Sciences, Shenyang , China 3 Shenyang Academy of Environmental Sciences, , Shenyang, China. Environ. Eng. Res Research Article

Transcription

1 Environ. Eng. Res Reserch Article pissn eissn X In Press, Uncorrected Proof KH 2 PO 4 -ided soil wshing for removing rsenic from wter-stle soil ggregtes collected in southern Chin Rnrn Zho 1,2, Xiojun Li 2, Zhiguo Zhng 1*, Gunghui Zho 3 * These uthors contriuted eqully to this work 1 College of Ecologicl Technique nd Engineering, Shnghi Institute of Technology, Shnghi201418, Chin 2 Key Lortory of Pollution Ecology nd Environmentl Engineering, Institute of Applied Ecology, ChineseAcdemy of Sciences, Shenyng , Chin 3 Shenyng Acdemy of Environmentl Sciences, , Shenyng, Chin Astrct Removl of rsenic (As) from soil ggregtes with prticle sizes of > 2.0, , , < mm y soil wshing of KH 2 PO 4 nd the kinetics of As relesing from soil ggregtes were investigted. Effects of KH 2 PO 4 concentrtion, rtio of liquid/soil nd wshing durtion on the removl were fully explored. The results showed tht the high As removl ws otined in > 2 mm ggregtes (48.56%) nd < mm ggregtes (42.88%) under the optimum condition (KH 2 PO 4 concentrtion of 0.1 mol/l, nd liquid/soil rtio (10 ml/g) for 360 min) % of As ws extrcted from ggregtes with size less thn 0.25 mm. Only 11.88% ws contriuted y the lrge ggregtes (> 2.0mm). Using KH 2 PO 4 wshing, it ws lso found tht extrcted As is minly in form of either specificlly sored As or As ssocited with oxides of Fe nd Al. Elovich model cn descrie the removl process of As more precisely thn Two-constnt kinetic models. The optimum wshing conditions nd removl process is lso pplied to ulk soil. This technique in this study is relile, cost-effective nd offers gret potentil for prcticl ppliction in soil remedition. Keywords: Arsenic (As), potssium dihydrogen phosphte (KH 2PO 4), Soil ggregtes, Soil wshing This is n Open Access rticle distriuted under the terms of the Cretive Commons Attriution Non-Commercil License ( which permits unrestricted non-commercil use, distriution, nd reproduction in ny medium, provided the originl work is properly cited. Received Mrch 4, 2016 Accepted My 24, 2016 Corresponding Author E-mil: lixijun@ie.c.cn Tel: Fx: Copyright 2016 Koren Society of Environmentl Engineers

2 1. Introduction The rsenic (As) contmintion of soil hs ecome n importnt environmentl issue throughout the world [1]. Contminted soil y As cn result in mssive nd persistent environmentl dmge, giving risk to humn helth [2, 3]. As such, reserch on the remedition of the As contminted soil hs ecome hot reserch topic. Mny techniques such s phytoextrction [4], iosurfctnt [5, 6], chemicl immoiliztion or stiliztion [7, 8], nd soil wshing [9, 10], hd een used to remedite the As-contminted soil. It hs een widely recognized tht soil wshing is one of the most promising techniques for removing the hevy metls from the contminted soil. Moreover, this method is still pplicle when there is multiple As concentrtions from region to region [11]. Mny studies hd een conducted in recent yers on the remedition of As contminted soil y soil wshing technique. Previous studies minly focused on the fesiility of different kinds of wshing solutions, the As removl efficiency from contminted soil, nd the chnges on soil toxicity fter wshing [12-14]. Phosphte (PO 3-4 ) hs een used for As removl from soil due to its cpility to replce rsenic or rsente (AsO 3-4 ). Alm et l. [15] found tht up to 40% of rsenic cn e extrcted from forest soil y 0.9 mol/l of KH 2 PO 4 nd less thn 0.4% of rsenic ws extrcted y other potssium-contining solutions such s KNO 3, K 2 SO 4 nd KCl. The As removl y 0.5 mol/l of KH 2 PO 4 (74.03%) ws higher thn tht y 0.5 mol/l citric cid (53.87%) or 0.1 mol/l EDTA (62.98%) [16].The ility of As removl etween 0.2 mol/l H 3 PO 4 nd KH 2 PO 4 ws similr from soil [13]. Importntly, KH 2 PO 4 hs limited interction with soil components such s C, Mg nd Si. Therefore, 1

3 KH 2 PO 4 is selected s n extrction regent in present study due to its efficiency nd environmentlly friendly. It is known tht chemicl forms of As could not only ffect the its removl efficiency, ut lso reflected the ecologicl risks of soils fter the tretment [12, 17]. The different chemicl forms of As cn result in different ecologicl risks. The different As chemicl forms trgeted y phosphte hd een reported. Jhoet l. [13] found tht phosphte-ided soil wshing cn only remove the non-specificlly sored nd specificlly sored As. While Alm et l. [15] reported tht phosphte solution could extrct the As ounded with Fe nd Al. However, Chen et l. [16] found tht phosphte-ided soil wshing could reduce ll the chemicl forms of As. Soil ggregtes were the sic unit of soil structure [18]. Hevy metls cn e dsored nd remined in soil ggregtes in different wys [19]. As such, soil wshing efficiency cn e gretly ffected y the prticle size of the soil ggregtes. The lrger the prticle size of soil ggregtes were, the more the removl of hevy metls ws during soil wshing [9, 20-22]. However, the investigtion of desorption of As from soil ggregtes of contminted soil y soil wshing is still in its infncy. The chnges of As forms in soil ggregtes fter soil wshing should e pid ttention to nd the resultnt risks should e crefully ssessed. Therefore, in this study, soil ggregtes with different prticle sizes nd ulk soil were used to investigte the As removl y KH 2 PO 4 -ided soil wshing method. The influence of the wshing conditions (KH 2 PO 4 concentrtion, liquid/soil rtio nd wshing durtion) on the removl of As from soil ggregtes nd ulk soil were comprehensively studied. The kinetic process of As removl were nlyzed nd the chnges of As chemicl forms were lso 2

4 compred mong soil ggregtes. The ph vlue of ll the KH 2 PO 4 solutions used in present study ws djusted the sme s tht of the ulk soil (4.60) to minimize the dmge of leching on soil properties. The results would e helpful to ccumulte the dt for the As removl from soil ggregtes nd to supply useful references for developing the remedition technique which ws cost-effective nd pplied esily. 2. Mterils nd Methods 2.1. Soil Smple Preprtion Surfce soil smples (0-20 cm) were collected from frmlnd close to smelter in Hunn Province, southern Chin. Soil smples were mintined their structure nd trnsported ck to the lortory within 2 dys of collection. Prt of the smples ws ir-dried t room temperture nd ws sieved through 2-mm polyethylene sieve s the ulk soil (S0).The rest fresh smples were kept in the refrigertor t 0-4 efore seprtion into soil ggregtes with different sizes Seprtion of Soil Aggregtes Frction of soil ggregtes ws otined y wet-sieving procedure [23]. In specific, the fresh soil ws immersed in wter on set of three nested sieves (2.0, 0.25, nd mm) nd shken verticlly 3 cm for 50 times during period of 2 min. The ggregtes retined in ech sieve, lrge-ggregtes (> 2.0 mm) (S1), mcro-ggregtes ( mm) (S2), 3

5 micro-ggregtes ( mm) (S3) nd silt + cly frctions (< mm) (S4) were collected fter centrifugtion Soil Wshing Tretment Procedure Effect of KH 2 PO 4 concentrtion 1.0 g of soil ws crefully dded in 50 ml centrifugl tue. 20 ml of KH 2 PO 4 solution with different concentrtions (0, 0.01, 0.05, 0.1, 0.5 nd 1 mol/l) ws slowly dded to chieve liquid/soil rtio of 20, nd then the tues ws shken t 25 ± 1 for 24 h. The suspension ws centrifuged for 10 min t 4000 rpm. The superntnt ws collected through 0.45 µm memrne. The filtrte ws nlyzed for As concentrtion Effect of liquid/soil rtio The liquid/soil rtios of 5, 10, 15, 20 nd 30 were otined y dding 5, 10, 15, 20 or 30 ml of 0.1 mol/l KH 2 PO 4 solution to 50 ml centrifugl tue with 1.0 g of soil. Otined solutions were shken t 25 ± 1 for 24 h nd then centrifuged for 10 min t 4000 rpm. The superntnt ws filtered through 0.45 µm memrne. The filtrte ws nlyzed for As concentrtion Effect of wshing time 4

6 The tues dded with 1.0 g of ulk soil or soil ggregtes nd then 10 ml of 0.1 mol/l KH 2 PO 4 solution were shken t 25 ± 1. The smples were collected t 20, 40, 60, 120, 240, 360, 720, 1440 min, nd then centrifuged t 4000 rpm for 10 min. The superntnt ws filtered through 0.45 µm memrne. The filtrte ws nlyzed for As concentrtion As Removl from Soil Aggregtes under the Optimum Wshing Conditions Soil slurries were prepred under the optimum wshing conditions (wshing period, concentrtion of KH 2 PO 4 nd liquid/soil rtio) sed on the results from The slurries were shken t 25 ± 1 for optimum wshing period nd then centrifuged t 4000 rpm for 10 min. The superntnt ws filtered through 0.45 µm memrne. The filtrte ws nlyzed for As concentrtion As Frctiontions in Soil Aggregtes fter Soil Wshing 10 ml KH 2 PO 4 (in 0.1 mol/l) solution ws dded into 50 ml centrifugl tues contined 1.0 g soil or soil ggregte smple. The suspension ws shken for 360 min t 25 ± 1 nd then centrifuged t 4000 r/min for 10 min. The superntnt ws discrded nd the precipittes were rinsed twice using deionized wter nd then dried for As extrction Chemicl Anlysis The ph of soil nd ggregtes ws mesured using ph electrode (phs-3c, Shnghi Jing Ke 5

7 Test Equipment Institute, Chin) (soil:wter = 1:2.5). The ction exchnge cpcity (CEC) ws determined in BCl 2 [24], orgnic mtter (OM) content ws determined y volumetric method of potssium dichromte heting. Free iron oxide ws mesured y the method descried y Lu et l [25]. Totl phosphorus (TP) of soil smples ws mesured ccording to the method introduced y Bo et l [26]. The As frctions in soil ggregtes nd ulk soil efore nd fter wshing were extrcted following the method descried y Wenzel et l [27]. As in soils were divided into five frctions (1) non-specificlly sored As (F1); (2) specificlly sored As (F2); (3) As ssocited with morphous hydrous oxides of Fe nd Al (F3); (4) As ssocited with crystlline hydrous oxides of Fe nd Al (F4) nd (5) residul As frction (F5). Totl rsenic content ws detected using the tomic fluorescence spectrometry (AFS-9700A, Higung Instrumentl Corp., Chin) fter qu regi extrction(0.2 g soil ggregtes nd 10 ml (1 + 1) qu regi digested for 2 h in oiling wter) in which the conditions were determined ccording to the Soil qulity-anlysis of totl Mercury, rsenic nd led contents - Atomic Fluorescence Spectrometry - Prt 2: Anlysis of totl rsenic contents in soils constituted y the Stte Environmentl Protection Administrtion of Chin(GB-T ) All ove tretments hd triplictes Dt Anlysis 6

8 The experimentl results were sujected to one-wy nlysis of vrince nd lest significnce difference using SPSS 16.0 sttisticl softwre t 95% confidence, SigmPlot 10.0 ws used to generte the grphs. 3. Results nd Discussion 3.1. Effect of KH 2 PO 4 Concentrtion on As Removl The optimum KH 2 PO 4 concentrtion ws 0.1 mol/l for oth soil ggregtes nd ulk soil (Fig. 1). It ws shown tht As removl from ll the ggregtes incresed drmticlly with KH 2 PO 4 concentrtion incresing from 0 to 0.1 mol/l. The As removl incresed t slow pce when the KH 2 PO 4 concentrtion ws higher thn 0.1 mol/l. When the KH 2 PO 4 solution ws 0.1 mol/l, As removls were 50.1%, 38.2%, 45.1% nd 47.0% for S1, S2, S3 nd S4, respectively. For comprison, the As removl ws only 11.4%, 4.5%, 11.3%, 11.5% for S1, S2, S3 nd S4 when no KH 2 PO 4 ws dded in the recipe. Using 0.5 mol/l KH 2 PO 4 only incresed As removl y nother 5-8% nd out % of the As removed using 1 mol/l KH 2 PO 4 could e removed using 0.1 mol/l KH 2 PO 4 in different size ggregtes. The similr trend of the As removl from ulk soil ws found. The As removl incresed rpidly when the concentrtion of KH 2 PO 4 ws lower thn 0.1 mol/l nd no significnt increse ws oserved fter tht. These results re consistent with the one reported y Zou et l. [28] in which of As removl is more efficient for higher concentrtion of N 2 EDTA. However, the liner reltionship etween As removl nd NH 4 H 2 PO 4 hd een reported too [12]. 7

9 Removl efficiency (%) S0 S1 S2 S3 S KH 2 PO 4 concentrtion (mol/l) Fig. 1. Effect of KH 2 PO 4 concentrtion on As removl y soil wshing tretment. S0: ulk soil, S1: lrge-ggregtes (> 2.0 mm), S2: mcro-ggregtes ( mm), S3: micro-ggregtes ( mm), S4: silt + cly frctions (< mm) Effect of Liquid/soil Rtio on As Removl The liquid/soil rtio could significntly ffect the efficiency of hevy metls removl from soils [28]. Bsed on the experiment, the optimum liquid/soil rtio should e 10 ml/g for ulk soil, S1, S3 nd S4, nd 15 ml/g for S2 (Fig. 2). The As removl from soil ggregtes, when the liquid/soil rtios were 10, 15 nd 20 ml/g, ws significntly higher thn tht when the liquid/soil rtios were 5, 25 nd 30 ml/g (p < 0.05). The As removl from S2 in the tretment of 15 ml/g ws higher thn tht of 10 nd 20 ml/g. However, the As removl from S3 nd S4 did not vry significntly mong these three concentrtions. The As removl from S1 decresed when the liquid/soil rtio incresed from 10 to 20 ml/g. Since the higher the liquid/soil rtio ws in soil wshing, the more lechte needed to e delt with [29], the optimum liquid/soil rtio ws selected to e 10 ml/g in present study. It ws lso found tht the As removl for S2, S3 nd S4 decresed with the increse of liquid/soil rtio when the 8

10 liquid/soil rtio ws higher thn 20 ml/g. The reson might e tht cid-sed KH 2 PO 4 wshing solutions dissolved soil frctions including metllic frctions (e.g., hydrous oxides of Fe nd Al). Thus, more soil frctions for re-dsorption of the extrcted As my e ville [16]. In ddition, high liquid/soil rtio incresed the contct re etween the extrcted As nd soil, which led to the re-dsorption of As [30].The highest As removl from ulk soil ws lso otined when the liquid/soil rtios were 10, 15 nd 20 ml/g. These results were different with those from the study of Yin et l [31] in which lower liquid/soil rtio removed more Cu, Zn, Ni from soils y wter ws found. The difference might e from tht the hevy metls dissolved in wter could not re-dsored on the surfces of soil solids during soil wshing under conditions of cidic nd circum-neutrl [30]. 60 Removl efficiency (%) c c d f c e d c c L/S=5 L/S=10 L/S=15 L/S=20 L/S=25 L/S=30 0 S0 S1 S2 S3 S4 Aggregte size (mm) Fig. 2. Effect of liquid/soil rtio on As removl y soil wshing tretment. Different smll letters in the sme ggregte indicted significnt difference mong the different tretments t 0.05 level.s0: ulk soil, S1: lrge-ggregtes (> 2.0 mm), S2: mcro-ggregtes ( mm), S3: micro-ggregtes ( mm), S4: silt + cly frctions (< mm). 9

11 The As removl ws different mong soil ggregtes with the chnges of L/S rtios. This ws similr with the study of Lee nd O [21]. The different compositions nd properties mong soil ggregtes might e relted to these kinds of results. Lee nd O [21] hd reported tht in the smples collected from the slope re, the P removl ws different from ech other with the chnge of L/S rtios. The optimum L/S rtios were 2 ml/g (> 2.8 mm), 3 ml/g ( mm), nd 4 ml/g (< mm). Compred to those from slop re, P removl efficiency from > 2.8 mm soil ggregtes collected from the lnd re ws not ffected y the L/S rtios. The optimum L/S rtios for nd < mm soil ggregtes were 4 ml/g Effect of Wshing Durtion on As Removl The effect of wshing durtion on As removl from soil ggregtes ws the sme s ulk soil (Fig. 3).The removl of As incresed significntly with the increse of wshing time from 0 to 360 min (p < 0.05), fter which only % of the As ws removed. A longer wshing period ment higher operting cost [32]. Hence, 360 min wshing could e chosen s the optimum wshing durtion to remedite As-contminted soil. This result ws similr with tht of Zeng et l. [13]. 10

12 Removl efficiency (%) S0 S1 S2 S3 S Time (min) Fig. 3. Kinetics of As removed from soil ggregtes. S0: ulk soil, S1: lrge-ggregtes (> 2.0 mm), S2: mcro-ggregtes ( mm), S3: micro-ggregtes ( mm), S4: silt + cly frctions (< mm). In the present study, the As removl mong the soil ggregtes ws similr with the wshing time extended. This ws different from some previous studies. Lio et l. [22] hd found tht the As removl in > 2 mm ggregtes slowly incresed with wshing durtion, which decresed in those of < 0.1 mm ggregtes when using the mixed solution ( mixture of 1.0% rhmnolipids in 2.0 mol/l HCl) for soil wshing. Menwhile, the P removl from nd < mm ggregtes were lmost not ffected y the wshing durtion ws lso reported [21]. The different results were proly ttriuted to the difference of extrction prmeters, trget hevy metls, the comined forms of hevy metls nd properties of studied soils which were highly relted to the relese of hevy metls. The kinetic dt of As removed from soil ggregtes were nlyzed y kinetic models. Desorption kinetic nlysis ws conducted y using Elovich model nd two-constnt model, which were generl equtions used to descrie the chrcteristics of the desorption 11

13 mechnism in soil [33]. Elovich model (Eq. (1)) nd two-constnt model (Eq. (2)) equtions were s follows: q t = 1/β s lnα s β s +1/β s ln t (1) q t = t (2) Where q t ws the As removl content t time t (in mg/kg); t ws the wshing time (min), α s nd were initil As removl rte (mg/kg/min), β s nd were As removl constnt [1/(mg/kg)]. The prmeters of kinetics equtions were presented in Tle 2.The result indicted tht the mechnism of As removl from soil ggregtes ws the sme s ulk soil. Compred to those of two-constnt model, coefficients of determintion (R 2 ) otined y Elovich model ( ) were high for ll the soil ggregtes nd ulk soil. Thus, Elovich model ws etter to descrie the process of As removl. This ws similr with the study of Zeng et l. [13].The removl of As from soil ggregtes nd ulk soil might e the rections composed of series of complex processes [34]. Tle 2. Prmeters of Removl Kinetics Equtions of As from Soil Aggregtesnd Bulk Soil Aggregte size/mm Kinetic models Prmeter S0 S1 S2 S3 S4 R Elovich α s /(mg/kg/min) β s /(1/mg/kg) R two-constnt /(mg/kg/min) /(1/mg/kg) R 2 : coefficients of determintion.s0: ulk soil, S1: lrge-ggregtes (> 2.0 mm), S2: mcro-ggregtes ( mm), S3: micro-ggregtes ( mm), S4: silt + cly frctions (< mm). 12

14 3.4. As Removl from Soil Aggregtes under Optimum Conditions Fig.4 compred the As removl from different soil ggregtes t the optimum conditions (0.1 mol/l of KH 2 PO 4, liquid/soil rtio 10 ml/g nd wshing for 360 min). The highest As removl ws otined in S1 (48.56%) resulting from the initil low contmintion, which ws followed y S4 (42.88%). The As removl from S2 ws difficult (31.38%). The result ws similr with tht of Lio et l [22]. Removl efficiency(%) c S0 S1 S2 S3 S4 Aggregte size (mm) Fig. 4. As removl from soil ggregtes with 0.1 mol/l KH 2 PO 4. Different smll letters indicted significnt difference mong the different soil ggregtes t 0.05 level.s0: ulk soil, S1: lrge-ggregtes (> 2.0 mm), S2: mcro-ggregtes ( mm), S3: micro-ggregtes ( mm), S4: silt + cly frctions (< mm). 13

15 Tle 1.The Properties of the Soil nd Soil Aggregtes Size (mm) ph OM (g/kg) TP/ (g/kg) Free iron oxide (g/kg) S ±1.40c 0.87± ±0.08 S ± ± ±0.02c S ±1.00e 0.83± ±0.02 S ±0.20d 1.28± ±0.03c S ± ± ±0.06 CEC (c mol/kg) 5.70± 0.03c 4.86± 0.03d 7.89± ± 0.01e 9.68± 0.03 As (mg/kg) Mss (%) ±6.01d ± ±3.80e ±4.56c ± Different smll letters in the sme column indicted significnt difference mong the different soil ggregtes (p < 0.05); S0: ulk soil, S1: lrge-ggregtes (> 2.0 mm), S2: mcro-ggregtes ( mm), S3: micro-ggregtes ( mm), S4: silt + cly frctions (< mm). In present study, the high As removl not only cme from S1 (> 2 mm) ut lso S4 (< mm). The content of Fe oxides nd orgnic mtter ws low in corse ggregtes, thus hevy metls were esier to e relesed (Tle 1). Therefore, the corse ggregtes usully otined high removl efficiency [22, 35]. Menwhile, some studies showed tht the fine ggregtes lso could chieved high hevy metls removl. Hnsen et l. [20] found tht copper removl ws nerly 90% in the fine prticles nd less thn 40% in corse prticles y 1 mol/l cid wshing. Hevy metl removl from soil ggregtes ws ffected not only y leching prmeters ut lso the comined form of hevy metls nd physicochemicl properties [22].The different removl of As mong soil ggregtes could e minly ttriuted to the rsenic specition, orgnic mtter contents nd initil As contents in soil ggregtes, which were highly responsile for As desorption from soils. The more the As ssocited with oxides of Fe nd Al ws, the more the As removl ws. Alm et l. [16] hd reported tht the 14

16 As extrcted from soil y KH 2 PO 4 wshing ws ll from the As ssocited with Fe nd Al. In present study, 71.8% % of the As removed cme from the As ssocited with oxides of Fe nd Al mong soil ggregtes nd ulk soil (Fig. 5). The removed As mounts ws in ccordnce with the mounts of As ssocited with oxides of Fe nd Al in soil ggregtes efore soil wshing. Therefore, rsenic specition might ffect the As removl in soil ggregtes, which ws similr to the result of Lee nd Ko [36]. The higher the orgnic mtter contents were, the more the As removl in soil ggregtes ws. Soil orgnic cids such s humic cids (HA), the most undnt frction of decomposed OM, my compete with phosphte for sorption on soil constituents [37].In turn, competition with rsente ffected As relese from soils [38]. In present study, the orgnic mtter contents in S4, S1 nd S3 were 3.0, 1.6 nd 1.2 times higher thn S2, respectively (Tle 1). Thus, the As removl from S4, S1 nd S3 ws lso 2.5, 2.1 nd 1.7 times higher thn tht from S2. Besides the orgnic mtter, the high initil concentrtion in soil ggregtes ws lso fctor tht ws likely to result in the greter As removl. Xu et l. [33] hd found tht the removl of hevy metls from medium pollution soil lod (P, mg/kg; Zn, mg/kg) ws 6.45%-12.35% higher thn tht in the slight pollution lod soil (P, mg/kg; Zn, mg/kg). In present study, the As concentrtions (Tle 1) in S4, S1 nd S3 were 82.8%, 41.8% nd 31.3% higher thn tht in S2 nd the removl efficiency were 136.7%, 154.8% nd 131.2% higher thn S2, respectively. The comined effects of these chrcteristics of soil ggregtes influenced the As desorption nd might result in different removl of As from soil ggregtes. 15

17 () S () S1 As content(mg/kg) F1 F2 F3 F4 F5 As frctions (C) S2 As content(mg/kg) F1 F2 F3 F4 F5 As frctions (d) S3 As content(mg/kg) F1 F2 F3 F4 F5 As frctions (e) S4 As content(mg/kg) F1 F2 F3 F4 F5 As frctions As content(mg/kg) F1 F2 F3 F4 F5 As frctions efore wshing fter wshing F1 non-specificlly sored As F2 specificlly sored As F3 As ssocited with morphous hydrous oxides of Fe nd Al F4 As ssocited with crystlline hydrous oxides of Fe nd Al F5 residul As Fig. 5. Distriution of As frctions in soil ggregtes efore nd fter wshing with KH 2 PO 4. Different smll letters in the sme ggregte indicted significnt difference mong the different tretmentst 0.05 level. KH 2 PO 4 concentrtion = 0.1 mol/l, wshing time = 360 min, temperture=25 ± 1. S0: ulk soil, S1: lrge-ggregtes (> 2.0 mm), S2: mcro-ggregtes ( mm), S3: micro-ggregtes ( mm), S4: silt + cly frctions (< mm). According to the proportion of different prticle size ggregtes in ulk soil, the mounts of As removed from ulk soil nd the contriution of ech prticle size ggregte to totl removed As could e clculted. The results indicted tht clculted mount of As removed from ulk soil (63.4 mg/kg) ws the sme s the results from the experiment (62.3 mg/kg). The results lso indicted tht the As removed minly cme from the < 0.25 mm ggregtes (S3 nd S4), which contriuted 62.8% for the totl As removed from the soil, mong which S3 contriuted 35.4%. Menwhile, S1 contriuted the lest (11.88%) for the totl As removed. 16

18 4. Conclusions Lortory tch experiments were conducted to study As removl with KH 2 PO 4 from soil ggregtes nd ulk soil. The effect of KH 2 PO 4 concentrtion, liquid/soil rtio nd wshing durtion on As removl from soil ggregtes nd ulk soil were discussed. The results indicted tht the As removl incresed with KH 2 PO 4 concentrtion nd wshing durtion incresing. The As removl from soil ggregtes decresed with the following sequence: S1 > S4 > S3 > S2 under the optimum condition (KH 2 PO 4 concentrtion (0.1 mol/l), liquid/soil rtio (10 ml/g) nd wshing period (360 min)). The removl of As in soil ws minly from S3 nd S4, which ccounted for 62.82% of the totl removed As, while S1 contriuted the lest (11.88%) for the totl As removed. Soil wshing reduced the specificlly sored As nd As ssocited with oxides of Fe nd Al. Compred to the two-constnt kinetic models, Elovich model ws etter to descrie the removl process of As from soil ggregtes. The similr optimum wshing conditions nd removl process were found in ulk soil. Acknowledgments The reserch ws finncilly supported y Ntionl Science Foundtion of Chin (NO , , ). We thnk Yicho Wng from RMIT University for ssistnce in English. References 17

19 1. Moon HD, Dermts D, Menounou N. Arsenic immoiliztion y clcium-rsenic precipittes in lime treted soils. Sci. Totl Environ. 2004;330: Wrren GP, Allowy BJ, Lepp NW, Singh B, BochereuFGM, Penny C. Field trils to ssess the uptke of rsenic y vegetles from contminted soils nd soil remedition with iron oxides. Sci. Totl Environ. 2003;311: Guo W, Zhu YG, Liu WJ, Ling YC, Geng CN, Wng SG. Is the effect of silicon on rice uptke of rsente (As V ) relted to internl silicon concentrtions, iron plque ndphosphte nutrition? Environ. Pollut. 2007;14: Leštn D, Luo CL, Li XD. The use of chelting gents in the remedition of metl-contminted soils: review. Environ. Pollut. 2008;153: Mullign CN. Environmentl pplictions for iosurfctnts. Environ Pollut. 2005;133: Mukhopdhyy S, Hshim MA, Allen M, Sen Gupt B. Arsenic removl from soil with high iron content using nturl surfctnt nd phosphte. Int. J. Environ. Sci. Technol. 2015;12: Theodortos P, Ppssiopi N, Xenidis A. Evlution of monosic clcium phosphte for the immoiliztion of hevy metls in contminted soils from Lvrion. J. Hzrd. Mter. 2002; 94: Kumpiene J, Lgerkvist A, Murice C. Stiliztion of As, Cr, Cu, P nd Zn in soil using mendments - review. Wste Mnge. 2008;28: Lee M, Pik IS, Do W, Kim I, Lee Y, Lee S. Soil wshing of As contminted strem 18

20 sediments in the vicinity of n ndoned mine in Kore. Environ. Geochem. Helth 2007;29: Moutstsou A, Gregou M, Mtss D, Protonotrios V. Wshing s remedition technology pplicle in soils hevily polluted y mining-metllurgicl ctivities. Chemosphere 2006;63: Elgh-Dlgren K, Arwidsson Z, Cmdzij A, et l. Lortory nd pilot scle soil wshing of PAH nd rsenic from wood preservtion site: chnges in concentrtion nd toxicity. J. Hzrd. Mter. 2009;172: Jho EH, Im J, Yng K, Kim YJ, Nm K. Chnges in soil toxicity y phosphte-ided soil wshing: Effect of soil chrcteristics, chemicl forms of rsenic, nd ctions in wshing solutions. Chemosphere 2015;119: Zeng M, Lio BH, Lei M, Zhng Y, ZengQR, OuYng B. Arsenic removl from contminted soil using phosphoric cid nd phosphte. J. Environ. Sci. 2008;20: Im J, Yng K, Jho EH, et l. Effect of different soil wshing solutions on iovilility of residul rsenic in soils nd soil properties. Chemosphere 2015;138: Alm MGM, Tokung S, Mekw T. Extrction of rsenic in synthetic rsenic - contminted soil using phosphte. Chemosphere 2001;43: Chen C, Chen XF, Li XM, et l. Study on wshing remedition technology of rsenic contminted soil using phosphte. Act Sci. Circumst. 2015;35: ChengSF, HungCY, TuYT. Remedition of soils contminted withchromium using citric nd hydrochloric cids: the role of chromiumfrctiontion in chromium leching. Environ. 19

21 Technol. 2011;32: SchultenHR, Leinweer P. New insights into orgnic-minerl prticles: composition, properties nd models of moleculr structure. Biol. Fertil. Soils. 2000;30: Wng F. Sorption - desorption of Cdmium, Copper nd Phennthene y Size frctions of micro ggregtes from pddy soils. Nnjing griculturl University, Hnsen HK, Yintos JB, Ottosen LM. Specition nd lechility of copper in mine tilingsfrom porphyry copper mining: Influence of prticle size. Chemosphere 2005;60: Lee Y, O SW. Desorption Kinetics nd removl chrcteristics of P- contminted soil y the soil wshing method: mixing rtios nd prticle size. Environ. Eng. Res. 2012;17: Lio XY, LiY, Yn XL. Removl of hevy metls nd rsenic from co-contmintedsoilysievingcominedwithwshingprocess [Internet]. Lio XY, LiY, Yn XL; c2015. Aville from: Cmrdell CA, Elliott ET. Cron nd nitrogen dynmics of soil orgnic mtter frctions from cultivted grsslnd soils. Soil Sci. Soc. Am. J. 1994;58: Wng H, Cui GX. Determintion of ction exchnge cpcity of soil y rium chloride uffer method. Soil. 1989;21: Lu RK. Anliticl methods for soil nd grochemistry. Beijing, Chin: Chinese griculturl Science nd Technology Press; p Bo SD. Soil nd Agriculture Chemistry Anlysis. 3rd ed. Beijing, Chin: Chinese 20

22 Agriculturl Press; p Wenzel WW, Kirchumer N, Prohsk T, Stingeder G, Lomi E, Adrino DC. Arsenic frctiontion in soils using n improved sequentil extrction procedure. Anlytic Chimic Act 2001;436: Zou ZL, Qiu RL, Zhng WH, et l. The study of operting vriles in soil wshing with EDTA. Environ. Pollut. 2009;157: Andrde MD, Prsher SO, Hendershot WH. Optimizing the molrity of EDTA wshing solution for sturted-soil remedition of trce metl contminted soils. Environ. Pollut. 2007;147: Ettler V, Mihljevič M, ŠeekO,Grygr T. Assessment of single extrctions for the determintion of moile forms of metls in highly polluted soils nd sediments - Anlyticl nd thermodynmic pproches. Anlytic Chimic Act 2007;602: Yin YJ, Impellitteri CA, You SJ, Allen HE. The importnce of orgnic mtter distriution nd extrct soil: solution rtio on the desorption of hevy metls from soils. Sci. Totl Environ. 2002;287: Hwng, JS, Choi SI, Jng M. Appliction of soil wshing technology for rsenic contminted soil. J. Soil Groundwter Environ. 2004;9: Xu C, XiBC, Lin Y. Kinetics of hevy metls in medium nd slight pollution lod soils under effects of citric wshing. Ecol. Environ. Sci. 2009;18: Tu C. Equtions of chemicl kinetics nd their ppliction to soil systerm. Trop. Su Trop. Soil Sci. 1994;3:

23 35. Lomi E, Sletten RS, Wenzel WW. Sequentilly extrcted rsenic from different size frctions of contminted soils. Wter Air Soil Pollut. 2000;124: Lee CS, Ko MM. Effects of extrcting regents nd metl specition on the removl of hevy metl contminted soils y chemicl extrction. J. Environ. Sci. Helth Prt A. 2004; 39: GeelhoedJS, Hiemstr T, Vn Riemsdijk WH. Competitive interctionetween phosphte nd citrte on goethite. Environ. Sci. Technol. 1998;32: Wng S, Mullign CN. Effect of nturl orgnic mtter on rsenic relesefrom soils nd sediments into groundwter. Environ. Geochem. Helth 2006;28:

24 23