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Removal and recovery of phosphate from water by lanthanum hydroxide materials
Accepted Manuscript Removal and recovery of phosphate from water by lanthanum hydroxide materials Jie Xie, Zhe Wang, Shaoyong Lu, Deyi Wu, Zhenjia Zhang, Hainan Kong PII: DOI: Reference:
S1385-8947(14)00697-4 http://dx.doi.org/10.1016/j.cej.2014.05.113 CEJ 12207
To appear in:
Chemical Engineering Journal
Please cite this article as: J. Xie, Z. Wang, S. Lu, D. Wu, Z. Zhang, H. Kong, Removal and recovery of phosphate from water by lanthanum hydroxide materials, Chemical Engineering Journal (2014), doi: http://dx.doi.org/ 10.1016/j.cej.2014.05.113
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1 2 3 4
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Removal and recovery of phosphate from water by lanthanum hydroxide materials
8
Jie Xie1, Zhe Wang 1, Shaoyong Lu 2, Deyi Wu*1, Zhenjia Zhang1, Hainan Kong11
9
(1. School of Environmental Science and Engineering, Shanghai Jiao Tong
5 6
10
University, No. 800, Dongchuan Rd., Shanghai 200240, China; 2. Research
11
Centre of Lake Environment, Chinese Research Academy of Environmental
12
Sciences, Beijing 100012, China)
13
1
Corresponding author. Tel: +86-21-54748529; Fax: +86-21-54740825; E-mail
address: [email protected] (D. Y. Wu) 1
14 15
Abstract: The adsorption of phosphate from water by two lanthanum hydroxides
16
(LHs), a commercial LH and a synthesized LH from waste alkaline solution, was
17
investigated. The amorphous synthesized LH had higher specific surface area, and
18
showed greater performance for phosphate adsorption than the crystalline commercial
19
LH. The phosphate adsorption data agreed well with the Langmuir model with the
20
calculated maximum capacity of 107.53 mg/g (dry weight) for synthesized LH and
21
55.56 mg/g (dry weight) for commercial LH, respectively. The affinity toward
22
phosphate was high over a wide pH value range, from about 2.5 to 9.0 for commercial
23
LH and from about 2.5 to 12.0 for synthesized LH, respectively. Release of La was
24
negligible when pH > 4.0. FTIR measurements showed that the monodentate surface
25
species of ≡La-OPO3 was formed via ligand exchange mechanism. In the coexistence
26
of chloride, nitrate, sulfate and hydrogen carbonate anions, phosphate removal by
27
LHs was only slightly affected. The uptake of phosphate (~5 mg/L) from real effluent
28
by LHs performs well at a dose of 1 kg/10m3, with the removal efficiency exceeding
29
99% for synthesized LH and 90% for commercial LH, respectively. The adsorbed
30
phosphate could be successfully recovered by hydrothermal treatment in NaOH
31
solution, and the regenerated LHs could be reused for phosphate removal.
32
Keywords: lanthanum hydroxide; wastewater; phosphate; adsorption; recycling
33
2
34
1. Introduction
35
Though a number of elements are required by organisms, just five elements
36
(carbon, oxygen, hydrogen, nitrogen and phosphorus) make up 93 to 97% of the
37
biomass of organisms, including plants, animals, fungi, and bacteria [1]. Phosphorus
38
is not very abundant in the biosphere. Sedimentary rocks that are especially rich in
39
phosphorus are mined for fertilizer and applied to agricultural soils since about 170
40
years ago [2, 3]. However, phosphate rock is a non-renewable resource and it is
41
predicted that current global reserves may be depleted in 50–100 years [2, 3]. What is
42
more, the global phosphorus cycle does not include a substantial atmospheric pool,
43
differing from other four elements. As a result, phosphorus entering into water bodies
44
could hardly be recycled to land for use in agriculture. It is also known that
45
accumulation of phosphorus is a leading cause of eutrophication for relatively
46
stagnant water bodies such as lakes and estuaries. Therefore, on one hand, phosphorus
47
removal from wastewater before discharge has to be considered to protect natural
48
waters from eutrophication. On the other hand, the depletion of phosphorus fertilizer
49
urges us to investigate the recovery and reuse of phosphorus from wastewater.
50
Adsorption is superior to chemical treatment (such as precipitation with iron salts,
51
alum, or lime) and biological process for phosphate removal from water/wastewater
52
in terms of initial cost, flexibility and simplicity of design, ease of operation, and
53
insensitivity to toxic pollutants, and reduced production of sludge [4-6]. More
54
importantly, adsorbed phosphorus may be recovered provided that the adsorption
55
amount is high and a suitable desorption method could be found. With the adsorption 3
56
technique, hence, the use of a good adsorbent is crucial to guarantee the efficiency of
57
wastewater treatment.
58
The utilization of industrial wastes or by-products as adsorbents for phosphate
59
removal has been widely investigated, including fly ash based materials [7, 8],
60
biosorbent from organic residues [9], blast furnace slag [10], red mud [11, 12], spent
61
alum sludge [13], ferric sludge [14], and iron-rich residues [15], etc. The major
62
advantage of using these kinds of adsorbents for wastewater treatment is
63
cost-effective. However, more effective adsorbent for phosphate removal with the
64
merit of phosphorus recovery is in urgent demand.
65
Lanthanum is a rare earth element that is considered to be environmentally
66
friendly and is relatively abundant in the earth’s crust [16-18]. Lanthanum is known to
67
have a high affinity for phosphate and the lanthanum–phosphate complex forms, even
68
when present in low concentrations of phosphate [19, 20]. As a result, considerable
69
attention has been focused on the use of lanthanum-containing materials for the
70
removal of phosphate in recent years [18-27].
71
Although a great number of works have been undertaken on the synthesis of
72
zeolite from coal fly ash as a productive measure for the reuse of the solid waste,
73
problems related to waste alkaline solution remains unsolved [28]. Thus, it is
74
important to search for a method to recycle the waste alkaline solution following
75
zeolite synthesis from coal fly ash.
76
The aim of our present study was to develop an efficient adsorbent from the waste 4
77
alkaline solution, for the removal and recovery of phosphate from wastewater. For
78
this purpose, lanthanum hydroxide (LH) was prepared by neutralization of lanthanum
79
chloride solution with the waste alkaline solution generated during the conversion of
80
coal fly ash into zeolite. The synthesized LH was investigated for its potential as an
81
adsorbent to remove and recover phosphate from wastewater. For comparison, a
82
commercial LH was also examined.
83
2. Material and methods
84
2.1 Materials
85
Coal fly ash used in this study was obtained from the Second Power Plant of
86
Minhang in Shanghai, China. For zeolite preparation, a conventional refluxing
87
method was used, with vigorous stirring, under the following reaction conditions:
88
reaction time 24 h, liquid/solid ratio 6 mL/g, NaOH concentration 2 M and
89
temperature 95 oC. After being cooled down to room temperature, waste alkaline
90
solution was recovered by centrifugation, and a ~0.67 M LaCl3 solution was added,
91
drop-wise (10 mL/min) with continuous stirring. The volume of LaCl3 solution was
92
equal to that of waste alkaline solution (200 mL). To guarantee a sufficient reaction of
93
LaCl3 with the alkaline solution, stirring was kept for 4 h, following the addition of
94
LaCl3. The formed lanthanum hydroxide was then washed three times with
95
double-distilled water and twice with ethanol. Finally, the product was dried in an
96
oven at 45°C, ground to pass through an 80-mesh (with the diameter of 180µm) sieve,
97
and stored in airtight containers until further use. 5
98
Besides the above synthesized lanthanum hydroxide, a pure commercial LH in its
99
power form was purchased from Aladdin Industrial Corporation (Shanghai, China)
100
and was used without any modification.
101
The chemical composition of materials was determined by X-ray fluorescence
102
analysis (PW2404, Philips). Moisture was measured by the loss of weight after
103
heating at 105°C for 24 h. CEC was determined by the ammonium acetate method
104
[29]. The X-ray diffraction (XRD) patterns were recorded using D8 ADVANCE
105
(BRUKER-AXS) with Cu Kα filtered radiation (30 kV, 15 mA). Particle morphology
106
was observed by SEM using a JEOL JSM-7401F microscope. The FTIR spectra were
107
recorded with a FT-IR spectrophotometer (SHIMAZU IRPrestige-21) using the KBr
108
method. BET surface area was determined by NOVA1200e (Quanta chrome) using
109
the nitrogen adsorption method. To determine pH value, 0.2 g of material and 40 mL
110
of distilled water were added to the centrifuge tubes and the final pH value was taken,
111
using a HachSension+ pH meter, after a 24 h equilibration period. The soluble
112
components in waste alkaline solution and the effluent following the treatment of
113
waste alkaline solution with LaCl3 were acidified for analysis by using an inductively
114
coupled plasma-atomic emission spectroscopy (ICAP 6000 Radial, Thermo
115
Company).
116
The batch adsorption experiments for phosphate were performed in duplicate and
117
the mean data are reported in this paper.
118
2.2 Adsorption isotherms 6
119
Adsorption isotherms of phosphate were performed in 50-mL centrifuge tubes.
120
About 0.1 g of material was put into centrifuge tubes containing 40 mL phosphate
121
solution with different concentrations of phosphate, ranging from 5 to 500 mg/L. The
122
suspensions were shaken in a thermostatic chamber at 25±1°C for 24 h at 180 rpm.
123
After 24 h, the equilibrium pH was measured and the suspensions were centrifuged.
124
The clear supernatants were determined for phosphate, using the molybdenum-blue
125
ascorbic acid method [30]. The amounts of phosphate adsorbed per unit mass of
126
adsorbent were calculated from the differences between the initial and the final
127
phosphate concentrations in solution:
128
Qe = (co - ce)V/m
129
where V is the sample volume in L, co is the initial phosphate concentration in mg/L,
130
ce is the equilibrium phosphate concentration in mg/L, and m is the dry weight of
131
adsorbent in g.
132
2.3 pH studies
133
The effect of solution pH on phosphate removal was measured in the same fashion
134
with the adsorption isotherm measurements, except that the initial phosphate
135
concentration was 100 mg/L. The suspensions were adjusted to the desired pH values
136
with 0.1 M HCl or NaOH. After 24 h, the equilibrium pH was measured and the
137
suspension was centrifuged for analyzing the residual phosphate concentration in
138
supernatant.
139
To assess the stability of LH materials under different pH conditions, 0.1 g sample 7
140
was added to each 250-mL Erlenmeyer flask containing 100 mL of doubly-distilled
141
water (DD water). The mixtures were adjusted to pH levels within the range of
142
2.0–7.5 and continuously shaken for 24 h at 25±1°C. After centrifugation, the
143
supernatant was collected and filtered prior to analysis for La by inductively coupled
144
plasma-atomic emission spectroscopy (ICAP 6000 Radial, Thermo).
145
2.4 Kinetic studies
146
The kinetic runs were carried out in a 1 L conical flask, into which 600 mL of
147
phosphate solution and 0.6 g of samples were added. The sample volume of 600 mL
148
was used so as to minimize the change in liquid-to-solid ratio due to the frequent
149
samplings. The initial phosphate concentration was 100 mg/L. The conical flask was
150
shaken in a thermostatic chamber at 25±1°C for 48 h at 180 rpm. After each specified
151
reaction time, aliquot of 5 mL sample was taken. The sample was then filtered
152
through 0.45 µm membrane filters and the filtrate was determined for phosphate
153
concentration. The pH was measured after 48 h equilibration time.
154
2.5 Dosage
155
To examine the effect of adsorbent dosage, a stock suspension of adsorbents was
156
prepared by continuous mixing of 1 g of LHs with 1 L DD water. From this
157
suspension subsamples were transferred to 1 L conical flasks to obtain the desired
158
adsorbent dose. The volume of solution was adjusted to 500 mL and the adopted
159
phosphate concentration was 5 mg/L so as to simulate real wastewater. After being
160
shaken for 24 h in the same fashion with kinetic studies, the suspensions were filtered 8
161
and the filtrate was determined for phosphate concentration.
162
2.6 Influence of coexisting anions and adsorption in real wastewater
163
The sample of 0.1 g of materials was put into 500 mL conical flasks and 200 mL
164
of 1 mmol/L phosphate solution with and without the coexisting anions of NO3–,
165
SO42–, Cl– and HCO3–, in the form of sodium salt, was added. The concentration of
166
each coexisting anion was 2.5 mmol/L. The flasks were then shaken for 24 h in the
167
same fashion with kinetic studies, the suspensions were filtered and the filtrate was
168
determined for phosphate concentration.
169
To further elucidate the performance of LH for phosphate removal in the presence
170
of competing anions, sample of an effluent from the Minhang Waste Water Treatment
171
Plant (Shanghai, China) was taken and it was filtered through 0.45 µm membrane
172
filters to remove suspended solid and/or organisms. The filtered effluent was then
173
spiked with KH2PO4 to the phosphate concentration level of about 5 mg/L. Phosphate
174
solution with the same phosphate concentration prepared from DD water was used as
175
control to test the effect of coexisting anions on phosphate removal. These phosphate
176
solutions were then reacted with adsorbent for 24 h in the same fashion with kinetic
177
studies, the suspensions were filtered and the filtrate was determined for phosphate
178
concentration. The soluble components of the effluent were analyzed by ion
179
chromatography (METROHM, MICI).
180
2.7 Phosphate desorption
9
181
The LHs were initially reacted, for 24 h, with the phosphate solution at a
182
phosphate concentration of 500 mg/L. After washing three times with DD water, the
183
adsorbed phosphate was recovered by NaOH treatment at different conditions of
184
temperature (100 ~ 250 oC), NaOH concentration (3 M and 12.5 M), and liquid/solid
185
ratio (6 ~ 80 mL/g). The percentage (%) of desorbed phosphate to adsorbed phosphate
186
was calculated, so as to assess the possibility of recovering adsorbed phosphate and
187
recycling LHs for further use.
188
To examine the possibility of the lanthanum hydroxide materials to recover both
189
the phosphate and the lanthanum hydroxide for further use, about 0.5 g of adsorbent
190
was initially reacted with 200 mL of phosphate solution (~200 mg/L) for 24 h. The
191
phosphate desorption (adsorbent regeneration) was conducted under the conditions of
192
NaOH concentration 3 M, temperature 250 °C, liquid/solid ratio 80 mL/g, desorption
193
time 5 h. Regenerated adsorbent was washed with DD water 3 times before the next
194
cycle of adsorption and desorption. The adsorption and regeneration procedures were
195
repeated for 5 cycles.
196
3. Results and discussion
197
3.1 Characterization of materials
198
The chemical composition of LHs is given in Table 1. The compositions of metal
199
elements were given in the form of oxides. It is shown that the commercial LH was
200
quite pure, with only traces of other components. However, the synthesized LH
201
contained a number of elements other than La. The La2O3 content was 68.01%, as a 10
202
result of the neutralization of LaCl3 with the waste alkaline solution. The
203
neutralization process also resulted in the formation of amorphous aluminasilicate
204
materials. The alkaline solution contained high concentrations of Si (6370 mg/L) and
205
Al (35.2 mg/L), which was dissolved from coal fly ash and was not incorporated into
206
zeolite structure during the crystallization process of zeolite. Neutralization by LaCl3
207
reduced the concentrations of Si and Al to 58.9 and 2.6 mg/L, respectively, by
208
forming the aluminasilicate material. Even after repeated washing with DD water,
209
synthesized LH contained high levels of Na+ and Cl-, due to the substantial cation
210
exchange capacity (Table 1) and high affinity toward anions.
211
The XRD patterns of LHs are illustrated in Fig. 1. Commercial LH was well
212
crystallized and it can be indexed as hexagonal La(OH)3 phase with cell parameters of
213
a＝b＝0.6547 nm, c＝0.3854 nm, α＝β＝90°, γ＝120° (JCPDS Card No. 83-2034)
214
from the 2 theta values and the radiation intensities of the peaks. In contrast, the
215
wide-angle XRD pattern of synthesized LH shows only two very broad and weak
216
peaks in the range of 25–35°and 40–50°, respectively. This indicates that both LH and
217
aluminasilicate in synthesized LH are amorphous. The difference in crystallinity
218
between commercial and synthesized LH can be clearly seen from their SEM images
219
as well (Fig. S1). The SEM images (Fig. S1) showed that amorphous lanthanum
220
hydroxides were loosely combined flocs while the crystal lanthanum hydroxides
221
were compact hexagonal phases. It is therefore not surprising that synthesized LH
222
had a considerably higher specific surface area than commercial LH (Table 1). 11
223
3.2 Adsorptive capacity
224
To determine the maximum adsorption capacity of LHs for phosphate, the
225
adsorption isotherms were measured in DD water containing different concentrations
226
of phosphate and the results are shown in Fig. 2. It can be seen that, at low
227
concentrations, phosphate was greatly adsorbed so that a very low residual phosphate
228
concentration was yielded, implying a high affinity of phosphate for LH. The
229
adsorption of phosphate increased as equilibrium phosphate concentration further
230
increases, but growth rate eventually slows and then begin to cease as uptake of
231
phosphate levels off, indicating the adsorption sites were close to be saturated.
232
The adsorption isotherm data for phosphate were fitted to the Langmuir,
233
Freundlich, Temkin, and Redlich-Peterson isotherm models and the fitting results are
234
given in Table S1. The Langmuir model gives the best fit, with r2 exceeding 0.99. The
235
maximum adsorption capacity was 107.53 mg/g (dw) for synthesized LH and 55.56
236
mg/g (dw) for commercial LH, respectively. These maximum phosphate capacities
237
are not only higher than those reported for cost-effective industrial wastes or
238
by-products [7-15], but also higher than those reported for functionalized adsorbents
239
aiming at efficient sequestration of phosphate from water [5, 6, 21-27, 31-34], as
240
listed in Table 2.
241
Though commercial LH had higher lanthanum content than synthesized LH
242
(Table 1), adsorptive capacity of synthesized LH was greater than commercial LH.
243
This difference did not arise from the components other than lanthanum hydroxide in 12
244
synthesized LH because separately prepared product through neutralization of waste
245
alkaline solution with hydrochloric acid (~0.67 M) showed trivial ability to absorb
246
phosphate (data not shown). Hence, higher affinity of synthesized LH toward
247
phosphate than commercial LH must be owing to the difference in crystallinity and
248
the resulted surface area, i.e., the amorphous phase and higher specific surface area of
249
synthesized LH resulted in its greater adsorptive capability than commercial LH. For
250
comparison, results of phosphate adsorption by the hydroxide products of iron and
251
aluminum in previous studies are listed in Table S2. Similarly to lanthanum hydroxide,
252
adsorption by amorphous phase of both iron and aluminum hydroxide was generally
253
greater than crystalline mineral phases, due to higher specific surface area of
254
amorphous phases [35-37]. Moreover, lanthanum hydroxide had considerably greater
255
adsorption capacity than the hydroxides of iron and aluminum (Table S2).
256
3.3 Phosphate complex at LH surface
257 258
The species of phosphate ion is different at different pH, as shown by the following equations:
259
H3PO4 ↔ H2PO4– + H+
pK1 = 2.13
260
H2PO4– ↔ HPO42– + H+
pK2 = 7.20
261
HPO42– ↔ PO43– + H+
pK3 = 12.33
262
The above pK constants allow us to calculate the distribution diagrams of
263
phosphate species (in percentages (%)) as a function of pH (Fig. S2). This indicated
264
that HPO4– is the main species (>90%) within the pH range from 8.2 to 11.2, which is 13
265
the case in the present study when no artificial adjustment for pH was done (Table
266
S3).
267
Previous studies have pointed up to five surface species by inner-sphere complex
268
formation of phosphate with metal (hydr)oxide:
269
MOH + H+ + PO43− ↔ MPO4 2− + H2O
(1)
270
MOH + 2H+ + PO43− ↔ MHPO4 − + H2O
(2)
271
MOH + 3H+ + PO43− ↔ MH2PO4 + H2O
(3)
272
2MOH + 2H+ + PO43− ↔ M2PO4− + 2H2O
(4)
273
2MOH + 3H+ + PO43−↔ M2H2PO4 + 2H2O
(5)
274
Where M represents a metal atom and OH, the hydroxyl group. (1) to (3) are
275
monodentate species while (4) and (5) are bidentate species [38]. FTIR spectra within
276
the region from 1200 to 800 cm−1 could be used for structural diagnosis of phosphate
277
complexes at the metal (hydr)oxides [38-41]. Fortunately no band appeared within
278
this region for commercial LH, so all bands observed for complexes of phosphate and
279
La on commercial LH would be due to surface complex itself, without the
280
interference of bands due to LH (Fig. 3). However, for synthesized LH, a strong band
281
centered at 975 cm−1 appeared, which could be assigned to the asymmetric internal
282
T-O stretching vibration mode of the TO4 tetrahedra, where T = Si or Al, due to the
283
occurrence of amorphous aluminasilicate materials in synthesized LH (Table 1). The
14
284
phosphate surface complexes include XOPO3 species with C3v symmetry, (XO)2PO2
285
species with C2v symmetry and (XO)3PO species with C3v symmetry (X = H or a
286
metal atom) , each has its characteristic FTIR bands [38-41]. The main FTIR bands of
287
phosphate-La complex appeared at 1058 and 1008 cm−1, with the latter appearing as a
288
shoulder for both synthesized and commercial LH (Fig. 3). The two bands can be
289
assigned to the vas and vs stretching vibration modes of P-O, respectively. Though the
290
signal-to-noise ratio is low, there is an indication of a band at around 860 cm−1 for
291
commercial LH which could be attributed to the P-O-La stretching mode and belongs
292
to the A1 symmetry species. The number of band and the frequency range where the
293
P-O bands are found together with their relative intensities contradicts a C2v
294
symmetry species, suggesting that the possibility of the nonprotonated bridging
295
complex (LaO)2PO2 and the protonated monodentate complex ( (≡LaO)(OH)PO2)
296
could be ruled out. The spectra are instead rather similar to the species with a C3v
297
symmetry. Furthermore, at the pH levels in this study, the only likely surface complex
298
with asymmetry close to C3v is a monodentatenonprotonated species, ≡LaOPO32−.
299
Thus, reaction (1) was probably the mechanism underlining the adsorption of
300
phosphate by LH. Our results agree well with previous studies [38-41], which showed
301
that ligand exchange process at high pH levels on iron (hydr)oxide surfaces gives rise
302
to the formation of monodentate nonprotonated species (≡FeOPO3).
303
3.4 Influence of pH value
304
The influence of pH value on the removal of phosphate by LHs was investigated 15
305
over a wide pH range 1.5–13.0, at a phosphate concentration of 100 mg/L. The results
306
are given in Fig. 4 and show that the adsorption of phosphate by LH performs well
307
over a wide pH range, from about 2.5 to 9.0 for commercial LH and from about 2.5 to
308
12.0 for synthesized LH, respectively. Within these pH ranges, the percentage
309
removal of phosphate approached 100%. This behavior is worthy of highlighting
310
when compared with other metal (hydr)oxides such as iron and aluminum whose
311
efficient removal for phosphate could only be achieved within a narrow pH range at
312
acidic pH levels [42-44]. The performance for phosphate removal by LHs decreased
313
sharply with increasing pH further (> about 9.0 for commercial LH and > about 12.0
314
for synthesized LH, respectively), indicating that OH- as well as carbonate and
315
bicarbonate ions could compete with phosphate for adsorption sites. Decreasing pH to
316
< about 2.5 also gave rise to the decrease in removal efficiency of phosphate, and this
317
was evidently caused by the release of La from adsorbents (Fig. 4).
318
Tests on the leachability of La from LH under different pH conditions indicated
319
that the extent of La release was greatly influenced by pH (Fig. 4). Desorption of La
320
was negligible when the pH was higher than 4.0, which is desirable for the use of the
321
material in water and wastewater treatment facilities. La release occurred under acidic
322
conditions (pH <4.0). It appeared that the dissolution of La started at a lower pH and
323
the degree of La release was lower for synthesized LH than for commercial LH.
324
3.5 Kinetic studies
325
The time-dependent adsorption (up to 48 h) of phosphate to LHs is shown in Fig. 16
326
5. The results indicated that removal of phosphate was initially very rapid and
327
was >80% in 1 h for each LH. The rate of phosphate uptake decreased with prolonged
328
reaction time. The reaction reached near equilibrium (>97%) after 4 h for commercial
329
LH and 6 h for synthesized LH, respectively. The data were fitted well to the
330
pseudo-second-order rate equation (r2>0.99), which assumes that adsorption follows
331
the Langmuir model. The pseudo-second-order kinetic model could be expressed as
332
following:
333
dqt/dt = k(q e-qt)2
334
where qt and q e are the amounts of phosphate adsorbed at time t and equilibrium
335
(mg/g), respectively, and the k is the equilibrium rate constant for second-order
336
adsorption (g/mg·min). The parameters obtained by fitting the data to the kinetic
337
model are summarized in Table S4. For comparison purposes, results obtained for
338
La-treated juniper bark (La/JB01 and La/JB02) are also shown. The rate constants (k)
339
for LH are comparable with those obtained for La-treated juniper bark [18].
340
3.6 Effect of dosage
341
The influence of adsorbent dosage on phosphate removal was studied by varying
342
the adsorbent dose from 0.05 to 0.2 g/L in DD water containing an initial phosphate
343
concentration of 5 mg/L to simulate real sewage. This phosphate concentration was in
344
the range of the average concentration in real waste waters, i.e., domestic waste
345
waters. Similar phosphate concentrations were used also by other workers for
346
investigating phosphate removal from wastewater [45, 46]. Increased adsorbent 17
347
dosage implied a greater surface area and a greater number of binding sites available
348
for the constant amount of phosphate. Therefore, as shown in Fig. 6, the percentage
349
removal (%) of phosphate initially increased sharply with increasing LHs dose,
350
reaching nearly 100%. It is worth noting that by adopting an appropriate LH dosage,
351
near complete removal of phosphate from aqueous solutions could be achieved. The
352
higher removal performance of phosphate by synthesized LH than that by commercial
353
LH was noticeable at low dosages. The newest discharge standard of pollutants for
354
municipal wastewater treatment plant (GB18918—2002) in China establishes
355
phosphorus limits at 0.5 mg/L (dotted line in Fig. 6). As can be seen in the figure, the
356
dose to attain this goal is 0.05-0.08 g/L for synthesized LH and 0.08-0.10 g/L for
357
commercial LH, respectively. This implies that the performance of phosphate removal
358
by LH is high and to treat 10 m3 of wastewater containing 5 mgP/L to reach a
359
phosphorus effluent level of 0.5 mgP/L, only 0.5 to 0.8 kg of synthesized LH or 0.8 to
360
1.0 kg of commercial LH is required.
361
3.7 Adsorption of phosphate in the presence of coexisting anions
362
Adsorptive removal of phosphate from natural water or real waste water could be
363
potentially interfered by other anionic species which may compete for adsorption sites.
364
Hence, the adsorption of phosphate from an effluent sample spiked with phosphate to
365
the concentration level of ~5 mgP/L was investigated at a dose of 0.1 g/L. The water
366
sample contained coexisting anions, with the concentration of some species listing in
367
Table S5. For comparison, adsorption of phosphate solution prepared in DD water 18
368
with the same dosage and phosphate concentration was also studied. Results in Table
369
3 show that, even in the presence of coexisting anions, removal efficiency of
370
phosphate by synthesized LH reached >99%, and for commercial LH, removal
371
efficiency exceeded 90%. The concentration of phosphate could be reduced to <
372
0.045 mg/L by the addition of synthesized LH, while the residual phosphate
373
concentration after treatment with commercial LH approached the phosphate limits
374
for effluent (0.5 mg/L).
375
However, the concentration of coexisting anions in the effluent was low. To better
376
elucidate the influence of coexisting anions on fixation of phosphate by LH,
377
adsorption of phosphate with and without the presence of common anionic species,
378
including chloride, nitrate, bicarbonate, and sulfate, was examined at a dosage of 0.5
379
g/L. The concentration of phosphate was 1 mmol/L, while that of other anions was 2.5
380
mmol/L. We choose to use this concentration of coexisting anions based on a
381
previous study by Tanada et. al. [44]. In this experiment, the concentration of each
382
anion species was 2.5 times greater than that of phosphate, i.e., the total number of
383
coexisting anions is ten-fold greater than phosphate in solution so as to see how the
384
coexisting anions impede the adsorption of phosphate. The total number of coexisting
385
anions is about five-fold (for synthesized LH) or ten-fold (for commercial) greater
386
than maximum adsorption sites calculated by assuming that one phosphate anion
387
corresponds to one adsorption site when maximum amount of phosphate was
388
adsorbed (determined from Langmuir model). It is shown in Table 3 that the 19
389
adsorption of phosphate by LH was quite selective, only a slight decrease in
390
adsorption capacity of phosphate was observed (<5%).
391
3.8 Recovery of phosphate
392
The phosphorus content in LH after being saturated with phosphate is high.
393
However, direct application in agriculture as fertilizer is not practical because the
394
formed ≡La-PO4 is insoluble in water and it would be desirable to recycle LH for
395
further use as adsorbent. Therefore, desorption of the phosphate as an important
396
resource of fertilizer in agriculture with the simultaneous regeneration of LH would
397
be much valuable. Though coexistence of OH− at high pH (> about 12.0) hindered the
398
adsorption of phosphate (Fig. 4), our results indicated that the complex could no
399
longer be readily desorbed by OH- once the phosphate was adsorbed by LH. In fact,
400
even by increasing the concentration of NaOH to 3 M at 100 oC for extraction, the
401
desorbed phosphate was still very slight (Table S6).
402
Processing of monazite — a lanthanum phosphate ore —generally involves
403
treatment by 50–70% (12.5–17.5 M) NaOH solution at 140–150°C for several hours
404
(Fig. S3). This results in the formation of insoluble LH and soluble Na3PO4, which
405
can be easily separated. The process thus allows for recovery of the valuable
406
phosphate from the ore, along with the separation of the rare earth element. Results
407
indicated that phosphate that was adsorbed by LH could be successfully recovered in
408
this way, with a percentage desorption of 96.52% for commercial LH and 96.73% for
409
synthesized LH, respectively (Table S6). Furthermore, we found that a high NaOH 20
410
concentration, a high liquid/solid ratio and a high temperature can facilitate phosphate
411
recovery (Table S6). Indeed, successful recovery of phosphate (95.78% for
412
commercial LH and 95.35% for synthesized LH, respectively) could be achieved at a
413
much lower NaOH concentration of 3 M, but at a higher temperature of 250°C, and a
414
higher liquid/solid ratio of 80:1 mL/g.
415
Regeneration of LHs (desorption of adsorbed phosphate on LHs) was performed
416
for 5 cycles. The adsorption amount of phosphate by original (0 circle) and
417
regenerated LHs (1st to 4th circle) is given in Fig. 7a while the desorption rate (the
418
percentage (%) of desorbed phosphate to adsorbed phosphate) is shown in Fig. 7b.
419
The experimental results in Fig. 7 show that the regenerated LHs could be used again
420
for phosphate removal. Compared with original LHs, amount of phosphate adsorbed
421
by regenerated LHs decreased gradually. But the amount of phosphate adsorbed by
422
synthesized LH after 4 circles of adsorption-desorption reached about 90% of the
423
original one, while for commercial LH, a 25% decrease was observed. At the same
424
time, the desorption rates of all the five circles exceeded 80% for synthesized LH and
425
85% for commercial LH, respectively, under the phosphate desorption (adsorbent
426
regeneration) conditions. More satisfactory results may be expected by further
427
optimization of the desorption/regeneration conditions.
428
We focused on the phosphate removal and recovery from wastewater in this
429
study. However, further study on the potential use of lanthanum hydroxide
430
materials in lakes and rivers needs to be done in future, before these materials can 21
431
be selected for use in natural ecosystems.
432
4. Conclusion
433
To develop a highly efficient adsorbent for phosphate removal from water and to
434
recycle the waste alkaline solution produced during the synthesis of zeolite from coal
435
fly ash, a lanthanum hydroxide was synthesized by neutralization with LaCl3. The
436
performance of phosphate removal by the synthesized lanthanum hydroxide, an
437
amorphous phase, was greater than the commercial lanthanum hydroxide with a
438
crystalline structure. The Langmuir adsorption capacity reached 107.53 mg/g and
439
55.56 mg/g, respectively, for synthesized and commercial materials. The uptake of
440
phosphate performs well over a wide pH range, and only slightly affected by common
441
competitive anions, such as chloride, nitrate, sulfate, and hydrogen carbonate anions.
442
No evident release of La was observed when the pH was higher than about 4.0. The
443
adsorbed phosphate could be extracted for application as fertilizer in agriculture, by
444
hydrothermal treatment in NaOH solution. Lanthanum hydroxide, particularly the
445
amorphous one synthesized from waste alkaline solution, is promising for the removal
446
and recovery of phosphate from water/waste water.
447 448
Acknowledgements: This research was supported by the National Key Project for Water Pollution Control (2013ZX07101-014, 2012ZX07105002-03).
449
Supporting Information Available
450
Supporting materials are available free of charge via the Internet. 22
451 452
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585
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586
Res. 33 (1999) 3595-3602.
587 588
29
589 590 591 Table 1. Chemical composition and some properties of LHs
Parameter
Synthesized LH
Commercial LH
SiO2 (%)
4.20
0.29
Al2O3 (%)
0.21
N.D.a)
CaO (%)
0.03
0.04
Na2O (%)
11.47
N.D.
K2O (%)
0.15
N.D.
SO3 (%)
0.09
0.05
Cl (%)
8.68
0.09
P2O5 (%)
0.13
0.02
La2O3 (%)
68.01
98.66
Moisture (%)
6.71
0.84
BET surface area (m2/g)
153.3
31.1
CEC b) (mmol/g)
0.182
0.038
pH
11.34
9.64
a)
Not detectable. b)cation exchange capacity.
592 593 594 30
595 596 597 598 599 600 601 602 603 604
Table 2. Comparison of phosphate adsorption maxima (Qm) of tested materials with some literature values Qm (mg/g)
SSA(m2/g)
References
Synthesized La(OH)3
107.53
153.3
This study
Commercial La(OH)3
55.56
31.1
This study
Fe-EDA-SAMMS
43.3
169
5
29.08-45.63
227~476
6
23.78
321
6
29.44
~1326
21
9.84-13.02
N.A. a)
22
8.59-11.60
39.3 or N.A.
23, 24, 25, 26
Modified inorganic bentonite (Zenith/Fe)
11.15
N.A.
26
Modified bentoniteBephos™
26.5
N.A.
27
Synthesized hydrotalcite
47.3
N.A.
31
Fe-Mn binary oxide adsorbent
36.0
309
32
Mesoporous ZrO2
29.7
232
33
Commercial zirconium ferrite
39.8
200
34
Material
Lanthanum loaded mesoporous silica SBA-15 (with different La content) Lanthanum loaded MCM-41 Hydroxyl-iron-lanthanum loaded activated carbon fiber Mixed lanthanum/aluminum pillared montmorillonite (at different temperatures for sorption) Bentonite product coated with lanthanum (Phoslock™)
605 606 607
a) N.A. = Not avalible
608 609 610 611 612 31
613 614 615 616 617 Table 3. Removal of phosphate by lanthanum hydroxide as affected by the presence of competing anions. Coexisting anionsa)
Spiked with phosphateb)
Materials and items No
Yes
DD water
Effluent
31.081
30.299
5.387
5.187
Residual concentration(mg/L)
0.489
0.489
0.035
0.043
Removal efficiency (%)
98.43
98.39
99.30
99.18
Amount adsorbed (mg/g)
65.58
63.91
57.37
55.15
Residual concentration(mg/L)
6.060
6.548
0.243
0.507
Removal efficiency (%)
80.50
78.39
95.49
90.23
Amount adsorbed (mg/g)
50.47
47.90
51.87
47.20
Phosphate concentration (mg/L)
Synthesized La(OH)3
Commercial La(OH)3
a)
initial phosphate concentration of ~1 mmol/L, dosage of 0.5 g/L, coexisting
anions include chloride, nitrate, sulfate and hydrogen carbonate with each concentration of 2.5 mmol/L.
b)
initial phosphate concentration of ~5 mg/L, dosage
of 0.1 g/L.
618 619 620 621 622
32
623 624 625 626 627 628 629
630 631
Fig. 1. XRD patterns of commercial (upper) and synthesized (lower) LHs.
632 633 634 635 636 637 638 33
639 640 641 642 643 644 645
646 647
Fig. 2. Adsorption isotherms of phosphate on commercial and synthesized LHs in DD water,
648
dosage of 2.5 g/L.
649 650 651 652 653 654 655 656 34
657 658 659 660 661 662 663
860 1058
1008
664 665
Fig. 3. FTIR spectra of (a) synthesized LH with adsorbed phosphate, (b) synthesized LH, (c)
666
commercial LH with adsorbed phosphate, and (d) commercial LH.
667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 35
682 683 684 685 686 687 688
689 690
Fig. 4. Adsorption of phosphate by LHs and La released from LHs under different pH conditions
691
in DD water, initial phosphate concentration of ~100 mg/L, dosage of 2.5 g/L for phosphate
692
adsorption and 1 g/L for La release. Circle: synthesized LH; triangle: commercial LH; open
693
symbol: phosphate adsorption; solid symbol: La release.
694 695 696 697 698 36
699 700 701 702 703 704 705
706 707
Fig. 5. Adsorption of phosphate by LHs in DD water as a function of time, initial phosphate
708
concentration of ~100mg/L, dosage of 1 g/L.
709 710 711 712 713 714 715 37
716 717 718 719 720 721 722
723 724
Fig. 6. Influence of adsorbent dosage on the adsorption of phosphate by LHs in DD water, initial
725
phosphate concentration of 5 mg/L, dosage of 0.05-0.2 g/L, and dotted line represents phosphate
726
discharge limit. Circle: synthesized LH; triangle: commercial LH; open symbol: removal
727
efficiency; solid symbol: residual phosphate concentration.
728 729 730 731 732 733 38
734 735 736 (a)
(b)
737 738 739 740 741 742
Fig. 7. Recovery of phosphate and regeneration of LHs. (a). adsorption of phosphate by regenerated LHs with initial phosphate concentration of ~200 mg/L, dose of 2.5 g/L, reaction time of 24 h; (b). desorption of phosphate adsorbed on LHs under the conditions of NaOH concentration 3 M, temperature 250°C, liquid/solid ratio 80:1 mL/g, and desorption time 5 h.
743 744 745 39
746
Graphical abstract
747
748 749 750
40
751 752 753 754 755 756 757 758 759
Highlights:
A lanthanum hydroxide adsorbent was prepared from waste alkaline solution. The Langmuir adsorption maximum for phosphate reached 107.53 mg/g. The affinity of lanthanum hydroxide toward phosphate was high over a wide pH range. The adsorption mechanism was explained by the ligand exchange process. Phosphate removal by lanthanum hydroxide performs well in real water/wastewater.
760
41
S1385-8947(14)00697-4 http://dx.doi.org/10.1016/j.cej.2014.05.113 CEJ 12207
To appear in:
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Please cite this article as: J. Xie, Z. Wang, S. Lu, D. Wu, Z. Zhang, H. Kong, Removal and recovery of phosphate from water by lanthanum hydroxide materials, Chemical Engineering Journal (2014), doi: http://dx.doi.org/ 10.1016/j.cej.2014.05.113
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1 2 3 4
7
Removal and recovery of phosphate from water by lanthanum hydroxide materials
8
Jie Xie1, Zhe Wang 1, Shaoyong Lu 2, Deyi Wu*1, Zhenjia Zhang1, Hainan Kong11
9
(1. School of Environmental Science and Engineering, Shanghai Jiao Tong
5 6
10
University, No. 800, Dongchuan Rd., Shanghai 200240, China; 2. Research
11
Centre of Lake Environment, Chinese Research Academy of Environmental
12
Sciences, Beijing 100012, China)
13
1
Corresponding author. Tel: +86-21-54748529; Fax: +86-21-54740825; E-mail
address: [email protected] (D. Y. Wu) 1
14 15
Abstract: The adsorption of phosphate from water by two lanthanum hydroxides
16
(LHs), a commercial LH and a synthesized LH from waste alkaline solution, was
17
investigated. The amorphous synthesized LH had higher specific surface area, and
18
showed greater performance for phosphate adsorption than the crystalline commercial
19
LH. The phosphate adsorption data agreed well with the Langmuir model with the
20
calculated maximum capacity of 107.53 mg/g (dry weight) for synthesized LH and
21
55.56 mg/g (dry weight) for commercial LH, respectively. The affinity toward
22
phosphate was high over a wide pH value range, from about 2.5 to 9.0 for commercial
23
LH and from about 2.5 to 12.0 for synthesized LH, respectively. Release of La was
24
negligible when pH > 4.0. FTIR measurements showed that the monodentate surface
25
species of ≡La-OPO3 was formed via ligand exchange mechanism. In the coexistence
26
of chloride, nitrate, sulfate and hydrogen carbonate anions, phosphate removal by
27
LHs was only slightly affected. The uptake of phosphate (~5 mg/L) from real effluent
28
by LHs performs well at a dose of 1 kg/10m3, with the removal efficiency exceeding
29
99% for synthesized LH and 90% for commercial LH, respectively. The adsorbed
30
phosphate could be successfully recovered by hydrothermal treatment in NaOH
31
solution, and the regenerated LHs could be reused for phosphate removal.
32
Keywords: lanthanum hydroxide; wastewater; phosphate; adsorption; recycling
33
2
34
1. Introduction
35
Though a number of elements are required by organisms, just five elements
36
(carbon, oxygen, hydrogen, nitrogen and phosphorus) make up 93 to 97% of the
37
biomass of organisms, including plants, animals, fungi, and bacteria [1]. Phosphorus
38
is not very abundant in the biosphere. Sedimentary rocks that are especially rich in
39
phosphorus are mined for fertilizer and applied to agricultural soils since about 170
40
years ago [2, 3]. However, phosphate rock is a non-renewable resource and it is
41
predicted that current global reserves may be depleted in 50–100 years [2, 3]. What is
42
more, the global phosphorus cycle does not include a substantial atmospheric pool,
43
differing from other four elements. As a result, phosphorus entering into water bodies
44
could hardly be recycled to land for use in agriculture. It is also known that
45
accumulation of phosphorus is a leading cause of eutrophication for relatively
46
stagnant water bodies such as lakes and estuaries. Therefore, on one hand, phosphorus
47
removal from wastewater before discharge has to be considered to protect natural
48
waters from eutrophication. On the other hand, the depletion of phosphorus fertilizer
49
urges us to investigate the recovery and reuse of phosphorus from wastewater.
50
Adsorption is superior to chemical treatment (such as precipitation with iron salts,
51
alum, or lime) and biological process for phosphate removal from water/wastewater
52
in terms of initial cost, flexibility and simplicity of design, ease of operation, and
53
insensitivity to toxic pollutants, and reduced production of sludge [4-6]. More
54
importantly, adsorbed phosphorus may be recovered provided that the adsorption
55
amount is high and a suitable desorption method could be found. With the adsorption 3
56
technique, hence, the use of a good adsorbent is crucial to guarantee the efficiency of
57
wastewater treatment.
58
The utilization of industrial wastes or by-products as adsorbents for phosphate
59
removal has been widely investigated, including fly ash based materials [7, 8],
60
biosorbent from organic residues [9], blast furnace slag [10], red mud [11, 12], spent
61
alum sludge [13], ferric sludge [14], and iron-rich residues [15], etc. The major
62
advantage of using these kinds of adsorbents for wastewater treatment is
63
cost-effective. However, more effective adsorbent for phosphate removal with the
64
merit of phosphorus recovery is in urgent demand.
65
Lanthanum is a rare earth element that is considered to be environmentally
66
friendly and is relatively abundant in the earth’s crust [16-18]. Lanthanum is known to
67
have a high affinity for phosphate and the lanthanum–phosphate complex forms, even
68
when present in low concentrations of phosphate [19, 20]. As a result, considerable
69
attention has been focused on the use of lanthanum-containing materials for the
70
removal of phosphate in recent years [18-27].
71
Although a great number of works have been undertaken on the synthesis of
72
zeolite from coal fly ash as a productive measure for the reuse of the solid waste,
73
problems related to waste alkaline solution remains unsolved [28]. Thus, it is
74
important to search for a method to recycle the waste alkaline solution following
75
zeolite synthesis from coal fly ash.
76
The aim of our present study was to develop an efficient adsorbent from the waste 4
77
alkaline solution, for the removal and recovery of phosphate from wastewater. For
78
this purpose, lanthanum hydroxide (LH) was prepared by neutralization of lanthanum
79
chloride solution with the waste alkaline solution generated during the conversion of
80
coal fly ash into zeolite. The synthesized LH was investigated for its potential as an
81
adsorbent to remove and recover phosphate from wastewater. For comparison, a
82
commercial LH was also examined.
83
2. Material and methods
84
2.1 Materials
85
Coal fly ash used in this study was obtained from the Second Power Plant of
86
Minhang in Shanghai, China. For zeolite preparation, a conventional refluxing
87
method was used, with vigorous stirring, under the following reaction conditions:
88
reaction time 24 h, liquid/solid ratio 6 mL/g, NaOH concentration 2 M and
89
temperature 95 oC. After being cooled down to room temperature, waste alkaline
90
solution was recovered by centrifugation, and a ~0.67 M LaCl3 solution was added,
91
drop-wise (10 mL/min) with continuous stirring. The volume of LaCl3 solution was
92
equal to that of waste alkaline solution (200 mL). To guarantee a sufficient reaction of
93
LaCl3 with the alkaline solution, stirring was kept for 4 h, following the addition of
94
LaCl3. The formed lanthanum hydroxide was then washed three times with
95
double-distilled water and twice with ethanol. Finally, the product was dried in an
96
oven at 45°C, ground to pass through an 80-mesh (with the diameter of 180µm) sieve,
97
and stored in airtight containers until further use. 5
98
Besides the above synthesized lanthanum hydroxide, a pure commercial LH in its
99
power form was purchased from Aladdin Industrial Corporation (Shanghai, China)
100
and was used without any modification.
101
The chemical composition of materials was determined by X-ray fluorescence
102
analysis (PW2404, Philips). Moisture was measured by the loss of weight after
103
heating at 105°C for 24 h. CEC was determined by the ammonium acetate method
104
[29]. The X-ray diffraction (XRD) patterns were recorded using D8 ADVANCE
105
(BRUKER-AXS) with Cu Kα filtered radiation (30 kV, 15 mA). Particle morphology
106
was observed by SEM using a JEOL JSM-7401F microscope. The FTIR spectra were
107
recorded with a FT-IR spectrophotometer (SHIMAZU IRPrestige-21) using the KBr
108
method. BET surface area was determined by NOVA1200e (Quanta chrome) using
109
the nitrogen adsorption method. To determine pH value, 0.2 g of material and 40 mL
110
of distilled water were added to the centrifuge tubes and the final pH value was taken,
111
using a HachSension+ pH meter, after a 24 h equilibration period. The soluble
112
components in waste alkaline solution and the effluent following the treatment of
113
waste alkaline solution with LaCl3 were acidified for analysis by using an inductively
114
coupled plasma-atomic emission spectroscopy (ICAP 6000 Radial, Thermo
115
Company).
116
The batch adsorption experiments for phosphate were performed in duplicate and
117
the mean data are reported in this paper.
118
2.2 Adsorption isotherms 6
119
Adsorption isotherms of phosphate were performed in 50-mL centrifuge tubes.
120
About 0.1 g of material was put into centrifuge tubes containing 40 mL phosphate
121
solution with different concentrations of phosphate, ranging from 5 to 500 mg/L. The
122
suspensions were shaken in a thermostatic chamber at 25±1°C for 24 h at 180 rpm.
123
After 24 h, the equilibrium pH was measured and the suspensions were centrifuged.
124
The clear supernatants were determined for phosphate, using the molybdenum-blue
125
ascorbic acid method [30]. The amounts of phosphate adsorbed per unit mass of
126
adsorbent were calculated from the differences between the initial and the final
127
phosphate concentrations in solution:
128
Qe = (co - ce)V/m
129
where V is the sample volume in L, co is the initial phosphate concentration in mg/L,
130
ce is the equilibrium phosphate concentration in mg/L, and m is the dry weight of
131
adsorbent in g.
132
2.3 pH studies
133
The effect of solution pH on phosphate removal was measured in the same fashion
134
with the adsorption isotherm measurements, except that the initial phosphate
135
concentration was 100 mg/L. The suspensions were adjusted to the desired pH values
136
with 0.1 M HCl or NaOH. After 24 h, the equilibrium pH was measured and the
137
suspension was centrifuged for analyzing the residual phosphate concentration in
138
supernatant.
139
To assess the stability of LH materials under different pH conditions, 0.1 g sample 7
140
was added to each 250-mL Erlenmeyer flask containing 100 mL of doubly-distilled
141
water (DD water). The mixtures were adjusted to pH levels within the range of
142
2.0–7.5 and continuously shaken for 24 h at 25±1°C. After centrifugation, the
143
supernatant was collected and filtered prior to analysis for La by inductively coupled
144
plasma-atomic emission spectroscopy (ICAP 6000 Radial, Thermo).
145
2.4 Kinetic studies
146
The kinetic runs were carried out in a 1 L conical flask, into which 600 mL of
147
phosphate solution and 0.6 g of samples were added. The sample volume of 600 mL
148
was used so as to minimize the change in liquid-to-solid ratio due to the frequent
149
samplings. The initial phosphate concentration was 100 mg/L. The conical flask was
150
shaken in a thermostatic chamber at 25±1°C for 48 h at 180 rpm. After each specified
151
reaction time, aliquot of 5 mL sample was taken. The sample was then filtered
152
through 0.45 µm membrane filters and the filtrate was determined for phosphate
153
concentration. The pH was measured after 48 h equilibration time.
154
2.5 Dosage
155
To examine the effect of adsorbent dosage, a stock suspension of adsorbents was
156
prepared by continuous mixing of 1 g of LHs with 1 L DD water. From this
157
suspension subsamples were transferred to 1 L conical flasks to obtain the desired
158
adsorbent dose. The volume of solution was adjusted to 500 mL and the adopted
159
phosphate concentration was 5 mg/L so as to simulate real wastewater. After being
160
shaken for 24 h in the same fashion with kinetic studies, the suspensions were filtered 8
161
and the filtrate was determined for phosphate concentration.
162
2.6 Influence of coexisting anions and adsorption in real wastewater
163
The sample of 0.1 g of materials was put into 500 mL conical flasks and 200 mL
164
of 1 mmol/L phosphate solution with and without the coexisting anions of NO3–,
165
SO42–, Cl– and HCO3–, in the form of sodium salt, was added. The concentration of
166
each coexisting anion was 2.5 mmol/L. The flasks were then shaken for 24 h in the
167
same fashion with kinetic studies, the suspensions were filtered and the filtrate was
168
determined for phosphate concentration.
169
To further elucidate the performance of LH for phosphate removal in the presence
170
of competing anions, sample of an effluent from the Minhang Waste Water Treatment
171
Plant (Shanghai, China) was taken and it was filtered through 0.45 µm membrane
172
filters to remove suspended solid and/or organisms. The filtered effluent was then
173
spiked with KH2PO4 to the phosphate concentration level of about 5 mg/L. Phosphate
174
solution with the same phosphate concentration prepared from DD water was used as
175
control to test the effect of coexisting anions on phosphate removal. These phosphate
176
solutions were then reacted with adsorbent for 24 h in the same fashion with kinetic
177
studies, the suspensions were filtered and the filtrate was determined for phosphate
178
concentration. The soluble components of the effluent were analyzed by ion
179
chromatography (METROHM, MICI).
180
2.7 Phosphate desorption
9
181
The LHs were initially reacted, for 24 h, with the phosphate solution at a
182
phosphate concentration of 500 mg/L. After washing three times with DD water, the
183
adsorbed phosphate was recovered by NaOH treatment at different conditions of
184
temperature (100 ~ 250 oC), NaOH concentration (3 M and 12.5 M), and liquid/solid
185
ratio (6 ~ 80 mL/g). The percentage (%) of desorbed phosphate to adsorbed phosphate
186
was calculated, so as to assess the possibility of recovering adsorbed phosphate and
187
recycling LHs for further use.
188
To examine the possibility of the lanthanum hydroxide materials to recover both
189
the phosphate and the lanthanum hydroxide for further use, about 0.5 g of adsorbent
190
was initially reacted with 200 mL of phosphate solution (~200 mg/L) for 24 h. The
191
phosphate desorption (adsorbent regeneration) was conducted under the conditions of
192
NaOH concentration 3 M, temperature 250 °C, liquid/solid ratio 80 mL/g, desorption
193
time 5 h. Regenerated adsorbent was washed with DD water 3 times before the next
194
cycle of adsorption and desorption. The adsorption and regeneration procedures were
195
repeated for 5 cycles.
196
3. Results and discussion
197
3.1 Characterization of materials
198
The chemical composition of LHs is given in Table 1. The compositions of metal
199
elements were given in the form of oxides. It is shown that the commercial LH was
200
quite pure, with only traces of other components. However, the synthesized LH
201
contained a number of elements other than La. The La2O3 content was 68.01%, as a 10
202
result of the neutralization of LaCl3 with the waste alkaline solution. The
203
neutralization process also resulted in the formation of amorphous aluminasilicate
204
materials. The alkaline solution contained high concentrations of Si (6370 mg/L) and
205
Al (35.2 mg/L), which was dissolved from coal fly ash and was not incorporated into
206
zeolite structure during the crystallization process of zeolite. Neutralization by LaCl3
207
reduced the concentrations of Si and Al to 58.9 and 2.6 mg/L, respectively, by
208
forming the aluminasilicate material. Even after repeated washing with DD water,
209
synthesized LH contained high levels of Na+ and Cl-, due to the substantial cation
210
exchange capacity (Table 1) and high affinity toward anions.
211
The XRD patterns of LHs are illustrated in Fig. 1. Commercial LH was well
212
crystallized and it can be indexed as hexagonal La(OH)3 phase with cell parameters of
213
a＝b＝0.6547 nm, c＝0.3854 nm, α＝β＝90°, γ＝120° (JCPDS Card No. 83-2034)
214
from the 2 theta values and the radiation intensities of the peaks. In contrast, the
215
wide-angle XRD pattern of synthesized LH shows only two very broad and weak
216
peaks in the range of 25–35°and 40–50°, respectively. This indicates that both LH and
217
aluminasilicate in synthesized LH are amorphous. The difference in crystallinity
218
between commercial and synthesized LH can be clearly seen from their SEM images
219
as well (Fig. S1). The SEM images (Fig. S1) showed that amorphous lanthanum
220
hydroxides were loosely combined flocs while the crystal lanthanum hydroxides
221
were compact hexagonal phases. It is therefore not surprising that synthesized LH
222
had a considerably higher specific surface area than commercial LH (Table 1). 11
223
3.2 Adsorptive capacity
224
To determine the maximum adsorption capacity of LHs for phosphate, the
225
adsorption isotherms were measured in DD water containing different concentrations
226
of phosphate and the results are shown in Fig. 2. It can be seen that, at low
227
concentrations, phosphate was greatly adsorbed so that a very low residual phosphate
228
concentration was yielded, implying a high affinity of phosphate for LH. The
229
adsorption of phosphate increased as equilibrium phosphate concentration further
230
increases, but growth rate eventually slows and then begin to cease as uptake of
231
phosphate levels off, indicating the adsorption sites were close to be saturated.
232
The adsorption isotherm data for phosphate were fitted to the Langmuir,
233
Freundlich, Temkin, and Redlich-Peterson isotherm models and the fitting results are
234
given in Table S1. The Langmuir model gives the best fit, with r2 exceeding 0.99. The
235
maximum adsorption capacity was 107.53 mg/g (dw) for synthesized LH and 55.56
236
mg/g (dw) for commercial LH, respectively. These maximum phosphate capacities
237
are not only higher than those reported for cost-effective industrial wastes or
238
by-products [7-15], but also higher than those reported for functionalized adsorbents
239
aiming at efficient sequestration of phosphate from water [5, 6, 21-27, 31-34], as
240
listed in Table 2.
241
Though commercial LH had higher lanthanum content than synthesized LH
242
(Table 1), adsorptive capacity of synthesized LH was greater than commercial LH.
243
This difference did not arise from the components other than lanthanum hydroxide in 12
244
synthesized LH because separately prepared product through neutralization of waste
245
alkaline solution with hydrochloric acid (~0.67 M) showed trivial ability to absorb
246
phosphate (data not shown). Hence, higher affinity of synthesized LH toward
247
phosphate than commercial LH must be owing to the difference in crystallinity and
248
the resulted surface area, i.e., the amorphous phase and higher specific surface area of
249
synthesized LH resulted in its greater adsorptive capability than commercial LH. For
250
comparison, results of phosphate adsorption by the hydroxide products of iron and
251
aluminum in previous studies are listed in Table S2. Similarly to lanthanum hydroxide,
252
adsorption by amorphous phase of both iron and aluminum hydroxide was generally
253
greater than crystalline mineral phases, due to higher specific surface area of
254
amorphous phases [35-37]. Moreover, lanthanum hydroxide had considerably greater
255
adsorption capacity than the hydroxides of iron and aluminum (Table S2).
256
3.3 Phosphate complex at LH surface
257 258
The species of phosphate ion is different at different pH, as shown by the following equations:
259
H3PO4 ↔ H2PO4– + H+
pK1 = 2.13
260
H2PO4– ↔ HPO42– + H+
pK2 = 7.20
261
HPO42– ↔ PO43– + H+
pK3 = 12.33
262
The above pK constants allow us to calculate the distribution diagrams of
263
phosphate species (in percentages (%)) as a function of pH (Fig. S2). This indicated
264
that HPO4– is the main species (>90%) within the pH range from 8.2 to 11.2, which is 13
265
the case in the present study when no artificial adjustment for pH was done (Table
266
S3).
267
Previous studies have pointed up to five surface species by inner-sphere complex
268
formation of phosphate with metal (hydr)oxide:
269
MOH + H+ + PO43− ↔ MPO4 2− + H2O
(1)
270
MOH + 2H+ + PO43− ↔ MHPO4 − + H2O
(2)
271
MOH + 3H+ + PO43− ↔ MH2PO4 + H2O
(3)
272
2MOH + 2H+ + PO43− ↔ M2PO4− + 2H2O
(4)
273
2MOH + 3H+ + PO43−↔ M2H2PO4 + 2H2O
(5)
274
Where M represents a metal atom and OH, the hydroxyl group. (1) to (3) are
275
monodentate species while (4) and (5) are bidentate species [38]. FTIR spectra within
276
the region from 1200 to 800 cm−1 could be used for structural diagnosis of phosphate
277
complexes at the metal (hydr)oxides [38-41]. Fortunately no band appeared within
278
this region for commercial LH, so all bands observed for complexes of phosphate and
279
La on commercial LH would be due to surface complex itself, without the
280
interference of bands due to LH (Fig. 3). However, for synthesized LH, a strong band
281
centered at 975 cm−1 appeared, which could be assigned to the asymmetric internal
282
T-O stretching vibration mode of the TO4 tetrahedra, where T = Si or Al, due to the
283
occurrence of amorphous aluminasilicate materials in synthesized LH (Table 1). The
14
284
phosphate surface complexes include XOPO3 species with C3v symmetry, (XO)2PO2
285
species with C2v symmetry and (XO)3PO species with C3v symmetry (X = H or a
286
metal atom) , each has its characteristic FTIR bands [38-41]. The main FTIR bands of
287
phosphate-La complex appeared at 1058 and 1008 cm−1, with the latter appearing as a
288
shoulder for both synthesized and commercial LH (Fig. 3). The two bands can be
289
assigned to the vas and vs stretching vibration modes of P-O, respectively. Though the
290
signal-to-noise ratio is low, there is an indication of a band at around 860 cm−1 for
291
commercial LH which could be attributed to the P-O-La stretching mode and belongs
292
to the A1 symmetry species. The number of band and the frequency range where the
293
P-O bands are found together with their relative intensities contradicts a C2v
294
symmetry species, suggesting that the possibility of the nonprotonated bridging
295
complex (LaO)2PO2 and the protonated monodentate complex ( (≡LaO)(OH)PO2)
296
could be ruled out. The spectra are instead rather similar to the species with a C3v
297
symmetry. Furthermore, at the pH levels in this study, the only likely surface complex
298
with asymmetry close to C3v is a monodentatenonprotonated species, ≡LaOPO32−.
299
Thus, reaction (1) was probably the mechanism underlining the adsorption of
300
phosphate by LH. Our results agree well with previous studies [38-41], which showed
301
that ligand exchange process at high pH levels on iron (hydr)oxide surfaces gives rise
302
to the formation of monodentate nonprotonated species (≡FeOPO3).
303
3.4 Influence of pH value
304
The influence of pH value on the removal of phosphate by LHs was investigated 15
305
over a wide pH range 1.5–13.0, at a phosphate concentration of 100 mg/L. The results
306
are given in Fig. 4 and show that the adsorption of phosphate by LH performs well
307
over a wide pH range, from about 2.5 to 9.0 for commercial LH and from about 2.5 to
308
12.0 for synthesized LH, respectively. Within these pH ranges, the percentage
309
removal of phosphate approached 100%. This behavior is worthy of highlighting
310
when compared with other metal (hydr)oxides such as iron and aluminum whose
311
efficient removal for phosphate could only be achieved within a narrow pH range at
312
acidic pH levels [42-44]. The performance for phosphate removal by LHs decreased
313
sharply with increasing pH further (> about 9.0 for commercial LH and > about 12.0
314
for synthesized LH, respectively), indicating that OH- as well as carbonate and
315
bicarbonate ions could compete with phosphate for adsorption sites. Decreasing pH to
316
< about 2.5 also gave rise to the decrease in removal efficiency of phosphate, and this
317
was evidently caused by the release of La from adsorbents (Fig. 4).
318
Tests on the leachability of La from LH under different pH conditions indicated
319
that the extent of La release was greatly influenced by pH (Fig. 4). Desorption of La
320
was negligible when the pH was higher than 4.0, which is desirable for the use of the
321
material in water and wastewater treatment facilities. La release occurred under acidic
322
conditions (pH <4.0). It appeared that the dissolution of La started at a lower pH and
323
the degree of La release was lower for synthesized LH than for commercial LH.
324
3.5 Kinetic studies
325
The time-dependent adsorption (up to 48 h) of phosphate to LHs is shown in Fig. 16
326
5. The results indicated that removal of phosphate was initially very rapid and
327
was >80% in 1 h for each LH. The rate of phosphate uptake decreased with prolonged
328
reaction time. The reaction reached near equilibrium (>97%) after 4 h for commercial
329
LH and 6 h for synthesized LH, respectively. The data were fitted well to the
330
pseudo-second-order rate equation (r2>0.99), which assumes that adsorption follows
331
the Langmuir model. The pseudo-second-order kinetic model could be expressed as
332
following:
333
dqt/dt = k(q e-qt)2
334
where qt and q e are the amounts of phosphate adsorbed at time t and equilibrium
335
(mg/g), respectively, and the k is the equilibrium rate constant for second-order
336
adsorption (g/mg·min). The parameters obtained by fitting the data to the kinetic
337
model are summarized in Table S4. For comparison purposes, results obtained for
338
La-treated juniper bark (La/JB01 and La/JB02) are also shown. The rate constants (k)
339
for LH are comparable with those obtained for La-treated juniper bark [18].
340
3.6 Effect of dosage
341
The influence of adsorbent dosage on phosphate removal was studied by varying
342
the adsorbent dose from 0.05 to 0.2 g/L in DD water containing an initial phosphate
343
concentration of 5 mg/L to simulate real sewage. This phosphate concentration was in
344
the range of the average concentration in real waste waters, i.e., domestic waste
345
waters. Similar phosphate concentrations were used also by other workers for
346
investigating phosphate removal from wastewater [45, 46]. Increased adsorbent 17
347
dosage implied a greater surface area and a greater number of binding sites available
348
for the constant amount of phosphate. Therefore, as shown in Fig. 6, the percentage
349
removal (%) of phosphate initially increased sharply with increasing LHs dose,
350
reaching nearly 100%. It is worth noting that by adopting an appropriate LH dosage,
351
near complete removal of phosphate from aqueous solutions could be achieved. The
352
higher removal performance of phosphate by synthesized LH than that by commercial
353
LH was noticeable at low dosages. The newest discharge standard of pollutants for
354
municipal wastewater treatment plant (GB18918—2002) in China establishes
355
phosphorus limits at 0.5 mg/L (dotted line in Fig. 6). As can be seen in the figure, the
356
dose to attain this goal is 0.05-0.08 g/L for synthesized LH and 0.08-0.10 g/L for
357
commercial LH, respectively. This implies that the performance of phosphate removal
358
by LH is high and to treat 10 m3 of wastewater containing 5 mgP/L to reach a
359
phosphorus effluent level of 0.5 mgP/L, only 0.5 to 0.8 kg of synthesized LH or 0.8 to
360
1.0 kg of commercial LH is required.
361
3.7 Adsorption of phosphate in the presence of coexisting anions
362
Adsorptive removal of phosphate from natural water or real waste water could be
363
potentially interfered by other anionic species which may compete for adsorption sites.
364
Hence, the adsorption of phosphate from an effluent sample spiked with phosphate to
365
the concentration level of ~5 mgP/L was investigated at a dose of 0.1 g/L. The water
366
sample contained coexisting anions, with the concentration of some species listing in
367
Table S5. For comparison, adsorption of phosphate solution prepared in DD water 18
368
with the same dosage and phosphate concentration was also studied. Results in Table
369
3 show that, even in the presence of coexisting anions, removal efficiency of
370
phosphate by synthesized LH reached >99%, and for commercial LH, removal
371
efficiency exceeded 90%. The concentration of phosphate could be reduced to <
372
0.045 mg/L by the addition of synthesized LH, while the residual phosphate
373
concentration after treatment with commercial LH approached the phosphate limits
374
for effluent (0.5 mg/L).
375
However, the concentration of coexisting anions in the effluent was low. To better
376
elucidate the influence of coexisting anions on fixation of phosphate by LH,
377
adsorption of phosphate with and without the presence of common anionic species,
378
including chloride, nitrate, bicarbonate, and sulfate, was examined at a dosage of 0.5
379
g/L. The concentration of phosphate was 1 mmol/L, while that of other anions was 2.5
380
mmol/L. We choose to use this concentration of coexisting anions based on a
381
previous study by Tanada et. al. [44]. In this experiment, the concentration of each
382
anion species was 2.5 times greater than that of phosphate, i.e., the total number of
383
coexisting anions is ten-fold greater than phosphate in solution so as to see how the
384
coexisting anions impede the adsorption of phosphate. The total number of coexisting
385
anions is about five-fold (for synthesized LH) or ten-fold (for commercial) greater
386
than maximum adsorption sites calculated by assuming that one phosphate anion
387
corresponds to one adsorption site when maximum amount of phosphate was
388
adsorbed (determined from Langmuir model). It is shown in Table 3 that the 19
389
adsorption of phosphate by LH was quite selective, only a slight decrease in
390
adsorption capacity of phosphate was observed (<5%).
391
3.8 Recovery of phosphate
392
The phosphorus content in LH after being saturated with phosphate is high.
393
However, direct application in agriculture as fertilizer is not practical because the
394
formed ≡La-PO4 is insoluble in water and it would be desirable to recycle LH for
395
further use as adsorbent. Therefore, desorption of the phosphate as an important
396
resource of fertilizer in agriculture with the simultaneous regeneration of LH would
397
be much valuable. Though coexistence of OH− at high pH (> about 12.0) hindered the
398
adsorption of phosphate (Fig. 4), our results indicated that the complex could no
399
longer be readily desorbed by OH- once the phosphate was adsorbed by LH. In fact,
400
even by increasing the concentration of NaOH to 3 M at 100 oC for extraction, the
401
desorbed phosphate was still very slight (Table S6).
402
Processing of monazite — a lanthanum phosphate ore —generally involves
403
treatment by 50–70% (12.5–17.5 M) NaOH solution at 140–150°C for several hours
404
(Fig. S3). This results in the formation of insoluble LH and soluble Na3PO4, which
405
can be easily separated. The process thus allows for recovery of the valuable
406
phosphate from the ore, along with the separation of the rare earth element. Results
407
indicated that phosphate that was adsorbed by LH could be successfully recovered in
408
this way, with a percentage desorption of 96.52% for commercial LH and 96.73% for
409
synthesized LH, respectively (Table S6). Furthermore, we found that a high NaOH 20
410
concentration, a high liquid/solid ratio and a high temperature can facilitate phosphate
411
recovery (Table S6). Indeed, successful recovery of phosphate (95.78% for
412
commercial LH and 95.35% for synthesized LH, respectively) could be achieved at a
413
much lower NaOH concentration of 3 M, but at a higher temperature of 250°C, and a
414
higher liquid/solid ratio of 80:1 mL/g.
415
Regeneration of LHs (desorption of adsorbed phosphate on LHs) was performed
416
for 5 cycles. The adsorption amount of phosphate by original (0 circle) and
417
regenerated LHs (1st to 4th circle) is given in Fig. 7a while the desorption rate (the
418
percentage (%) of desorbed phosphate to adsorbed phosphate) is shown in Fig. 7b.
419
The experimental results in Fig. 7 show that the regenerated LHs could be used again
420
for phosphate removal. Compared with original LHs, amount of phosphate adsorbed
421
by regenerated LHs decreased gradually. But the amount of phosphate adsorbed by
422
synthesized LH after 4 circles of adsorption-desorption reached about 90% of the
423
original one, while for commercial LH, a 25% decrease was observed. At the same
424
time, the desorption rates of all the five circles exceeded 80% for synthesized LH and
425
85% for commercial LH, respectively, under the phosphate desorption (adsorbent
426
regeneration) conditions. More satisfactory results may be expected by further
427
optimization of the desorption/regeneration conditions.
428
We focused on the phosphate removal and recovery from wastewater in this
429
study. However, further study on the potential use of lanthanum hydroxide
430
materials in lakes and rivers needs to be done in future, before these materials can 21
431
be selected for use in natural ecosystems.
432
4. Conclusion
433
To develop a highly efficient adsorbent for phosphate removal from water and to
434
recycle the waste alkaline solution produced during the synthesis of zeolite from coal
435
fly ash, a lanthanum hydroxide was synthesized by neutralization with LaCl3. The
436
performance of phosphate removal by the synthesized lanthanum hydroxide, an
437
amorphous phase, was greater than the commercial lanthanum hydroxide with a
438
crystalline structure. The Langmuir adsorption capacity reached 107.53 mg/g and
439
55.56 mg/g, respectively, for synthesized and commercial materials. The uptake of
440
phosphate performs well over a wide pH range, and only slightly affected by common
441
competitive anions, such as chloride, nitrate, sulfate, and hydrogen carbonate anions.
442
No evident release of La was observed when the pH was higher than about 4.0. The
443
adsorbed phosphate could be extracted for application as fertilizer in agriculture, by
444
hydrothermal treatment in NaOH solution. Lanthanum hydroxide, particularly the
445
amorphous one synthesized from waste alkaline solution, is promising for the removal
446
and recovery of phosphate from water/waste water.
447 448
Acknowledgements: This research was supported by the National Key Project for Water Pollution Control (2013ZX07101-014, 2012ZX07105002-03).
449
Supporting Information Available
450
Supporting materials are available free of charge via the Internet. 22
451 452
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586
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587 588
29
589 590 591 Table 1. Chemical composition and some properties of LHs
Parameter
Synthesized LH
Commercial LH
SiO2 (%)
4.20
0.29
Al2O3 (%)
0.21
N.D.a)
CaO (%)
0.03
0.04
Na2O (%)
11.47
N.D.
K2O (%)
0.15
N.D.
SO3 (%)
0.09
0.05
Cl (%)
8.68
0.09
P2O5 (%)
0.13
0.02
La2O3 (%)
68.01
98.66
Moisture (%)
6.71
0.84
BET surface area (m2/g)
153.3
31.1
CEC b) (mmol/g)
0.182
0.038
pH
11.34
9.64
a)
Not detectable. b)cation exchange capacity.
592 593 594 30
595 596 597 598 599 600 601 602 603 604
Table 2. Comparison of phosphate adsorption maxima (Qm) of tested materials with some literature values Qm (mg/g)
SSA(m2/g)
References
Synthesized La(OH)3
107.53
153.3
This study
Commercial La(OH)3
55.56
31.1
This study
Fe-EDA-SAMMS
43.3
169
5
29.08-45.63
227~476
6
23.78
321
6
29.44
~1326
21
9.84-13.02
N.A. a)
22
8.59-11.60
39.3 or N.A.
23, 24, 25, 26
Modified inorganic bentonite (Zenith/Fe)
11.15
N.A.
26
Modified bentoniteBephos™
26.5
N.A.
27
Synthesized hydrotalcite
47.3
N.A.
31
Fe-Mn binary oxide adsorbent
36.0
309
32
Mesoporous ZrO2
29.7
232
33
Commercial zirconium ferrite
39.8
200
34
Material
Lanthanum loaded mesoporous silica SBA-15 (with different La content) Lanthanum loaded MCM-41 Hydroxyl-iron-lanthanum loaded activated carbon fiber Mixed lanthanum/aluminum pillared montmorillonite (at different temperatures for sorption) Bentonite product coated with lanthanum (Phoslock™)
605 606 607
a) N.A. = Not avalible
608 609 610 611 612 31
613 614 615 616 617 Table 3. Removal of phosphate by lanthanum hydroxide as affected by the presence of competing anions. Coexisting anionsa)
Spiked with phosphateb)
Materials and items No
Yes
DD water
Effluent
31.081
30.299
5.387
5.187
Residual concentration(mg/L)
0.489
0.489
0.035
0.043
Removal efficiency (%)
98.43
98.39
99.30
99.18
Amount adsorbed (mg/g)
65.58
63.91
57.37
55.15
Residual concentration(mg/L)
6.060
6.548
0.243
0.507
Removal efficiency (%)
80.50
78.39
95.49
90.23
Amount adsorbed (mg/g)
50.47
47.90
51.87
47.20
Phosphate concentration (mg/L)
Synthesized La(OH)3
Commercial La(OH)3
a)
initial phosphate concentration of ~1 mmol/L, dosage of 0.5 g/L, coexisting
anions include chloride, nitrate, sulfate and hydrogen carbonate with each concentration of 2.5 mmol/L.
b)
initial phosphate concentration of ~5 mg/L, dosage
of 0.1 g/L.
618 619 620 621 622
32
623 624 625 626 627 628 629
630 631
Fig. 1. XRD patterns of commercial (upper) and synthesized (lower) LHs.
632 633 634 635 636 637 638 33
639 640 641 642 643 644 645
646 647
Fig. 2. Adsorption isotherms of phosphate on commercial and synthesized LHs in DD water,
648
dosage of 2.5 g/L.
649 650 651 652 653 654 655 656 34
657 658 659 660 661 662 663
860 1058
1008
664 665
Fig. 3. FTIR spectra of (a) synthesized LH with adsorbed phosphate, (b) synthesized LH, (c)
666
commercial LH with adsorbed phosphate, and (d) commercial LH.
667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 35
682 683 684 685 686 687 688
689 690
Fig. 4. Adsorption of phosphate by LHs and La released from LHs under different pH conditions
691
in DD water, initial phosphate concentration of ~100 mg/L, dosage of 2.5 g/L for phosphate
692
adsorption and 1 g/L for La release. Circle: synthesized LH; triangle: commercial LH; open
693
symbol: phosphate adsorption; solid symbol: La release.
694 695 696 697 698 36
699 700 701 702 703 704 705
706 707
Fig. 5. Adsorption of phosphate by LHs in DD water as a function of time, initial phosphate
708
concentration of ~100mg/L, dosage of 1 g/L.
709 710 711 712 713 714 715 37
716 717 718 719 720 721 722
723 724
Fig. 6. Influence of adsorbent dosage on the adsorption of phosphate by LHs in DD water, initial
725
phosphate concentration of 5 mg/L, dosage of 0.05-0.2 g/L, and dotted line represents phosphate
726
discharge limit. Circle: synthesized LH; triangle: commercial LH; open symbol: removal
727
efficiency; solid symbol: residual phosphate concentration.
728 729 730 731 732 733 38
734 735 736 (a)
(b)
737 738 739 740 741 742
Fig. 7. Recovery of phosphate and regeneration of LHs. (a). adsorption of phosphate by regenerated LHs with initial phosphate concentration of ~200 mg/L, dose of 2.5 g/L, reaction time of 24 h; (b). desorption of phosphate adsorbed on LHs under the conditions of NaOH concentration 3 M, temperature 250°C, liquid/solid ratio 80:1 mL/g, and desorption time 5 h.
743 744 745 39
746
Graphical abstract
747
748 749 750
40
751 752 753 754 755 756 757 758 759
Highlights:
A lanthanum hydroxide adsorbent was prepared from waste alkaline solution. The Langmuir adsorption maximum for phosphate reached 107.53 mg/g. The affinity of lanthanum hydroxide toward phosphate was high over a wide pH range. The adsorption mechanism was explained by the ligand exchange process. Phosphate removal by lanthanum hydroxide performs well in real water/wastewater.
760
41