Abstract

Sterile phosphate (SP) was investigated for phosphorus removal from wastewater using batch adsorption experiments. The novel adsorbent is a mining by-product obtained from the phosphate mining plants having a strong affinity with phosphorus ions present in wastewater. The results of the batch adsorption experiments indicated that 30 min of contact time between the adsorbent and wastewater was sufficient for attaining equilibrium. The phosphorus removal from wastewater increased with increasing initial phosphorus concentration, adsorbent dose and temperature, while it decreased with increasing initial pH values. The maximum phosphorus removal efficiency was noted to be 94.4%. It was achieved in slightly acidic conditions (pH = 4), with an adsorbent dose and initial phosphorus concentration of 3 g L−1 and 20 mg L−1, respectively, and at room temperature. Kinetic analysis showed that phosphorus adsorption onto sterile phosphate was best fitted with the pseudo-second order kinetic model. The adsorption equilibrium data fitted well to the Langmuir model equation, indicating monolayer coverage of the adsorbent. The adsorption capacity calculated from the Langmuir model equation was found to be 7.962 mg g−1. Comparing with some industrial products and natural mineral adsorbents, sterile phosphate was found to be the most efficient adsorbent for phosphorus removal from wastewater.

INTRODUCTION

Anthropogenic activities generate very large volumes of waste water and industrial effluents which, without adequate treatment, contaminate the receiving environment.

Uncontrolled phosphorus-laden discharges, the origin of which can be domestic, industrial or agricultural, are considered to be the most undesirable form of pollution for the receiving aquatic environment and cause a significant ecological imbalance in aquatic ecosystems, known as eutrophication. This phenomenon is practically due to an enrichment of nutrients (nitrogen and phosphorus) in the water and can lead to the increased presence of algae and the depletion of dissolved oxygen, which compromises the use of water.

The need to avoid the uncontrolled release of phosphorus in effluents is now integrated into the approach of most phosphorus users: industrialists, industrialists specializing in water treatment, farmers and even individuals; awareness of the need for better management of phosphorus reserves is more recent and not yet shared by all. However, more and more convinced that in the future this element will not be an inexhaustible resource, and also under socio-environmental pressures, the phosphorus and water treatment industries in developed countries have set the objective of recovering the phosphorus contained in wastewater (urban, industrial or agricultural) as part of a sustainable development policy.

LITERATURE REVIEW

Phosphorus is one of the most important elements for the normal functioning of ecosystems (Song et al. 2011). Besides the nitrogen and potassium, phosphorus is considered as a major nutrient essential for plant growth (Yan et al. 2010; Vohla et al. 2011) and for all forms of life (Cheng et al. 2009). Phosphorus has a primordial role in DNA/RNA molecules. It is also related to the transport of cellular energy via ATP (Cheng et al. 2009). Phosphorus in wastewater occurs in various forms such as orthophosphates (ortho-P), condensed phosphates (condensed- P) and organic phosphates (organic-P) (Gao et al. 2013a). It is delivered to aquatic systems as a mixture of dissolved and particulate inputs, each of which is a complex mixture of these different molecular forms of pentavalent P (Van Wazer & Holst 1950).

It is well known that high phosphorus concentrations in wastewater can lead to hazardous environmental impacts largely due to the eutrophication phenomenon (Ryther & Dunstan 1971; Song et al. 2011; Gao et al. 2013b). Economic losses attributed to eutrophication include the costs of water purification for human use, losses of fish and wildlife production, and losses of recreational amenities (Rabotyagov et al. 2014). Unfortunately, phosphorus is considered a non-renewable resource (Lewis et al. 2013). The growing awareness that phosphorus elements will be depleted within the next 100 years requires an effective means to secure such as a mineral resource from contaminated water (et al. 2011). Therefore, it is necessary to recover and reuse this limited resource for the global sustainable development (Cheng et al. 2009). However, the phosphorus element is generally removed from wastewater by converting the phosphorus ions to a solid fraction in different forms, such as an insoluble salt precipitate, a microbial mass in an activated sludge or a plant biomass in constructed wetlands (Luo et al. 2017). These approaches are not eco-friendly because the phosphorus element is converted to various other wastes, some of which are toxic. The disposal of non-solubilized phosphates can be carried out either by burying it in landfills after incineration of the organic matter or using it as a sludge fertilizer in the case of the treatment facility eliminating human pathogens and toxic compounds (de-Bashan & Bashan 2004).

Several methods have been applied for phosphorus removal from wastewater (Wang et al. 2016; Gao et al. 2017; Marques et al. 2018) such as ion exchange (Nur et al. 2014), membrane separation (Xu et al. 2014; Park et al. 2017; Geng et al. 2018) and biodegradation (Fytianos et al. 1998; Conley et al. 2009; Hussain et al. 2015). The methods of phosphorus removal are classified in three categories: (i) the biological methods (Yi et al. 2017; Yang et al. 2018) involve the over-accumulation of phosphorus in the biomass (Wu et al. 2013), (ii) the chemical treatment methods consist of adding a reagent into the phosphorus solution to precipitate insoluble phosphorus salt, which is then separated from the liquid phase by filtration or decantation (Ruixia et al. 2002), and (iii) the adsorption processes from liquid discharges using adsorbents such as natural phosphate (de-Bashan & Bashan 2004), aluminum and aluminum oxides (Tang et al. 1997; Johnson et al. 2002; Lee et al. 2015), iron and iron oxides (Hongshao & Stanforth 2001; Huang 2004; Juang & Chung 2004; Lin et al. 2004), fly ash (Tsitouridou & Georgiou 1988; Ugurlu & Salman 1998), blast furnace slag (Chen et al. 2007), slag (Oguz 2005), red mud (Koumanova et al. 1997; Akay et al. 1998), bauxite (Altundoğan & Tümen 2001), silicates (Kasama et al. 2004; Shin et al. 2004), activated carbon (Bhargava & Sheldarkar 1993) and anion exchangers (Jia et al. 2016). In this study, we have investigated the adsorption potential of SP as a specific calcic adsorbent for phosphorus removal from wastewater. It is an abundant by-product generated by the phosphate mining plants in considerable quantities, which is generally discharged into a sensitive receiving environment. Adsorption experiments were performed in a batch reactor to study the effect of various operational parameters on phosphorus adsorption efficiency. Moreover, kinetic and equilibrium models were used to fit the experimental data. A comparison based on adsorption capacity was also made to highlight our novel adsorbent among those examined in our knowledge.

METHODS

Sampling, preparation and characterization

The adsorbent used in this study was obtained from the discharged by-products of the phosphate mining plant. The adsorbent was dried in an oven at 40 °C for 24 h, to a constant weight, and converted into fine powder by mechanical grinding.

Phase composition was identified by X-ray diffraction (XRD) with a diffractometer X'Pert PRO PANALATYCAL, equipped with a scintillation detector with X'Celerator Ultrafast radiation beam Kα (λ = 0.154060 nm), operating at the voltage of 40 kV and current of 30 mA with a copper target. Data were collected in 2θ-range 10° to 80°.

Analysis of the XRD spectrum of SP (Figure 1) indicates that it is made up mainly of calcite (CaCO3), fluorapatite (Ca5(PO4)3F and some trace of quartz (SiO2), which was confirmed by chemical analysis by X-ray fluorescence XRF; OXFORDMDX 1000; the results are mentioned in Table 1.

Table 1

Elemental analysis of SP

Major elements (%)SP
SiO2 18.37 
P2O5 20.35 
F 2.60 
Al2O3 0.51 
Fe2O3 0.65 
CaO 39.26 
MgO 1.94 
SO3 1.18 
LOI 15.14 
Major elements (%)SP
SiO2 18.37 
P2O5 20.35 
F 2.60 
Al2O3 0.51 
Fe2O3 0.65 
CaO 39.26 
MgO 1.94 
SO3 1.18 
LOI 15.14 
Figure 1

XRD analysis of SP.

Figure 1

XRD analysis of SP.

LOI: loss on ignition; SP was considered as a porous material, which was demonstrated by the BET analysis (Micromeritics Instrument Corporation 3Flex surface characterization) and shows a surface area about 14.18 m2 g−1, to visualize the texture of this by-product (Figure 2) a SEM analysis was carried out by Scanning Electron Microscope (SEM) (FEI QUANTA 200).

Figure 2

SEM analysis of SP.

Figure 2

SEM analysis of SP.

In this shot, we observe an image of the morphology of the by-product composed of small particles with diameters of some microns, this texture offers an interesting adsorption capacity for our material.

Phosphorus solutions preparation and analysis

Potassium di-hydrogen phosphate (KH2PO4) was used during adsorption experiments as the source of phosphorus ions in wastewater. A stock phosphorus solution of 100 mg L−1 was prepared by dissolving 439.5 mg of KH2PO4 in 1,000 mL of deionized water. Experimental solutions for various experiments were then prepared by appropriate dilution of the stock solution. The analysis of phosphorus concentrations in wastewater was performed spectrophotometrically at 880 nm, following the ascorbic acid method (ISO 6878 2005).

Batch adsorption studies

Effect of initial phosphorus concentration and contact time

The effect of initial phosphorus concentration and contact time on the adsorption process onto SP was carried out through kinetics and equilibrium studies. Four initial phosphorus concentrations were chosen: 10, 20, 30 and 40 mg L−1, and their progress in time was followed at seven contact times: 5, 10, 20, 30, 40, 50 and 60 min. The adsorbent dose and initial pH solution were fixed at 3 g L−1 and 7 respectively.

Effect of initial pH solution

The effect of initial pH solution on phosphorus adsorption was examined in a series of experiments varying pH between 4 and 10. During these experiments, the initial phosphorus concentration and the adsorbent dose were fixed at 20 mg L−1 and 3 g L−1 respectively. The pH adjustment was carried out by using 0.1 N (HCl) or 0.1 N (NaOH).

Effect of adsorbent dose

The effect of the adsorbent dose on phosphorus adsorption onto SP was determined for 20 mg L−1 initial phosphorus concentration at pH 7. The tested adsorbent doses varied from 3 to 12 g L−1.

Effect of temperature

The effect of temperature was investigated at 25, 35, 45 and 55 °C in a constant temperature shaker. For all these experiments, the initial phosphorus concentration, adsorbent dose and initial pH solution were fixed at 20 mg L−1, 3 g L−1 and 7 respectively.

Phosphorus removal calculation

The specific amount of the adsorbed phosphorus, Qe (mg g−1), was calculated from the decrease of the phosphorus concentration in the aqueous solutions, as expressed in the following equation: 
formula
(1)
where Qe is the adsorption capacity (mg g−1) in the solid at equilibrium; C0, Ce are initial and equilibrium concentrations of phosphorus ions (mg L−1), respectively; V is volume of the aqueous solution (L) and W is the mass (g) of the adsorbent used in the experiments.
The adsorption removal efficiency (ARE), at equilibrium, is calculated by using the following equation: 
formula
(2)

Modeling studies

Kinetic models

The adsorption speed is largely influenced by several parameters, mainly the status of the adsorbent and the physicochemical conditions under which the adsorption takes place. The aim of the adsorption kinetics study is to understand the mechanism involved and the rate controlling steps affecting the adsorption kinetics. Several models are exploited to fit the kinetic sorption tests. In this study, the kinetics of phosphorus adsorption onto SP were verified using pseudo-first order and pseudo-second order equations. The pseudo-first order is a kinetic model described by the following equation (Bhattacharyya & Gupta 2008): 
formula
(3)
The linear form of the pseudo-first order kinetic model can be expressed by the following equation: 
formula
(4)
where, and are the amount of phosphorus adsorbed (mg g−1) at equilibrium and at time ‘t’, respectively. K1 (min−1) represents the rate constant of the pseudo-first order adsorption reaction. A straight line of log(Qe − Qt) against t suggests the applicability of these kinetic models. Both Qe and K1 can be determined from the intercept and slope of the curve, respectively.
The linear form of the pseudo-second order kinetic model can be expressed by the following equation (Unuabonah et al. 2007): 
formula
(5)
where, K2 is the rate constant for the pseudo-second order reaction (g mg−1 min−1). Qe and Qt are the amounts of phosphorus ions adsorbed at equilibrium and at any time ‘t’ (mg g−1), respectively. The straight line plot of t/Qt against t for the kinetic data gives the values for Qe and K2 from the slope and intercept, respectively.

Adsorption isotherms

Adsorption equilibrium (the ratio between the adsorbed amount with what remains in the solution) is defined to be a state of dynamic equilibrium, where both adsorption and desorption rates are equal. Therefore, the Langmuir and Freundlich models were used to describe the equilibrium data. The Langmuir model is based on the hypothesis that uptake occurs on a homogenous surface by monolayer adsorption without interaction between adsorbed molecules, and can be written in linear form as it is represented in the following equation (Langmuir 1916): 
formula
(6)
where, Ce is the equilibrium concentration of phosphorus ions (mg L−1), Qe is a solid phase concentration of phosphorus ions (mg g−1), Qm (mg g−1) and KL (L mg−1) are empirical constants which can be evaluated from the slope and intercept of the linear plot of Ce/Qe against Ce.
The Freundlich model proposes a multilayer adsorption with a heterogeneous energetic distribution of active sites and with interaction between adsorbed molecules. It is expressed mathematically in linear form as it is represented in the following equation (Freundlich 1906): 
formula
(7)
where, KF is the Freundlich characteristic constant (mg g−1) and 1/n is the heterogeneity factor of sorption, obtained from the intercept and slope of the logQe versus logCe linear plot respectively.

RESULTS AND DISCUSSION

Effect of initial phosphorus concentration and contact time

As shown in Figure 3, the amount of adsorbed phosphorus (Qt) increased rapidly in the first 10 min of reaction. The percentage of the total amount adsorbed at this time reached about 76.4% for an initial phosphorus concentration of 10 mg L−1. In the first 10 min of reaction, the adsorption rate was relatively fast. This was due to the fact that phosphorus ions were mainly adsorbed on the exterior surfaces of SP. When these latter reached saturation, the phosphorus ions entered into the adsorbent particles where they were adsorbed on their interior surfaces. On the other hand, the phosphorus removal increased with increasing initial phosphorus concentration. It was observed that the adsorbent increased its adsorption capacity from 2.083 to 5.167 mg g−1 while raising the initial phosphorus concentration from 10 to 40 mg L−1, respectively. This could be explained by the fact that the higher the initial phosphorus concentrations, the higher the concentration gradient between the aqueous solution and the solid phase, which results in more important diffusion rates. However, 30 min of contact time between SP and wastewater was sufficient to attain the equilibrium.

Figure 3

Effect of initial phosphorus concentration and contact time on phosphorus adsorption onto SP (adsorbent dose = 3 g L−1; initial pH solution = 7; T = 25 °C; shaken speed = 400 rpm).

Figure 3

Effect of initial phosphorus concentration and contact time on phosphorus adsorption onto SP (adsorbent dose = 3 g L−1; initial pH solution = 7; T = 25 °C; shaken speed = 400 rpm).

Effect of initial pH solution

Figure 4 shows the effect of initial pH solution on phosphorus adsorption onto SP. At equilibrium (Qe), the amount of the adsorbed phosphorus increased with the decrease of pH values. It was observed that SP increased its adsorption capacity from 3 mg g−1 at pH 10 to 6.293 mg g−1 at pH 4. This may be due to the formation of positively charged sites on the adsorbent surface in the acidic conditions, which consequently enhanced phosphorus adsorption by electrostatic attraction. At this pH, the phosphorus removal efficiency reached 94.4%. Furthermore, the phosphorus adsorption decreased for alkaline pH. This can also be attributed to the competition between hydroxyl and phosphorus anions for adsorption sites. These results were found to be in concordance with previous studies on phosphorus removal from wastewater using mineral adsorbents (Wei et al. 2008; Jellali et al. 2011). Fortunately, its pH as a by-product while discharging it into the environment is slightly acid. Therefore, the phosphorus removal from wastewater using SP doesn't require the supply of chemicals, which makes the by-product a suitable and efficient mineral adsorbent.

Figure 4

Effect of initial pH solution on phosphorus adsorption onto SP (adsorbent dose = 3 g L−1; initial phosphorus concentration = 20 mg L−1; contact time = 30 min; T = 25 °C; shaken speed = 400 rpm).

Figure 4

Effect of initial pH solution on phosphorus adsorption onto SP (adsorbent dose = 3 g L−1; initial phosphorus concentration = 20 mg L−1; contact time = 30 min; T = 25 °C; shaken speed = 400 rpm).

Effect of adsorbent dose

Figure 5 shows the effect of the adsorbent dose on phosphorus removal from wastewater. It was observed that phosphorus removal efficiency increased rapidly by increasing the amount of SP. The phosphorus removal efficiency increased from 60.75% to 83.65% for adsorbent doses of 3 and 6 g L−1, respectively. However, the increase of the adsorbent dose, especially from 9 to 12 g L−1, didn't show any considerable increase in the percentage of phosphorus removal. This is probably due to the overlapping phenomenon of the active sites at a higher adsorbent dose resulting in a reduction of surface area. At this stage, the efficiency of phosphorus removal reached 91.75% for an average residual P concentration at equilibrium of 1.65 mg L−1.

Figure 5

Effect of adsorbent dose on the phosphorus adsorption onto SP (initial phosphorus concentration = 20 mg L−1; initial pH solution = 7; contact time = 30 min; T = 25 °C, shaken speed = 400 rpm).

Figure 5

Effect of adsorbent dose on the phosphorus adsorption onto SP (initial phosphorus concentration = 20 mg L−1; initial pH solution = 7; contact time = 30 min; T = 25 °C, shaken speed = 400 rpm).

Effect of temperature

The effect of temperature on phosphorus removal from wastewater was investigated. As shown in Figure 6, the parameter of temperature strongly influenced the adsorption of phosphorus onto SP. At equilibrium (Qe), the amount of adsorbed phosphorus increased significantly when the temperature was raised from 25 to 55 °C. This finding proved that high temperatures could enhance phosphorus adsorption. This latter may involve both physical and chemical sorption processes. On the one hand, the increase of adsorption capacity with temperature results in the rise of the kinetic energy of the adsorbent particles. Thus, the frequency of collision between SP and phosphorus ions increased so that the adsorption process could be enhanced onto the adsorbent surface. On the other hand, high temperatures could induce the bond rupture of functional groups, increasing the number of active adsorption sites, which may enhance the phosphorus adsorption process.

Figure 6

Effect of temperature on the phosphorus adsorption onto SP (adsorbent dose = 3 g L−1; initial phosphorus concentration = 20 mg L−1; initial pH solution = 7; contact time = 30 min, shaken speed = 400 rpm).

Figure 6

Effect of temperature on the phosphorus adsorption onto SP (adsorbent dose = 3 g L−1; initial phosphorus concentration = 20 mg L−1; initial pH solution = 7; contact time = 30 min, shaken speed = 400 rpm).

Kinetic studies

The two models (pseudo-first order and pseudo-second order) were used to investigate the kinetics of the phosphorus adsorption onto SP. The aim was the understanding of the mechanism and the rate controlling steps affecting the adsorption kinetics. As shown in Figure 7, the pseudo-first order (a) presented low correlation coefficients for all initial phosphorus concentrations compared with that of the pseudo-second order (b). It can be clearly seen that for all initial phosphorus concentrations, the adsorption of phosphorus onto SP fitted well to the pseudo-second order (R2 > 0.99).

Figure 7

Adsorption kinetics for phosphorus onto SP material.

Figure 7

Adsorption kinetics for phosphorus onto SP material.

Adsorption isotherm

The adsorption capacity Qe (mg g−1) of SP material was examined by determining the equilibrium sorption of phosphorus ions as a function of residual phosphorus concentration in the liquid phase. The variation of the adsorption capacity of SP material for phosphorus ions is presented in Figure 8. According to the equilibrium curve, the adsorption capacity increased from 1.983 to 5 mg g−1 when the residual phosphorus concentration increased from 4.05 to 25 mg L−1. It was also observed that the adsorption capacity at equilibrium increased progressively at lower initial phosphorus concentration because of the availability of excess adsorption sites. On the other hand, as the phosphorus concentration increased, the adsorption capacity at equilibrium progressively decreased until reaching saturation. The availability of adsorption sites at high phosphorus concentration becomes the limiting factor as the adsorbent surface reaches maximum adsorption capacity. The results obtained from the equilibrium curve were fitted to Langmuir and Freundlich adsorption isotherms using the least squares fit method as shown in Figures 9 and 10.

Figure 8

Adsorption isotherm of phosphorus sorption onto SP (adsorbent dose = 3 g L−1; initial pH solution = 7; contact time = 30 min; T = 25 ± 2 °C, shaken speed = 400 rpm).

Figure 8

Adsorption isotherm of phosphorus sorption onto SP (adsorbent dose = 3 g L−1; initial pH solution = 7; contact time = 30 min; T = 25 ± 2 °C, shaken speed = 400 rpm).

Figure 9

Langmuir isotherm plot for phosphorus adsorption onto SP (initial pH solution = 7; adsorbent dose = 3 g L−1; contact time = 30 min; T = 25 °C; shaken speed = 400 rpm).

Figure 9

Langmuir isotherm plot for phosphorus adsorption onto SP (initial pH solution = 7; adsorbent dose = 3 g L−1; contact time = 30 min; T = 25 °C; shaken speed = 400 rpm).

Figure 10

Freundlich isotherm plot for phosphorus adsorption onto SP (initial pH solution = 7; adsorbent dose = 3 g L−1; contact time = 30 min; T = 25 °C; shaken speed = 400 rpm).

Figure 10

Freundlich isotherm plot for phosphorus adsorption onto SP (initial pH solution = 7; adsorbent dose = 3 g L−1; contact time = 30 min; T = 25 °C; shaken speed = 400 rpm).

The results of fitting the experimental data to the Langmuir and Freundlich models are presented in Table 2. It was observed that the Langmuir model exhibited clearly a better fit for phosphorus adsorption onto sterile phosphate compared to the Freundlich model. A high correlation coefficient (R2 > 0.98) was observed when using the Langmuir equation model. The adsorption capacity calculated from this latter was found to be 7.962 mg g−1. The experimental data suggested that phosphorus removal occurs on a homogenous surface by monolayer adsorption without interaction between adsorbed molecules. The Langmuir isotherm model expect uniform energies of sorption onto the surface with no transmigration of the adsorbate in the plane of the surface.

Table 2

Langmuir and Freundlich adsorption isotherm constants for phosphorus ions

IsothermLangmuir
Freundlich
Qm (mg g−1)KL(L mg−1)R2nKF(mg g−1)R2
Sterile phosphate 7.962 0.083 0.989 1.947 1.048 0.938 
IsothermLangmuir
Freundlich
Qm (mg g−1)KL(L mg−1)R2nKF(mg g−1)R2
Sterile phosphate 7.962 0.083 0.989 1.947 1.048 0.938 

Table 3 illustrates a comparison made between our novel adsorbent and others (industrial by-products and natural mineral adsorbents) in terms of phosphorus adsorption capacity (mg g−1). The comparison highlighted our novel adsorbent (SP) as a promising and effective material for phosphorus removal from wastewater. It was observed that our novel adsorbent is more than 79 times higher than opoka (Johansson 1999), 46 times higher than dolomite (Prochaska & Zouboulis 2006), 18 times higher than sand (Pant et al. 2001), 7 times higher than Spodosol (Johansson 1999) and 3 times higher than natural zeolite (Wu et al. 2006).

Table 3

Comparison between SP and other mineral adsorbents in terms of adsorption capacity

AdsorbentAdsorption capacity (mg g−1)
Sterile phosphate (this study) 7.96 
Phosphate mine slimes (Jellali et al. 20117.45 
Pyrite calcinate sorbent (Chen et al. 20155.36 
Date palm fibers (Riahi et al. 20094.35 
Wood fiber (Eberhardt et al. 20064.32 
Natural zeolite (Wu et al. 20062.19 
Natural zeolite (Sakadevan & Bavor 19982.15 
Apatite (Bellier et al. 20061.09 
Spodosol (Johansson 19991.00 
Limestone (Drizo et al. 19990.67 
Bauxite (Drizo et al. 19990.61 
Sand (Pant et al. 20010.42 
Dolomite (Prochaska & Zouboulis 20060.17 
Opoka (Johansson 19990.10 
AdsorbentAdsorption capacity (mg g−1)
Sterile phosphate (this study) 7.96 
Phosphate mine slimes (Jellali et al. 20117.45 
Pyrite calcinate sorbent (Chen et al. 20155.36 
Date palm fibers (Riahi et al. 20094.35 
Wood fiber (Eberhardt et al. 20064.32 
Natural zeolite (Wu et al. 20062.19 
Natural zeolite (Sakadevan & Bavor 19982.15 
Apatite (Bellier et al. 20061.09 
Spodosol (Johansson 19991.00 
Limestone (Drizo et al. 19990.67 
Bauxite (Drizo et al. 19990.61 
Sand (Pant et al. 20010.42 
Dolomite (Prochaska & Zouboulis 20060.17 
Opoka (Johansson 19990.10 

CONCLUSIONS

The control of phosphorus in wastewater is an important part of environmental protection from eutrophication. In this study, phosphorus removal from wastewater was carried out by adsorption of phosphorus ions onto sterile phosphate using a batch experiments study. The obtained results indicated that 30 min of contact time between the adsorbent and the wastewater was sufficient to achieve equilibrium. The adsorption process effectiveness increased with increasing initial phosphorus concentrations, adsorbent dose and temperature while it decreased with increasing initial pH values. The maximum phosphorus removal efficiency (94.4%) was achieved in slightly acidic conditions, with an adsorbent dose and initial phosphorus concentration of 3 g L−1 and 20 mg L−1, respectively, and at room temperature. The phosphorus adsorption onto the novel calcic adsorbent followed the pseudo-second order kinetic model. The equilibrium data fitted well to the Langmuir isotherm model, indicating monolayer coverage of the adsorbent without interaction between adsorbed molecules. The adsorption capacity determined from the Langmuir model equation was found to be 7.962 mg g−1. According to the experimental data, sterile phosphate was found to be the most efficient adsorbent for phosphorus removal from wastewater compared to some natural and industrial by-products reported in the literature. This may open a novel way of revaluing SP material, as a mining by-product, cogenerated in considerable quantities by the phosphate mining plants.

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