Review: Low Cost, Environmentally Friendly Humic Acid Coated Magnetite Nanoparticles (HA-MNP) and Its Application for the Remediation of Phosphate from Aqueous Media

Phosphate is a primary nutrient required for the normal functioning of many organisms in the ecosystem. However, presence of excess phosphate into the aquatic systems leads to eutrophication which can promote harmful algal growth and decrease the amount of dissolved oxygen in water. Municipal, industrial and agricultural run-off wastewaters are the major point sources for phosphate discharges. There are different methods to remove phosphates from water. Among these, adsorption is the most widely accepted method for phosphate removal because of its high efficiency, minimum cost, easy and simple operation and applicability at lower concentrations. The emphasis of this review, is to consolidate low cost, environmentally friendly humic acid coated magnetite nanoparticles (HA-MNP) and its application for the remediation of phosphate from aqueous media. The magnetic nanoparticles could be easily separated from the reaction mixture by using a simple hand held magnet and adsorption studies demonstrate the fast and effective separation of phosphate with maximum removal efficiency > 90% at pH 6.6. The adsorption behavior follows the Freundlich isotherm and the removal of phosphate is found higher at acidic and neutral pH compared to basic conditions. The nanoparticles exhibit good selectivity and adsorption efficiency for phosphate in the presence of co-existing ions such as Cl, 4 SO − and 3 NO − with some inhibition effect by 2 3 CO − and finally, the effect of temperature on the adsorption reveals that the process is endothermic and spontaneous.


Introduction
Phosphorus is the mineral nutrient which is essential for all living species. It is one of the more understood nonrenewable available nutrients for fertilizers production [1]. Phosphate is a primary nutrient required for the normal functioning of many organisms in the ecosystem. However, the presence of trace amounts of phosphate (even below 1 mg/L) in treated wastewater from municipalities and industries is often accountable for eutrophication which leads to short-and long-term environmental and esthetic problems in lakes, ponds, reservoirs, coastal areas, and other confined water bodies [2]. Nuisance algal blooms in eutrophic water bodies cause great negative consequences for ecosystem functioning and ecosystem services. In fact, eutrophication in relatively static water bodies has become a worldwide water quality issue. Among essential nutrients, phosphorus (P) is widely recognized to regulate the algal blooms and to protect against nuisance in which average phosphorus concentration should not surpass 0.02 mg/L [3] [4]. Generally, the excessive presence of phosphorus in water bodies, which is mainly considered as a result of untreated sewage effluent and agricultural run-off, causes eutrophication problem which can promote harmful algal growth and decrease the amount of dissolved oxygen in rivers, lakes and seas. Eutrophication induces overgrowth of phytoplankton, thus deteriorating water quality, depopulating aquatic species and accelerating water scarcity. According to the Australian Water Quality Guidelines for Fresh and Marine Waters, the total phosphorus contaminant level in rivers and streams is controlled in the range of 0.010 -0.100 mg/L; whilst the requirement for lakes and reservoirs is more stringent as 0.005 -0.050 mg/L [2] [3] [5] [6].
Nowadays eutrophication has already become a globalized environmental problem in relatively stagnant water bodies and can cause comprehensive ecological crisis of aquatic environment, such as the decrease of biological diversity, the loss of landscape function and the potential health risk to human beings [7] [8]. Phosphorus is well recognized as the limiting factor for eutrophication.
Thus, the removal of phosphate, the main species of phosphorus, from wastewater before discharge is one of the most effective ways for eutrophication control [9]. Adsorption is a promising technology for the removal of phosphate. Therefore, in recent years, investigations on adsorbents for immobilizing phosphate from water have been undertaken. As a consequence, phosphate removal, from wastewater has been considered as an important environmental sustainability concern [2]- [9].

1) Biological, Chemical, Physical and Adsorption Methods
Phosphate removal from water has attracted considerable research interest in the last few decades. Until now, a range of methods have been developed, mainly including biological chemical and physical treatments [10] [11] [12] [13] [14].
Biological methods, i.e. the conventional activated sludge process, can achieve T. Damena which is ascribed to the fact that the presence of insufficient phosphate lowers metabolism of microorganisms. Furthermore, specific care and strict control are often needed during implementation of biological methods [10] [11].
Chemical treatments using lime, alum, and iron salts have been mostly utilized in phosphate removal, which suffer difficulty in sludge disposal and effluent neutralization [10] [12]. Physical methods, as in the case of reverse osmosis and electro dialysis, have been proven to be either too expensive or inefficient in removal [13] [14]. In comparison to the aforementioned methods, the adsorption process is promising for phosphate removal attributed to its attractive advantages, which is simple operation, high removal efficiency and fast adsorption rate, especially at low phosphate concentration. In general adsorption is one of the most attractive approaches in water treatment, with the advantage of having high removal efficiency without yielding harmful bi-products [4]- [9].
It is well known that phosphates have a strong affinity for mineral surfaces. Its affinity for hydroxide surfaces depends on one hand on the anions' complexing capacity, which allows binding to surface groups by ligand exchange reactions, and on the other hand on electrostatic interactions with the charged hydroxide surfaces. The total annual economic cost for the process incorporated with chemical precipitation was approximately 10% lower than that with adsorption.
However, the adsorption process could reduce phosphate concentration in the discharge to a much lower level, as compared with the chemical method [13] [14] [15]. The development of materials with superior adsorption capacity could further enhance phosphate removal, minimize sludge disposal and eventually make adsorption cost-effective for practical application [16]. Furthermore, the adsorption is able to be used for not only phosphate removal, but also phosphate recovery. In recent years, there has been tremendous interest focusing on the fabrication of mesoporous materials and their potential in different practical ap- for effective removal of dyes, heavy metals and foulants using carbon nanotube membranes, electrospun nanofibers and hybrid nano-membranes. Finally, the integration of nanotechnology with biological processes such as algal membrane bioreactor, anaerobic digestion microbial fuel cell and bio-chemically active materials found in soil such as humic acid were developed with respect to its potential for wastewater purification [33].
In recent years, iron based nanoparticles have been widely applied for environmental remediation as mentioned above. The strong magnetic property of such nanoparticles enables separation of adsorbent and adsorbate by using a simple magnet. Magnetite, an iron oxide (Fe 3 O 4 ) material shows the highest magnetism among all the naturally available minerals [34]. Application of bare magnetite nanoparticles (MNP) for the removal of toxic water contaminants have been reported in the literature (Table 1). However, the susceptibility to auto-oxidation, tendency to agglomerate and concerns over toxicity are the main challenges for the real life water treatment applications of bare MNP [32] [35].
The coating of natural organic matter (NOM) on the bare MNP surface has been 12 Co-precipitation ---As(V) [72] 13 Commercially available magnetite-maghemite mixture nanoparticles --65 As(V), As(III), Cr(VI) [73] 14 Commercially available Magnetite --99 As(V), As(III) [74] 15 Modification of co-precipitation method Starch Hydroxyl group 91 As(V) [75] found useful in making the nanoparticles less toxic and more environmental friendly. Such thin coatings can also inhibit the agglomeration and auto-oxidation, which are the primary drawbacks associated with the use of bare MNP. In addition coating with NOM can potentially increase the adsorption capacity and selectivity of the nanoparticles [36].

5) Humic Acid (HA)
Humic acid (HA), a ubiquitous natural organic matter (NOM), is derived from plants, lignite, coal, soil, leonardite and microbial residues ( Figure 2). HA possesses a number of organic functional groups including carboxylic acids, carbonyl groups and phenolic hydroxyl groups which can promote its complexation with a variety of metal oxides [37] [38] [39] [40]. HA has a high affinity for magnetite (Fe 3 O 4 ) and effectively coats bare MNP most likely through the surface complexation ligand exchange reactions. Some studies have been reported on the removal of water contaminants using HA-MNP with primary focus on the removal of metal cations [41] [42].
Recent research activities demonstrated that the potential use of HA-MNP to successfully remove metal oxyanions (chromate), metal ions such as, Cu(II), Cd(II), Pb(II), Hg(II) and Rhodamine B from the aqueous media [33] [43] [44] [45] [61]. Humic acid coated magnetite nanoparticle (HA-MNP) can have tremendous application (Table 1). Among these it is used to remove heavy metals from soil and west water in different contact time and dosage of adsorbate. This seminar report has reviewed the uses of adsorption to remove phosphate from the aqueous solutions using humic acid coated magnetic iron oxide (magnetite) nanoparticles as adsorbent.

Synthesis of Bare Fe3O4 and Humic Acid Coated Magnetite Nanoparticles (HA-MNP)
For different applications, several chemical methods can be used to synthesize magnetic nanoparticles: co-precipitation, reverse micelles and micro-emulsion technology, sol-gel syntheses, sonochemical reactions, hydrothermal reactions, hydrolysis and thermolysis of precursors, flow injection syntheses, and electrospray syntheses [46]. The synthesis of superparamagnetic nanoparticles is a complex process because of their colloidal nature. For metal removal applications, an adequate surface modification of the nanoparticles is a critical aspect regarding both selectivity and aqueous stability of these materials.

Characterization of the Magnetite Nanoparticles and Coated Nanoparticles
The TEM images of HA-MNP showed high crystallinity of magnetite nanoparticles inside (Fe 3 O 4 core) and disordered structure outside which is due to the coating of humic acid on the surface of iron oxide nanoparticles [45].   [42]. From these it can be concluding that approximately similar spectra were recorded within the different study procedure. The magnetite is an amphoteric solid, which can develop charges in the protonation and deprotonation reaction of Fe-OH sites on surface. This process is controlled by the pH and ion strength in aqueous medium. At pH lower than the PZC (the point of zero charge) (pH < 7.9) the surface charge is positive, electrostatic interaction between HA and Fe 3

Adsorption Experiments
Stock solution of phosphate (P) with concentration of 1000 mg of P/L (ppm) was where Q e is the amount of phosphate adsorbed (mg/g) at equilibrium, C o and C e corresponds to the initial and equilibrium concentration of phosphate in solution respectively, expressed in mg/L; m is the mass (g) of the adsorbent (HA-MNP) and V is the total volume (L) of the solution. Reproducibility of the collected data was ensured by taking the average of triplicate run of the experiments [45].

T. Damena, T. Alansi Journal of Encapsulation and Adsorption Sciences
The phosphate removal efficiency or phosphate removal percentage (η) was calculated through the following equation:

Adsorption Kinetics
The adsorption of phosphate on HA-MNP surface increased with time and dosage. As different studies indicated adsorption equilibrium reached within a given minute with >90% of phosphate removed from the solution in 3 h for an initial phosphate concentration of 2 ppm [46]. Table 2 shows maximum adsorption ability of different adsorbent. Among different adsorbent humic acid coated magnetite nanoparticles economical, simple and ease of separation of adsorbent and adsorbate. Surface saturation occurs at higher initial phosphate concentration and the adsorption rate became slower. The kinetic data could be fit to both pseudo-first order and pseudo-second order kinetic models (Equation (5) and Equation (6) respectively) and compared, to provide insights of adsorption mechanisms such as mass transfer and chemical reaction. It is well established that the pseudo-first order kinetic model fits better in the initial stage of reaction processes especially those with rapid adsorption, whereas the pseudo-second order model considers adsorption behavior over longer contact times with chemisorption as the rate controlling process [ [62]. In the typical pseudo-second order process, an initial surface reaction occurs until all the surface sites are occupied; subsequently diffusion and molecular reorganization can take place at the surface for additional complexation. In addition the pseudo-first order kinetic model also exhibits a good correlation after fitting the experimental data, however, significant deviation was observed between the two Q e values indicating that this model is not consistent with the observed phosphate adsorption [45] [56]. To further explore the adsorption process, the kinetic data were also examined using the Weber and Morris intraparticle diffusion model that expresses the fraction of solute adsorbed as a function of square root of time [45].
where Q t is the amount of phosphate adsorbed at time t (minute), k id is the intraparticle diffusion rate constant and C is the intercept. A plot of Q t vs t yielded a linear relationship which did not pass through the origin. The results suggest that intraparticle diffusion is involved in the overall adsorption process although it is not the rate limiting step. Additionally, the observed multilinearity can be attributed to the involvement of two or more steps in the adsorption process [1] [34] [43] [56]. The external adsorption process is assumed to be the fastest and instantaneous where a significant concentration of initial phosphate was adsorbed in the HA-MNP surface within 10 min. The second stage of adsorption was controlled by intraparticle diffusion mechanism over a period of approximately 60 min. The final stage is the equilibrium adsorption where intraparticle diffusion no longer dominates due to low phosphate concentration and the process approaches the equilibrium [

Adsorption Isotherms
The adsorption isotherm describes the adsorbent-adsorbate relationship at equilibrium critical for determining optimal parameters for the application of the adsorbent. A range of concentrations of phosphate were mixed with a fixed amount of HA-MNP (1 g/L). The experiment was carried out for 3 h in the orbit shaker at temperature 298 K, pH 6.6 and a mixing speed of 250 rpm. The concentrations of phosphate obtained were plotted as adsorption isotherms and fit to Langmuir and Freundlich models to determine the equilibrium adsorption and the maximum adsorption capacity. The Langmuir isotherm assumes that monolayer adsorption occurs on homogeneous adsorbent surface and there is no interaction between the adsorbate molecules [76]. In the case of magnetite the  (8) where, C e is the concentration of phosphate (mg/L) in solution at equilibrium, Q e is the amount of phosphate adsorbed (mg/g) on the adsorbent surface at equilibrium, Q max is the maximum adsorption capacity (mg/g) and b is the The theoretical maximum adsorption capacity for phosphate was calculated to be 28.9 mg/g (P) which is comparable to other similar types of adsorbents used for phosphate removal which listed Table 1 at a given temperature and pH. The authors also showed that the Freundlich isotherm model explains (R 2 > 0.93) the adsorption process better than the Langmuir isotherm (R 2 = 0.80) and thus indicative of the formation of multilayer adsorption and heterogeneous surface sites. The value of 1/n was found to be less than unity (0.48) which corresponds to an adsorption mechanism involving chemisorption, supported by kinetic study [45].

Effect of pH
Solution pH can have a pronounced effect on adsorption since the adsorbent surface charge strongly influence the adsorption of charged phosphate species.
The leaching of humic acid from the HA-MNP surface is also affected by solution pH. With this in mind, the adsorption of phosphate was studied as a function of initial solution pH from acidic to alkaline range [34] [45] [59]. The amount of phosphate removal decreased with the increase of pH values, Figure   6. The lower adsorption at basic pH can be explained by considering the phosphate speciation in aqueous medium. Phosphate is polyacidic (pK 1 = 2.12, pK 2 = 7.21, and pK 3 =12.67) prevail as H 3 PO 4 , 2 4 H PO − ,  lower pH, sorbent surface will be positively charged while the phosphate species will be predominantly monoanionic which is 2 4 H PO − [45] [60]. Thus electrostatic attraction between 2 4 H PO − and HA-MNP-H + results in higher adsorption of 3 4 PO − on HA-MNP. As the pH increases, more hydroxyl ion exists in the solution which might compete with phosphate species for the sorbent site.
Another possibility is that, at higher pH, the sorbent surface and the 3 4 PO − species becomes more negative which introduces greater repulsion and as a consequence, the adsorption of 3 4 PO − decreases [1] [34] [45] [52]. This implies that by using a strongly basic solution, it was expected that most of the adsorbed phosphate would be desorbed because of the effect of pH on adsorption [1] [34]. The amount of phosphate desorbed was calculated by the following equation: The desorption efficiency was calculated by the following equation: where C d is the concentration of phosphate in supernatant after desorption (mgP/L); V is the volume of NaOH solution added as desorption reagent (L); Q e is the amount of phosphate adsorbed (mg/g) (Figure 7).

Effect of Temperature
Thermodynamic study of phosphate adsorption was conducted by measuring the adsorption at different temperatures) with an initial concentration of phos- where R is the gas constant, K C is the equilibrium constant and T is the temperature in K. The K C value is calculated from Equation (10):  (Figure 8 & Figure 9).

Effect of Coexisting Ions
In groundwater and wastewater, anions such as sulfate, nitrate chloride and carbonate often coexist along with 3 4 PO − , so the effect of coexisting ions was also studied by separately adding 1 mM of each of the anions into the reaction mixtures containing affixed amount of 3 4 PO − and HA-MNP. Different authors indicated that after the reaction was continued for more than an hour, except   CO − ) made the overall phosphate solution alkaline by changing the pH from the initial 6.6 to 10.1. This higher pH might be the reason for the decrease of phosphate uptake by HA-MNP as observed the similar trend in pH effect study section [1] [9] [34] [41] [45] [60]. Furthermore, increasing both NaOH concentration and temperature resulted in an enhancement of desorption efficiency.
Thus NaOH solution could be used to desorb phosphate adsorbed on the material for reuse, by adopting a high NaOH concentration and/or a high temperature [9] [60].

Conclusion
Effective removal of phosphate from water is critical to counteract eutrophication and restore water quality. Eutrophication has become a worldwide environmental problem and removing phosphate from water/wastewater by adsorption before discharge is essential. This review, focused on an environmentally friendly magnetic adsorbent has been applied to effectively separate phosphate present in the aqueous media and focused on humic acid coated magnetite nanoparticle (HA-MNP). The materials were characterized with X-ray diffraction, transmission electron microscope; vibrating sample magnetometer and Fourier transform infrared spectra. The FT-IR spectra reveal that HA has been successfully coated onto surface of SO − ) and temperature on phosphate adsorption were also investigated, which showed that the adsorbent had good selectivity for phosphate.
The increase of temperature also exerts a positive influence on phosphate adsorption efficiency and investigations of adsorption kinetics and adsorption isotherm suggest that the adsorption mainly occurs through chemisorption and thus indicative of strong bonding between phosphate and the adsorbent nanoparticles. Thermodynamic study identified the removal process as endothermic and spontaneous.