R2

qe (mmol g− 1)

k2 (g mmol− 1 h− 1)

R2

qe (mmol g− 1)

k′

n

R2

0.027 0.029 0.024

0.063 0.060 0.080

0.961 0.978 0.974

0.033 0.036 0.029

2.12 1.71 2.94

0.981 0.989 0.989

0.040 0.034 0.028

0.006 0.016 0.016

1.95 1.58 1.73

0.997 0.992 0.995

the name the modified pseudo-n-order (MPnO) equation, was used to fit the experimental data. The MPnO equation can be written as ! n−1  n dqt q n = k tn−1 qe −qt dt qe

ð5Þ

qe =

or in its integral form:
 −nk′ t 1n qt = qe 1−e

The sorption isotherms were measured using KH2PO4 or H3PO4 as sorbate (Fig. 3). The Langmuir and Langmuir–Freundlich isotherm equations that are frequently used to describe sorption processes in environmental chemistry [29] were used to evaluate the experimental dependencies. The Langmuir isotherm can be expressed as

ð6Þ

qm Kce 1 + Kce

ð7Þ

whereas the Langmuir–Freundlich isotherm can be written as m

qe =

qm Kce 1 + Kcm e

ð8Þ

1 where n is a constant and k′ = kqn− . e The PFO, PSO and MPnO rate equations were used to evaluate the experimental qt−t dependencies for the phosphate sorption on IH. The model parameters obtained by a method of nonlinear regression are listed in Table 1. A comparison of the models is shown also in Fig. 2 for the sorbent dose 2.0 g. As can be seen, the MPnO model provides the best fit to the experimental data — see also the R2 values in Table 1. Figs. 1 and 2 demonstrate clearly that the phosphorus uptake is a rather slow process requiring several days to approach the equilibrium. This long equilibrating time differs markedly from relatively short equilibrating times reported in literature for the phosphorus sorption on metal oxides or hydroxides [5,7]. The mechanisms governing the sorption of phosphate anions on IH include the ionexchange and ligand-exchange, similarly to the sorption on iron hydroxide [4], but some additional sorption mechanisms should be considered, such as co-precipitation or surface precipitation, which slow down the overall process [28]. The kinetic measurements were used to estimate a time period necessary to attain the sorption equilibrium for the phosphorus removal. For further equilibrium (or “quasi-equilibrium”) experiments, the contact time was fixed at 96 h. This contact time is sufficient to approach closely (up to 95%) the maximum (equilibrium) sorbed amounts as estimated from the time dependencies calculated from the MPnO kinetic model.

qe is the phosphate concentration on the sorbent (mmol g− 1 or mg g− 1), ce is the phosphate concentration in solution (mM or mg L− 1), qm, K and m are parameters of the models. qm is the maximum sorption capacity under given conditions, m is usually called a “heterogeneity parameter” [29] reflecting various kinds of non-ideal behavior that may occur during the sorption on heterogeneous surfaces [30]. The isotherm parameters were calculated from the experimental data using a non-linear regression and are listed in Table 2. The sorption capacities (the qm values) estimated from the models ranged from ca. 0.11–0.37 mmol g− 1 (3.4– 11.5 mg Pg− 1); these values are quite comparable with the sorption capacities achieved on other non-conventional sorbents — Yan et al. [8] found the capacities ranging from 0.91 to 29.5 mg Pg− 1 for various kinds of coal fly ashes, whereas Cyrus and Reddy [31] reported the capacity of 0.5 mg Pg− 1 for the phosphate retention on shales. Humate by-product from titanium mining industry exhibited the sorption capacity of 0.99 mg g− 1 for phosphorus removal from solutions [16]. As can be seen from Table 2, the K and m parameters of the isotherms differ for the KH2PO4 and H3PO4 sorption, which may be related to the pH changes during the sorption and possible changes in the sorption

Fig. 2. Comparison of various kinetic models used to fit the sorption of phosphate on IH. Weight of the sorbent 2.0 g, other conditions as in Fig. 1.

Fig. 3. Sorption isotherms for the sorption of KH2PO4 and H3PO4 on IH. Solid lines calculated from the parameters of the Langmuir–Freundlich isotherms.

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Table 2 Parameters of sorption isotherms. Sorbate

KH2PO4 H3PO4

Langmuir isotherm

Langmuir–Freundlich isotherm

qm (mmol g− 1)

K (L mmol− 1)

R2

qm (mmol g− 1)

K (Lm mmol− m)

m

R2

0.109 0.169

2.17 1.02

0.949 0.974

0.373 0.177

0.240 0.929

0.49 0.96

0.968 0.974

mechanisms. It was observed during the isotherm measurements that the equilibrium pH values remained nearly constant (varied very slightly and irregularly in the range of 3.95–4.08) in the case of KH2PO4 sorption, whereas they decreased monotonously (from 4.02 to 3.10) with an increasing initial concentration of sorbate in the case of H3PO4 sorption. To elucidate the effect of pH on the phosphate removal, the sorption of KH2PO4 was measured as function of pH for different initial phosphate concentrations. As can be seen from Fig. 4, an effectiveness of the phosphate sorption was not influenced by pH in a slightly acidic region; a certain decrease in the sorption efficiency was observed only at very low pH values close to pH 1. It is generally expected that the sorption of anionic species on metal (hydr)oxides decreases with increasing pH if ion-exchange mechanisms are solely responsible for the sorption, due to the fact that surface charge of the sorbent becomes more negative as the pH increases [10,32]. The phosphate binding to iron oxides leads usually to the formation of bidentate surface complexes with Fe ions [10,32], and is accompanied by the pH changes caused by the exchange reaction between the surface hydroxyl groups and phosphate anions [33]. However, this effect was not observed during the sorption of KH2PO4 on IH. It was therefore assumed that some other phosphate binding mechanisms are effective, in which humic substances play a significant role. The possible mechanism of this binding involves formation of tertiary complexes, in which Fe ions in IH act as a bridge between humic substances and phosphate anions [34]. The decrease in the phosphate sorption at low pH values may be associated also with structural changes of the sorbent, such as its “coiling” at very low pH values, which results in an inaccessibility of some binding sites. IH can be used for the phosphate removal from aqueous solutions in a slightly acidic to neutral pH region. It is less stable in an alkaline region, similarly to other humate-based sorbents, although its resistance in alkaline solutions and its working pH range are better than those of e.g. solid humic acids. With increasing pH, humic substances are leached increasingly from IH, although the sorbent remains virtually undestroyed up to pH ca. 9 [22].

Fig. 4. Dependencies of the phosphate sorption on equilibrium pH for various initial concentrations of KH2PO4.

A leachability of phosphorus from the spent (loaded) sorbent was examined with the aid of a standard leaching test for granular wastes and sediments. In this test, deionised water was used as an extraction agent without any pH adjustment. Under these conditions, a leachability of phosphorus with water was rather low, representing only a few percent of the total phosphorus bound to the sorbent (Fig. 5). A more detailed picture about an extractability of phosphorus from the loaded sorbent may be obtained from a sequential extraction test. As can be seen from Fig. 5, only a small portion of phosphorus was liberated with the NH4Cl solution as an extraction agent. This fraction is only weakly bound to the sorbent (mainly by ion-exchange mechanisms) and is considered as most labile and easily mobilizable [24]. A major fraction of phosphorus was extracted in the second step with NaOH as an extraction agent. In this step, a fraction of phosphorus bound more strongly to amorphous and poorly-crystalline Fe compounds is liberated predominantly [24]. As a humate matrix of the sorbent is altered substantially in an alkaline medium, it is expected that also phosphorus associated with humic acids is liberated in this step [35]. 4. Conclusions It was shown that IH is capable to retain phosphate anions from aqueous solutions in a slightly acidic to neutral pH range and can be used for the removal of inorganic phosphorus from wastewaters. The removal efficiency (sorption capacity) was comparable with that reported for other non-conventional sorbents. The sorption of phosphates is a relatively slow process requiring several days to approach equilibrium. Although ion-exchange and ligand-exchange mechanisms are effective in the phosphate retention, some other mechanisms play also a significant role, such as co-precipitation or surface precipitation. This conclusion is supported by the fact that a major portion of phosphorus is bound rather strongly to the sorbent matrix and may be hardly liberated into the environment, e.g by

Fig. 5. Leachability of phosphorus from the loaded sorbent. Left column – sequential extraction test utilizing NH4Cl, NaOH and HCl as extraction agents, right column – extraction with deionised water.

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leaching with water. As a non-toxic material containing a significant amount of organic matter, IH could also be used as an amendment capable to modify a mobility and bioavailability of phosphorus in soils. Acknowledgements Humatex Comp., Bilina, is thanked for providing iron humate for this study. Financial support from the Czech Science Foundation (Grant No. 104/08/0758) is gratefully acknowledged. Additional financial support was obtained from the Internal Grant Agency of the University of Jan Evangelista Purkyně in Ústí nad Labem. Jiřina Petráková and Dr. Jiří Čmelík from the Research Institute of Inorganic Chemistry, Ústí nad Labem, are thanked for their assistance in the ICP-OES analyses. References [1] C.-Y. Wu, Y.-Z. Peng, S.-Y. Wang, Y. Ma, Enhanced biological phosphorus removal by granular sludge: from macro- to micro-scale, Water Res. 44 (2010) 807–814. [2] K.-G. Song, J. Cho, K.-W. Cho, S.-D. Kim, K.-H. Ahn, Characteristics of simultaneous nitrogen and phosphorus removal in a pilot-scale sequencing anoxic/anaerobic membrane bioreactor at various conditions, Desalination 250 (2010) 801–804. [3] L. Zeng, X. Li, J. Liu, Adsorptive removal of phosphate from aqueous solutions using iron oxide tailings, Water Res. 38 (2004) 1318–1326. [4] S. Mustafa, G. Nawab, A. Naeem, N. Rehana, B. Dilara, Temperature effect on phosphate sorption by iron hydroxide, Environ. Technol. 25 (2004) 1–6. [5] J.A. Rentz, I.P. Turner, J.L. Ullman, Removal of phosphorus from solution using biogenic iron oxides, Water Res. 43 (2009) 2029–2035. [6] G. Zhang, H. Liu, R. Liu, J. Qu, Removal of phosphate from water by a Fe–Mn binary oxide adsorbent, J. Colloid Interface Sci. 335 (2009) 168–174. [7] O.R. Harvey, R.D. Rhue, Kinetic and energetics of phosphate sorption in a multicomponent Al(III)–Fe(III) hydr(oxide) sorbent system, J. Colloid Interface Sci. 322 (2008) 384–393. [8] J. Yan, D.W. Kirk, C.Q. Jia, X. Liu, Sorption of aqueous phosphorus onto bituminous and lignitous coal ashes, J. Hazard. Mater. 148 (2007) 395–401. [9] M. Hojamberdiev, Y. Kameshima, A. Nakajima, K. Okada, Z. Kadirova, Preparation and properties of materials from paper sludge, J. Hazard. Mater. 151 (2008) 710–719. [10] S. Mustafa, M.I. Zaman, S. Khan, pH effect on phosphate sorption by crystalline MnO2, J. Colloid Interface Sci. 301 (2006) 370–375. [11] S. Mustafa, M.I. Zaman, S. Khan, Temperature effect on the mechanism of phosphate anions sorption by ß-MnO2, Chem. Eng. J. 141 (2008) 51–57. [12] S. Hamoudi, A. El-Nemr, K. Belkacemi, Adsorptive removal of dihydrogenphosphate ion from aqueous solutions using mono, di- and tri-ammoniumfunctionalized SBA-15, J. Colloid Interface Sci. 343 (2010) 615–621. [13] W. Chouyyok, R.J. Wiacek, K. Pattamakomsan, T. Sangvanich, R.M. Grudzien, G.E. Fryxell, W. Yantasee, Phosphate removal by anion binding on functionalized nanoporous sorbents, Environ. Sci. Technol. 44 (2010) 3073–3078. [14] K.C. Makris, W.G. Harris, G.A. O'Connor, T.A. Obreza, Phosphorus immobilization in micropores of dringing-water treatment residuals: implications for long-term stability, Environ. Sci. Technol. 38 (2004) 6590–6596.

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