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Recovery of Ammonium and Phosphate from Wastewater by Wheat Straw-based Amphoteric Adsorbent and Reusing as a Multifunctional Slow-Release Compound Fertilizer Xinggang Wang, Shaoyu Lü, chunmei gao, Chen Feng, Xiubin Xu, xiao bai, Nannan Gao, Jinlong Yang, Mingzhu Liu, and Lan wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01494 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 5, 2016

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ACS Sustainable Chemistry & Engineering

Recovery of Ammonium and Phosphate from Wastewater by Wheat Straw-based Amphoteric Adsorbent and Reusing as a Multifunctional Slow-Release Compound Fertilizer Xinggang Wang a, b, Shaoyu Lü a, *, Chunmei Gao a, Chen Feng a, Xiubin Xu a, Xiao Bai a, Nannan Gao a, Jinlong Yang a, Mingzhu Liu a, *, and Lan Wu c a

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal

Chemistry and Resources Utilization of Gansu Province and Department of Chemistry, Lanzhou University, 222 Tianshui South Road, Lanzhou 730000, People’s Republic of China b

Research Institute of Lanzhou Petrochemical Corporation, Petrochina Lanzhou

Petrochemical company, 17 Qinshui Road, Lanzhou 730060, People’s Republic of China c

College of Chemical Engineering, Northwest University for Nationalities, 1 Northwest

Village, Lanzhou 730030, People’s Republic of China

KEYWORDS: Biomass; Wheat Straw; Amphoteric straw cellulose; Adsorption; Multifunctional slow-release compound fertilizer

*S. Lü. Tel.: +86-931-8912387. FAX: +86-931-8912582. E-mail: [email protected] *M. Liu. Tel.: +86-931-8912387. FAX: +86-931-8912582. E-mail: [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT

In order to minimize the negative impact of nitrogen and phosphorus pollution from wastewater and improve fertilizer use efficiency, a novel multifunctional slow-release compound fertilizer was prepared by recovery of NH4+ and H2PO4− from aqueous solutions onto amphoteric straw cellulose (ASC) adsorbent. The effects of adsorbent dosage, solution pH, contact time and ionic strength on adsorption were investigated. The adsorption quickly reached equilibrium. The maximum NH4+ adsorption capacity of ASC was 68.4 mg g-1 at around pH 7.0, while it was 38.6 mg g-1 for H2PO4− adsorption at pH 5.0. Moreover, the feasibility of reusing the nutrients-laden carrier material as a multifunctional slow-release compound fertilizer was determined. The study demonstrated that the product with excellent slow-release and water-retention properties. Thus it could improve soil moisture content and reduce soil moisture evaporation rate and was economical and environmentally-friendly for application in horticulture and agriculture.

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INTRODUCTION Over the past century, the development of industrialization and agriculture has resulted in the degradation of ecological environment, which has a negative influence on the survival and development for the mankind. In aquatic ecosystems, water enrichment of nitrogen (N) and phosphorus (P) is the most universal environmental pollution problem in China 1. In terms of river and lake quality, these nutrients cause diverse problems such as loss of oxygen, toxic algal blooms, fish kill and impairs the use of water for drinking

2,3

. The loss of the

agricultural N and P is the main reason for their over enrichment, therefore, it is desirable that these nutrients should be controlled released into the soil, and removed from the natural water 4

. Various methods have been applied for removing both NH4+ and H2PO4−, including

chemical precipitation 5, supercritical water oxidation 6, reverse osmosis 7, electrochemical and biological treatments 9, coagulation-flocculation

10

and adsorption

8

11

. Among these

approaches, adsorption is considered as one of the most reliable and effective techniques for removing N and P contaminants from wastewater due to its high efficiency, easy operation, low-cost and avoidance of chemical sludge. Therefore, an effective way to alleviate the eutrophication is to develop an amphoteric adsorbent to recover phosphate and ammonium from aqueous solutions by adsorption.

Biosorption is a relatively new technique that has proven to be very promising in the removal of contaminants of NH4+ and H2PO4− from wastewater12-14. Recently, biomass materials have been applied to prepare adsorbents for wastewater treatment, especially 3



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agricultural

byproducts,

since

it

is

abundant

in

resources,

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economical

and

environmentally-friendly 15-18. Wheat straw is a low-cost, abundantly available and renewable biological resource, which has been widely used to remove contaminants from industrial sewage and sanitary wastewater

19-21

. Wheat straw mainly consists of lignin, hemicelluloses,

and cellulose. These components contain multiple functional groups, e.g., carboxyl, hydroxyl, ether, amino and phosphate

22

. Because of the existence of these groups, wheat straw has a

great advantage in the preparation of adsorbent for wastewater treatment

23-25

. Furthermore,

the recycling of crop residues could provide organic matter and a large number of elements 26 as fertilizer to improve soil fertility 27.

However, there are several problems brought by the direct application of wheat straw as adsorbents, such as low adsorption efficiency and adsorption capacity for chemical contaminants, the release of water soluble secondary pollutants contained in the raw materials 28

. Therefore, it is necessary that wheat straw is pretreated

applied for adsorption

29

and modified before being

30

. Many methods have been developed to modify wheat straw for

NH4+ and H2PO4− removal

31,32

. However, only a few researchers have investigated the

preparation of amphoteric adsorbents from wheat straw for recovery of NH4+ and H2PO4− from wastewater and then reused them as an environmentally friendly fertilizer for nutrients slow release.

In this study, a wheat straw based amphoteric adsorbent was developed and the adsorption properties for NH4+ and H2PO4− were studied. In addition, the nutrients release characteristics of the adsorbed materials were assessed. The product was expected to minimize N and P

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pollution and improve fertilizer use efficiency, and enhance soil moisture retention capacity. Therefore, the aims of this study were: (a) to study the effect of the adsorbent dosage, ionic strength and solution pH values on the adsorption capacity of amphoteric straw cellulose (ASC) for NH4+ and H2PO4−; (b) to determine the adsorption kinetics, equilibrium isotherms and mechanism of the adsorption; (c) to investigate the potential of using the straw cellulose based adsorbents as the carrier for nutrients slow release.

MATERIALS AND METHODS

Materials. Wheat straw (WS) was obtained from Baiyin city Gansu of western china, which was collected between august and April 2014. It was chopped and then washed several times with distilled water to remove soluble impurities and surface dirty stuff and dried in an oven at 100 oC for 48 h and sieved to retain the 0.135-0.170 mm (90-110 meshes) fraction. Chloroacetic acid (CA, analytical grade, Tianjin Chemical Reagent Factory, Tianjin, China), 3-chloro-2-hydroxypropyl trimethylammonium chloride (CTA, analytical grade, Aladdin Reagent Co., Ltd., Shanghai, China) and hydrogen peroxide solution (H2O2, analytical grade, Chengdu Aikeshiji Reagent Co., Ltd., Chengdu, China) were used as received. Sodium chloride (NaCl), sodium hydrate (NaOH), hydrochloric acid (HCl), urea, ethanol and other chemical reagents used in this work were all analytical reagents and were used without further purification.

Extraction of cellulose from wheat straw. Wheat straw was cut into small pieces (about 5 mm in length) and washed several times with distilled water. Then it was subjected to water extraction at 90 oC for 6 h. In addition, it was delignified with 1.3% sodium chlorite to 5



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remove the water-soluble free samples at pH 3.5-5.0, and the solution pH values were adjusted with 10% acetic acid, at 80 oC for 2 h. After this process, the intermediate product was treated with 10 wt% NaOH at 75 oC for 3 h to remove lignin with residual pectin. Finally, the mixture solution of 2% (v/v) H2O2 and 2% (w/v) sodium hydroxide was employed to extract holocellulose, at pH 10, for 24 h, under stirring at room temperature, in a 1:25 solid to liquor ratio (g mL-1). After filtration, straw cellulose was successively treated with 95% ethanol and distilled water until neutral and then dried in an oven at 70 oC for 12 h. The overall yield of cellulose departing from wheat straw was 22 wt%. The purified straw cellulose sample is a white powder.

Preparation of the ASC. A typical procedure for preparing ASC was performed as follows: 1.0 g straw cellulose extracted from wheat straw was initially added into a three-necked flask containing of 25 g NaOH aqueous solution (14 wt %) precooled to -12 oC with vigorous stirring. Then, the same amount of 24 wt% urea solution precooled to -12 oC was poured into the mixture solution with vigorous stirring for 20 min at -12 oC. After mixing, the obtained transparent cellulose solution was heated to 70 oC. A certain amount of the CTA aqueous solution (60 wt%) was added drop-wisely into the above mixed solution, and the mixture was kept stirring for 2 h. CA (6.69 g, 9.2 mmol) was continuously added into the previous mixed solution and reacted for another 1 h at 70 oC. After the reaction completed, the reaction liquid was neutralized with concentration of 10 wt% CH3COOH solution. The obtained precipitate was filtered and then washed several times with 80 wt% ethanol solution to neutral, and the resultant product was dried in a vacuum oven at 60 oC for 48 h.

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Characterization. The Fourier transform infrared (FTIR) spectra of wheat straw cellulose, cationic straw cellulose, anionic straw cellulose and amphoteric straw cellulose were obtained using a NEXUS 670 FTIR Spectrophotometer (USA) with a KBr pellet in the range of 400-4000 cm-1. Scan electron microscopy (SEM, JSM-5600LV, JEOL, Ltd.) was used to examine the surface morphology of the solid samples. Before observation, the samples were fixed on aluminum stubs and coated with a thin layer of palladium gold alloy. The concentration of NH4+ was determined by nessler’s reagent colorimetric method and the concentration of H2PO4− was determined by UV spectrometer at 440 nm 33. Each sample was evaluated in triplicate. The nitrogen content of solid samples was determined using an elemental analysis instrument (Germany Elemental Vario EL Corp., Model 1106) and the total exchange capacity (TEC) was calculated by the nitrogen content of samples solid 32. The phosphate content of slow release fertilizer was digested with HNO3-HClO4 and analyzed based on the method of reduction of molybdophosphate (GB5413.22-2010, China).

Batch adsorption investigates. Batch adsorption experiments were carried out to obtain adsorption kinetics rate and equilibrium isotherm of NH4+ and H2PO4− adsorption onto ASC adsorbent. The effects of adsorbent dosage, solution pH, ionic strength, and adsorbate ion concentration on the adsorption performance of the ASC adsorbent were measured. The effect of solution pH on NH4+ and H2PO4− adsorption capacity was examined with pH ranging from 2 to 12, adjusting by 0.1 mol L-1 HCl or NaOH solutions. The initial concentrations of NH4+ and H2PO4− were both 50 mg L-1. 0.1 g ASC was added into 50 mL adsorbate ion solution with various pH values and then shaken at 25 oC for 1 h.

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In order to evaluate the effects of solution ionic strength for NH4+ and H2PO4− adsorption, background electrolytes used in these experiments were 0～0.2 mol L-1 NaCl, CaCl2 or Na2SO4. After that, the suspension of aqueous solution was filtered and the resulting solution was taken for analyzing the residual adsorbate ions concentration. Adsorption kinetics of NH4+ and H2PO4− onto ASC were measured by mixing 0.1 g of the ASC adsorbent in 50 mL of the adsorbate ions solution (50 mg L-1 for NH4+ and 25 mg L-1 for H2PO4−) at room temperature (25 oC). The mixed solution was shaken in an incubator shaker (HNY-2102C) at 150 rpm under room temperature. At appropriate time intervals, the suspension was collected by filtration and then analyzed quantitatively, respectively. For the adsorption isotherm studies, the initial NH4+ and H2PO4− concentrations were ranged from 50 to 200 mg L-1 with a contact time of 2 h. The samples were filtrated and the filtrate solution samples were analyzed for residual adsorbate ion concentrations. The adsorption capacity of NH4+ and H2PO4− adsorbed by ASC at equilibrium (qe) and percentage removal efficiency (Removal%) were determined through the following equation: qe =

(C0 − Ce )V m

Removal % =

(1)

(C0 − Ce ) × 100% Ce

(2)

where C0 and Ce (mg L-1) are the initial and equilibrium concentration of NH4+ or H2PO4− in solution, respectively. V is the volume of the adsorbate ions solution (mL), and m is the dry weight of amphoteric adsorbent (g). The amount of NH4+ or H2PO4− adsorbed at time t, qt (mg g-1) was calculated based on the equation as follow:

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qt =

(C0 − Ct )V m

(3)

where Ct is the NH4+ or H2PO4− concentration in solution at time t (min). For co-adsorption of NH4+ and H2PO4− in binary systems, NH4+ and H2PO4− mixture solutions (50 mL) at certain concentrations were mixed with 0.1 g of SAC. The co-adsorption experiments contain three parts: (i) Effect of H2PO4− on the adsorption of NH4+, the initial adsorbate ion concentration of NH4+ is fixed to 100 mg L-1, whereas the H2PO4- concentration is ranged from 0 to 200 mg L-1. (ii) Effect of NH4+on the adsorption of H2PO4−, the initial adsorbate ion concentration of H2PO4− is constant at 50 mg L-1, and the NH4+ concentration is ranged from 0 to 100 mg L-1. (iii) The competitive adsorption between NH4+ and H2PO4− under the fixed total concentration at 120 mg L-1 in the same experimental condition. Each type of adsorbate ions concentration varied.

Synthesis of multifunctional slow-release compound fertilizer. The multifunctional slow-release compound fertilizer (MSCF) was prepared as following: 0.2 g of ASC particles with diameter of 0.090-0.110 mm were filled into a flask with 300 mL of 200 mg L-1 NH4+ and H2PO4− aqueous solutions at 25 oC for 2 h, and then filtrated and dried at 40 oC until to a constant weight. The dried adsorbed nutrients ASC particles with a nitrogen content of 7.81% and phosphorus content of 13.06 mg g-1 were used to investigate the nutrients release property in soil.

Slow-release behavior of MSCF in soil. In order to investigate the release behavior of NH4+ and H2PO4− from the MSCF in soil, the soil testing experiment was performed. 0.2 g MSCF was encapsulated into a nonwoven plastic mesh bag and buried 6 cm above the 9



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bottom of soil and packed in a ceramic beaker filled with 200 g of dry sandy loam soil (below 26-mesh) under room temperature. Throughout the soil testing experiment, the moisture content of the soil samples was maintained at 30 wt%, and the water-holding ratio of soil was weighed and added a certain amount of tap water if necessary, periodically. After 1, 3, 5, 7, 10, 15, 20 and 30 days, the mesh bags were picked out and dried at room temperature in a plastic dish and then taken out to determine the nutrient contents of nitrogen (N) and phosphorus (P2O5). Eight fertilizer mesh packages were simultaneously evaluated at the same environmental conditions. The total and remaining content of nitrogen (N) and phosphorus (P2O5) were evaluated by nessler’s reagent colorimetric method and the method of the vanadium molybdate colorimetry, respectively.

Evaluation of the water-retention capacity of soil with MSCF. The sandy loam soils were used for the testing experiment which were collected from the cultivated field at western of China. The soil samples mixed with different application rates of MSCF (0%, 0.5%, 1.0% and 2.0%) were evaluated for the water-retention ratio. The samples were divided into four different groups: 200 g of dry soil only (control); 1.0 g of MSCF mixed with 200 g of dry soil; 2.0 g MSCF mixed with 200 g of dry soil; and 4.0 g MSCF mixed with 200 g of dry soil. PVC tubes with diameter of 4.5 cm were used to fill with the dry soil samples, and their bottom was encapsulated with three layers of nylon fabric (200-mesh). The tubes were weighed and marked W0. Tap water was slowly drenched from the top of the PVC tube until water seeped out. After that there was no water seeping out from the soil samples, the PVC tube was weighed again and marked W1. The tubes were simultaneously kept at same environment conditions, and weighed every day (Wi). The measurements were continuous 10


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monitored for a period of 30 days. The water-retention ratio (WR%) of the soil samples were calculated from the equation as follows:

 W − W0  WR(%) =  i  ×100%  W1 − W0 

(4)

RESULTS AND DISCUSSIONS

Characterization. Fig. 1 shows the FTIR spectra of wheat straw cellulose, a cationic straw cellulose (CTA-grafted), anionic straw cellulose (carboxymethylated) and amphoteric straw cellulose. For wheat straw cellulose (Fig. 1a), the characteristic adsorption peaks at around 3437, 2908 and 1031 cm-1 are attributed to O-H stretching vibration of hydroxyl, C-H asymmetrical stretching vibrations of methylene, and C-O-C bending vibrations of glucose ring, respectively. Those were appeared in all the spectrum of straw cellulose and its derivatives. For cationic straw cellulose (Fig. 1b), the presence of the quaternary ammonium groups is indicated by the peak at 1470 cm-1. For anionic straw cellulose (Fig. 1c), the presence of –COO– groups is indicated by the peaks at 1607 cm-1. For amphoteric straw cellulose (Fig. 1d), the characteristic absorption peaks of quaternary ammonium and carboxyl groups are both appeared in the spectrum of ASC. The above results indicated that the quaternary ammonium and carboxyl groups are successively grafted on the wheat straw cellulose.

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Figure 1. FTIR spectra of wheat straw cellulose (a), cationic straw cellulose (b), anionic straw cellulose (c), amphoteric straw cellulose (d).

Elemental changes and total exchange capacity (TEC) of unmodified and modified wheat straw cellulose are displayed in Table 1. Compared with wheat straw cellulose, the carbon and hydrogen contents of modified wheat straw cellulose were slight decreased due to some organic monomers that were grafted on the cellulose skeleton. However, the nitrogen content of ASC was significantly increased from 0 to 0.84%. The results indicated that considerable number of quaternary ammonium groups have been grafted onto wheat straw cellulose and the chemical modification performed efficiently. The TEC values of unmodified and modified wheat straw are estimated from the nitrogen contents of samples, as shown in Table 1. The TEC values of ASC are significantly higher than that of unmodified wheat straw cellulose, which indicated ASC has excellent adsorption capacity.

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Table 1. Elemental changes and total exchange capacity (TEC) of unmodified and modified wheat straw cellulose. Sample

N (%)

C (%)

H (%)

TEC (mEq g-1)

Wheat straw cellulose

0

42.68

5.74

0

Amphoteric straw cellulose

0.84

39.43

5.61

0.6

The surface morphology of the wheat straw, pretreated wheat straw, ASC and ASC after adsorption is determined by SEM, as shown in Fig. 2. Compared with wheat straw (Fig. 2A), the surface of extracted wheat straw cellulose (Fig. 2B) became smooth and the impurities on the surface have been removed after pretreatment. The surface of amphoteric wheat straw cellulose (Fig. 2C) became roughness after the grafting reaction with cationic and anionic functional groups. The results indicated that the functional groups were grafted on the skeleton of wheat straw cellulose after modification. The amphoteric chemical modification resulted in a porous structure and roughness surface, which increased the surface area of the adsorbent. Porous and coarse structure was also observed for the ASC adsorbent after adsorption (Fig. 2D), which would increase the surface area of amphoteric adsorbent and facilitate for water molecule permeation. When aqueous solution diffused into the internal pores of the ASC adsorbent, adsorbate ions of NH4+ and H2PO4− can be easily captured by active adsorption sites within the amphoteric adsorbents.

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Figure 2. SEM images of the wheat straw (A), pretreatment wheat straw (B), amphoteric straw cellulose (C) and amphoteric adsorbent after adsorption (D).

Effect of ASC dosage on adsorption. Fig. 3 shows the effect of the adsorbent dosage on adsorption capacity and removal efficiency of NH4+ (a) and H2PO4− (b) onto ASC adsorbent in solution. As shown in the figure, with the increase in amphoteric adsorbent usages, the adsorption capacity for NH4+ and H2PO4− decreased. However, the removal efficiency of NH4+ increased significantly from 29.9% to 96.1%, and that of H2PO4− increased from 14.0% to 84.7% when the amount of adsorbent was increased from 0.2 to 3 g/L. This is because that with increasing amphoteric adsorbent dosage more adsorption sites is available for NH4+ and H2PO4− adsorption. However, the adsorption capacity of ASC decreased with the increase in adsorbent dosage of amphoteric. This may be due to the concentration of adsorbate ions solution decreased with increasing in per unit mass of adsorbent dosage

34

. Considering the

effect of qe and Removal% on adsorption, the ASC dosage of 2 g L-1 was choosed. Comparing Fig. 3a and b, we found that when adsorbent dosage was 2 g L-1, 82.8% and 66.7% removal efficiency were achieved for NH4+ and H2PO4−, respectively. Simultaneously, the adsorption 14


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capacity of NH4+ and H2PO4− were 20.7 and 16.7 mg g-1, respectively. It indicated that ASC was more efficiency for NH4+ removal than H2PO4−.

Figure 3. Effect of the adsorbent dosage on adsorption capacity and removal efficiency of NH4+ (a) and H2PO4− (b) onto ASC adsorbent in solution (solution concentration: 50 mg L-1 for NH4+ and H2PO4−; pH: 6.5-7.0 for NH4+, 5.0 for H2PO4−; contacting time 1 h; shaking speed of 150 rpm at 20oC).

Effect of solution pH on adsorption. pH is one of the most important factors that influence the adsorption capacity of ionic-type adsorbents. The effect of pH on NH4+ and H2PO4− adsorption by ASC was investigated, and the obtained results are shown in Fig. 4. From the figure, we can see that with pH increasing from 2.0 to 4.0, the adsorption capacity of ASC for NH4+ increased and then kept almost constant in the pH range from 4.0 to 9.0. However, when pH is greater than 10.0, the adsorption capacity decreased. This is because that the adsorption of ionic nutrients onto the amphoteric adsorbent is primarily due to electrostatic interactions. At lower pH values, most of the –COO− groups on ASC are protonized, inhibiting the adsorption of NH4+. At pH 4.0, the –COOH is deprotonized, active sites on the adsorbent are occupied by NH4+, thus, the adsorption capacity increases. The –

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COOH groups gradually deprotonize with increase in pH, and NH4+ is easily adsorbed until all of the –COOH is deprotonized. Therefore, the adsorption capacity is slightly increased and keeps almost constant at pH 4.0-9.0. The adsorption capacity of NH4+ is decreased when the pH value of aqueous solution is higher than 9.0. It can be attributed to NH4+ is converted to NH3·H2O at higher pH values, making it uncharged and resulting in the concentration of NH4+ decreased 35.

Figure 4. Effect of solution pH on equilibrium adsorption capacities of NH4+ and H2PO4− onto ASC adsorbent in solution (solution concentrations: 50 mg L-1 for NH4+ and H2PO4−; ASC dosage: 2 g L-1; contacting time: 1 h; shaking speed of 150 rpm at 20oC). As shown in Fig. 4, the uptake of H2PO4− is initially increased and then decreased after reaching the maximum adsorption capacity of 16.8 mg g-1 at around pH 5.0. The quaternary ammonium groups on ASC could bind with H2PO4− easily. However, at pH ranges of 12, phosphate can be existed in the form of H3PO4, H2PO4−, HPO42− and PO43−, respectively

36

. Therefore, at lower pH conditions, the dominant species of phosphate form

were H3PO4, which had a weak interaction with the cationic groups. At high pH range, the

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dominant species are HPO42− and PO43−, which necessitated more adsorption sites from ASC for the adsorption of single phosphate ions

37

. In addition, OH− ions may compete strongly

with phosphate for the active sites of ASC. This resulted in the adsorption capacity of H2PO4− decreasing at higher pH values.

Effect of ionic strength on adsorption. As we know, wastewater contains various kinds of co-existing ions, which may remarkably affect the adsorbate-adsorbent interaction. Therefore, the effect of common salts, such as NaCl, CaCl2 and Na2SO4 on the adsorption of NH4+ and H2PO4− onto ASC was studied, as shown in Fig. 5a and Fig. 5b. It can be also observed from Fig. 5, there is more negative effect with higher electric charge, such as Ca2+ for NH4+, SO42- for H2PO4− in aqueous solution. The study results revealed that with the increasing of the concentration of NaCl, CaCl2 and Na2SO4 ranged from 0.05 to 0.2 mol L-1, and the adsorption of NH4+ and H2PO4− exhibits similar decreasing trends in the presence of other co-existing ions. This indicates that the presence of other co-existing ions will hinder for the adsorption of NH4+ and H2PO4- onto ASC.

Figure 5. Effect of ionic strength on adsorption capacities of NH4+ (a) and H2PO4− (b) onto

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ASC in solution (solution concentrations: 50 mg L-1 for NH4+ and H2PO4−; ASC dosage: 2 g L-1; pH: 7.0 for NH4+, 5.0 for H2PO4−; contact time: 1 h; shaking speed of 150 rpm at 20oC). Abundance of Na+ and Ca2+ co-existing cations compete with NH4+ for adsorption sites on ASC surface, which has a negative impact on the interaction between NH4+ and ASC. Thus, the adsorption capacity of NH4+ decreased with increasing of Na+ and Ca2+ concentrations. For H2PO4− anions, with the increasing of concentration of competitive anions Cl- and SO4-, the ion activity of H2PO4− reduced, resulting in the effectiveness of ASC adsorbent for H2PO4− adsorption removal reduced. Adsorption kinetics. Fig. 6 shows the adsorption kinetics of NH4+ and H2PO4− adsorption onto ASC adsorbent. As shown in Fig. 6, the adsorption rate of both NH4+ and H2PO4− onto ASC is very rapid, and the adsorption process can be almost reached equilibrium within 20 and 30 min, respectively. The adsorption efficiency increased rapidly initially, which may be attributed to the quantity of existing active sites. Then, the number of active sites gradually decreased, therefore, the adsorption rate become slower. Finally, the adsorption reached equilibrium, demonstrating that all of the available active sites have reached adsorption saturation. The adsorption rate of H2PO4− was slower than that of NH4+, because the molecular mass of H2PO4− is larger than NH4+, resulting in the diffusion rate of H2PO4− entered into the pores of ASC is slower.

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Figure 6. Adsorption kinetics of NH4+ and H2PO4− adsorption onto ASC adsorbent in solution (solution concentrations: 50 mg L-1 for NH4+ and 25 mg L-1 for H2PO4−, ASC dosage: 2 g L-1, shaking speed of 150 rpm at 25oC).

To provide valuable insights into the adsorption mechanism and kinetics for sorting out the rate-controlling step of adsorption process, the pseudo-first-order 38, pseudo-second-order

39

and intra-particle diffusion model 40 were employed to study the kinetics experimental data of NH4+ and H2PO4− adsorption onto ASC. The linear form of these three models equation can be expressed as

ln(qe − qt ) = ln qe − k1t t 1 1 = + t 2 qt k2 qe qe

qt = kid t 1 2 + Ci

(5) (6)

(7)

where qe and qt represent the amount of NH4+ or H2PO4− (mg g-1) adsorbed at equilibrium time and at any time t (min), respectively; k1 represents the kinetic equation constant of pseudo-first-order kinetic equation constant (min-1); k2 is the adsorption rate constant of pseudo-second-order kinetic equation (g mg-1 min-1) determined by plotting t/qt versus t; kid is

19



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the rate constant of intra-particle diffusion (mg g-1 h-1/2); values of Ci give an idea about the thickness of the boundary layer.

In order to investigate the adsorption mechanism and further understand the time dependence of adsorption process, the adsorption experimental data of NH4+ and H2PO4− adsorption onto ASC were fitted with pseudo-first-order and pseudo-second-order model, and the determined kinetic parameters and correlation coefficient R2 are listed in Table 2. It can be found that the correlation coefficient (R2>0.99) obtained for NH4+ and H2PO4− indicated the adsorption process is better fit to pseudo-second-order model than pseudo-first-order model, and the values of qe calculated from the pseudo-second-order model agree better with the experimental data. It indicates that experimental kinetic data for NH4+ and H2PO4− adsorption onto ASC followed the pseudo-second-order model, which suggests that the adsorption process involved chemisorption in addition physisorption. The chemisorption might be the rate-controlling step where valency forces are involved through sharing or exchange of electrons between adsorbate ions and amphoteric adsorbent 41. Table 2. The kinetic parameters of NH4+ and H2PO4− adsorption onto ASC adsorbent.

The intra-particle diffusion model was employed to analyze the adsorption kinetic results and to verify whether the intra-particle diffusion was the only rate-controlling step. In the study of adsorption kinetic process, if the intra-particle diffusion was the rate-limiting step, adsorption plot of qt versus t1/2 should be linear and go through the origin. If the plots can be separated into multi-linearity, it indicated that other adsorption kinetic steps and some degree 20


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of boundary diffusion control and two or three steps may be functioning simultaneously 42. As shown in Fig. 7, the intra-particle diffusion plots of NH4+ and H2PO4− adsorption onto ASC adsorbent exhibit multi-linearity, indicating that three stages are involved in adsorption process. The first stage (the curve with a larger slope) corresponds to migration of adsorbate ions (NH4+ and H2PO4−) from the bulk solution to the external surface of ASC by boundary diffusion. The second stage is the gradual adsorption stage, in which the diffusion of the adsorbate ions through the boundary layer into the internal pores of the amphoteric adsorbent, and intra-particle diffusion dominates throughout the adsorption process. The third stage shows a small slope, indicating the adsorption gradually reached equilibrium and the adsorption rate becomes slow 30. The parameters obtained from intra-particle diffusion model are listed in Table 2. It can be seen from Fig. 7 and Table 2, the fitting lines of NH4+ and H2PO4− intra-particle diffusion plots are multi-linearity over the whole time range, and the values of Ci are not zero, which confirms that the rate-limiting step was not sole, and the transporting of adsorbate ions to the external also determined the adsorption rate.

Figure 7. The intra-particle diffusion plots of NH4+ and H2PO4− adsorption onto ASC 21



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adsorbent in solution (solution concentrations: 50 mg L-1 for NH4+ and 25 mg L-1 for H2PO4−, ASC dosage: 2 g L-1, contact time: 90 min, shaking speed of 150 rpm at 25oC).

Adsorption isotherm. The plots of the equilibrium adsorption capacity (qe) and initial NH4+ and H2PO4− concentration (Ci) in solution were studied, as shown in Fig. 8. It can be observed that the adsorption capacity of amphoteric adsorbent increased with increasing initial adsorbate ions concentration in solution and then gradually reached adsorption equilibrium. The mass transfer resistance between the solid phase and aqueous solution would be overcome by the powerful driving force generated by the initial adsorbate ions concentration in solution

43

. The plots of the equilibrium adsorption capacity (qe) and

equilibrium NH4+ and H2PO4− concentration (Ce) in solution are shown in Fig. 9. The maximum adsorption capacity of ASC for NH4+ is about 68.4 mg g-1, which is much higher than that of H2PO4− (38.6 mg g-1). These results indicate more effective interaction occurred between adsorption sites and NH4+ than H2PO4−.

Figure 8. Effect of initial solution concentration on adsorption capacity of NH4+ and H2PO4− 22


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onto ASC adsorbent in solution (ASC dosage: 2 g L-1, pH: 7.0 for NH4+, 5.0 for H2PO4−, contact time: 2 h, shaking speed of 150 rpm at 25oC).

Figure 9. Equilibrium adsorption isotherm of NH4+ and H2PO4− onto ASC adsorbent (ASC dosage: 2 g L-1, pH: 7.0 for NH4+, 5.0 for H2PO4−, contact time: 2 h, shaking speed of 150 rpm at 25oC).

To evaluate the performance of an adsorbent and further better understand the thermodynamics of NH4+ and H2PO4− adsorption onto ASC, three typical adsorption isotherm models of Langmuir

44

, Freundlich

45

and Temkin

46

were examined based on the batch

adsorption equilibrium results. The equation of Langmuir model is as follow:

Ce Ce 1 = + qe qm qm K L

(8)

where Ce is the equilibrium concentration of the adsorbate solution (mg L-1); qe is the amount of NH4+ or H2PO4− adsorbed at equilibrium (mg g-1); qm is the theoretical saturation adsorption capacity corresponding to complete monolayer coverage of active adsorption sites (mg g-1); KL is the values for Langmuir constant related to the energy of adsorption and the 23



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affinity of the binding sites (L mg-1).

The Freundlich isotherm was an empirical equation which assumes that the adsorption process occurred on heterogeneous surfaces. The equation can be expressed as follows: 1 ln qe = ln K F + ln Ce n

(9)

where KF is a Freundlich isotherm constant which indicates the adsorption capacity (L mg-1); 1/n is the heterogeneity factor of the adsorption intensity suggesting whether the adsorption process is favorable.

The Temkin isotherm assumes that the heat of adsorption decreases linearly with coverage, due to the indirect electrostatic interaction between adsorbate ions and charged adsorbent. The linearized form of this model can be expressed as:

qe = BT ln KT + BT ln Ce

(10)

where BT=RT/bT and bT is the Temkin constant related to the heat of adsorption; KT is the equilibrium binding constant (L mg-1) related to maximum binding energy.

The equilibrium adsorption isotherm data were fitted to the Langmuir, Freundlich, and Temkin models, and the correlation coefficient (R2) with obtained parameters are listed in Table 3. The Langmuir model with high correlation coefficients (R2>0.99) indicated that the adsorption of NH4+ and H2PO4− onto ASC best fitted the Langmuir model. The obtained values of 1/n (0.1

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