Effect of ionic strength on adsorption of As(Ⅲ) and As(V) on variable charge soils

The study was to investigate the adsorption behavior of arsenite （As（HI）） and arsenate （As（V）） on two variable charge soils, i.e., Haplic Acrisol and Rhodic Ferralsol at different ionic strengths and pH with batch methods. Results indicated that the amount of As（HI） adsorbed by these two soils increased with increasing solution pH, whereas it decreased with increasing ionic strength under the acidic condition. This suggested that As（Ⅲ） was mainly adsorbed on soil positive charge sites through electrostatic attraction under the acidic condition. Moreover, intersects of As（Ⅴ） adsorption-pH curves at different ionic strengths （a characteristic pH） are obtained for both soils. It was noted that above this pH, the adsorption of As（Ⅴ） was increased with increasing ionic strength, whereas below it the reverse trend was true. Precisely the intersect pH was 3.6 for Haplic Acrisol and 4.5 for Rhodic Ferralsol, which was near the values of PZSE （soil point of zero salt effect） of these soils. The effects of ionic strength and pH on arsenate adsorption by these soils were interpreted by the adsorption model. The results of zeta potential suggested that the potential in adsorption plane becomes less negative with increasing ionic strength above soil PZSE and decreases with increasing ionic strength below soil PZSE. These results further supported the hypothesis of the adsorption model that the potential in the adsorption plane changes with ionic strength with an opposite trend to surface charge of the soils. Therefore, the change of the potential in the adsorption plane was mainly responsible for the change of arsenate adsorption induced by ionic strength on variable charge soils.

Preparation of Fe3O4-octadecyltrichlorosilane for Removal of Methyl Orange and Methylene Blue:Influence of p H and Ionic Strength on Competitive Adsorption

Fe3O4-octadecyltrichlorosilane（Fe3O4-OTS）was synthesized and used to remove dyes in a competitive system.Fe3O4-OTS was prepared by slow hydrolysis of OTS in cyclohexane on the surface of Fe3O4obtained through coprecipitation method.Scanning electron microscope（SEM）,energy dispersive spectrometer（EDS）,and contact angle analyzer（CA）were used to analyze the properties of Fe3O4-OTS.Methyl orange（MO）and methylene blue（MB）were selected as model molecules to study the influence mechanism of p H and ionic strength on competitive adsorption.The results of EDS and CA indicated that Fe3O4 was modified successfully with OTS on the surface.Silicon appeared and carbon content increased obviously on the surface of adsorbent.Contact angle of adsorbent increased from 0~o to 107~o after being modified by OTS.Fe3O4-OTS showed good separation for MO and MB in competitive system,which has potential to separate dyes in sewage.Separation factor（β~OB）changed from 18.724 to 0.017,when p H changed from 7 to 12,revealing that MO and MB could be separated almost thoroughly by Fe3O4-OTS.p H could change the surface charge of Fe3O4-OTS and structure of dyes,and thus change the interactions of competitive system indirectly.Even though hydrophobic interaction was enhanced,ionic strength reduced the difference of electrostatic interaction between dyes and Fe3O4-OTS.So it is unfavorable to separate dyes with opposite charges when ionic strength increases.These findings may provide theoretical guidances to separate two-component dye pollutants.

Adsorption Kineties of Pb￣(2+) and Cu￣(2+) on Variable Charge Soils andMinerals V .Effects of Temperature and Ionic Strength￣*1

The study results of the effects of temperature and ionic strength on the adsorption kineties of Pb￣2+ and Cu￣2+ bylatosol, red soil and kaolinte coated with Mn oxide showed that Pb￣2+ and Cu￣2+ adsorption by all samples, as awhole, increased with missing temperature. Temperature also increased both values of X_m (the amount of ionadsorbed at equilibrium) and k (kinetica constant) of Pb￣2+ and Cu￣2+. The activation energies of Pb￣2+ adsorption werekaolin-Mn >red soil>goethite and those of Cu￣2+ were latosol> red soil > kaolin-Mn >goethite. For a given singlesample the activation energy of Cu￣2+ was greater than that of Pb￣2+. Raising ionic strength decreased the adsorptionof Pb￣2+ and Cu￣2+ by latosol, red soil and kaolinite coated with Mn oxide but increased Pb￣2+ and Cu￣2+ adsorption bygoethite. The contrary results could be explained by the different changes in ion forms of Pb￣2+ or Cu￣2+ and in surfacecbarge characteristics of latosol, red soil, kaolin-Mn and goethite. Increasing supporting electrolyte concentration in-creased X_m and k in goethite systems but decreased X_m and k in kaolin-Mn systems. All the time-dependent data fit-ted the surface second-order equation very well.

Influence of particle size and ionic strength on the freeze-thaw stability of emulsions stabilized by whey protein isolate

The influence of particle size and ionic strength on the freeze-thaw(FT) stability of emulsions stabilized by whey protein isolate(WPI) was investigated in this study. The destabilization of emulsions during the FT process could be suppressed in a way by decreasing the particle size of the initial emulsions, which was the result of retarding the coalescence between oil droplets. To further improve the FT stability of emulsions, different amounts of Na Cl were added before emulsification. The emulsions with the ionic strength at 30–50 mmol/L exhibited good FT stability. Notably, the ionic strength in this range would not lower the freezing point of emulsions below the freezing temperature used in this study. Salt addition could improve the structural properties of proteins, which was available to strengthen the rigidity and thickness of interfacial layers, sequentially building up the resistance that the destruction of ice crystals to emulsions. Moreover, stronger flocculation between emulsion droplets could promote the formation of a gel-like network structure dominated by elasticity in the emulsion system, which might effectively inhibit the movement of droplets, and improve the FT stability of emulsions eventually. The result was of great significance for the preparation of emulsion-based foods with improved FT stability.

Protein adsorption onto diethylaminoethyl dextran modified anion exchanger:Effect of ionic strength and column behavior

Our previous studies on bovine serum albumin （BSA） adsorption to diethylaminoethyl dextran （DEAE dextran, DexD, grafting-ligand） and DEAE （D, surface-ligand） modified Sepharose FF resins found that all the grafted resins （FF-DexD and FF-D-DexD） exhibited extremely fast uptake rate （effective diffusivity, De, De/Do 〉 1.4）, which was six times greater than the ungrafted resins （De/Do 〈 0.3）. In this work, the influence of ionic strength （IS） on 6 typical DEAE dextran-grafted resins was investigated. Bath adsorption equilibria and kinetics, breakthrough, and linear gradient elution experiments were conducted. Commercial DEAE Sepharose FF was used for comparison. It is found that protein adsorption capacities on DEAE dextran-FF resins and the commercial resin decreased with increasing IS, but DEAE dextran-FF resins exhibited much higher capacity sensitivity to salt concentration. Besides, steeper decrease of adsorption capacities could be obtained at higher graftingligand or surface-ligand density. It is worth noting that the facilitating role of surface-ligand to the ＂chain delivery＂ effect was weakened after adding salt, leading to the less improvement in uptake rate by increasing surface-ligand density at higher IS. Although the uptake rates of the DEAE dextran-FF resins increased first and then decreased with increasing fS, they kept the extremely high level of De values （De/Do 〉 1.1 ） at the their working/binding IS range. Moreover, the DEAE dextran-FF resin displayed much higher adsorption capacities and De values than commercial ungrafted resin in their working condition. Furthermore, the column results of DEAE dextran-FF resins presented higher dynamic binding capacities than and similar elution ISs with DEAE Sepharose FF to achieve similar （or even higher） recoveries suggest the excellent chromatographic column performance of the DEAE dextran-FF resins. Finally, both high recovery and purity of BSA and γ-globulin could be easily achieved using the typical DEAE dextran-FF column, FF-D60-DexD160, to separate their binary mixtures, by step gradient elution. The research has provided new insights into the practical application of the series of DEAE-dextran grafted resins in protein chromatography and proved their superiority.

Oxidation of Solid Phase and Ionic Strength Effect to the Cesium Adsorption on Pumice Tuff

In-situ oxidation of solid phase was considered to investigate adsorption behavior under different geochemical parameters like pH, initial concentration and ionic strength. Pumice tuff, a potential host rock for low and intermediate radioactive wastes, has been affected by the redox zone. The characterization of the fresh and oxidized tuff was performed by X-ray diffractometer, scanning electron microscope and mercury intrusion porosimetry. In order to compare the difference of distribution coefficient (Kd) in fresh and oxidized pumice tuffs, a batch adsorption study was carried out at the range of pH (4 - 12), ionic strength (0.003, 0.1, 1.0 and 3.0 mol/dm3) and initial cesium concentration (10dž, 10LJ, 10Lj and 10lj mol/dm3). Based on experimental Kd values, ionic strength was found to be the most influential factor, whereas the effects of pH, initial Cs concentration and weathering condition of pumice tuff were negligible. The recalculated Kd values suggest that the existing surface complexation model is applicable to explain the sorption coefficients through the wide range of solution conditions.

Characterization of DNA Intercalator Bound to Calf Thymus DNA:Ionic Strength and pH Value Effects

By means of circular dichroism（CD） spectrum coupled with UV-Vis and fluorescence spectra,the binding model of DNA intercalator A1{4-（2-diethylamino-ethylamino）-8-oxo-8H-acenaphtho[1,2-b]pyrrole-9-carbonitrile} to calf thymus（CT） DNA was investigated,depending on the values of R（R is defined as the ratio of the concentration of A1 to CT DNA base pairs） and different outer factors.Molecules A1 were intercalated into the CT DNA base pairs in different orientations in the intercalation pocket at a lower R value（R≤0.20）,while A1 molecules aggregated on the surface of the helix of the CT DNA as the R value increased.The influence of NaCl on the binding was smaller because the electrostatic interaction only provided approximately 16% of the overall free energy of binding.The protonated diethylamine substitution would influence the binding geometry greatly at a low pH value via forming hydrogen bonds with the exposed C=O group on DNA surface.

Potentiometric Study of Dissociation Constants of Dihydroxybenzoic Acids at Reduced Ionic Strengths and Temperatures

The dissociation behavior of two dihydroxybenzoic acid isomers, 2,3-DHBA and 3,4-DHBA, at 281 K and 293 K was determined by potentiometric titrations in 0.01 M NaCl and 0.03 M NaCl. Results showed that the dissociation enthalpy for the carboxylic group in DHBA is close to zero, resulting in dissociation constants that do not vary appreciably with temperature, whereas the dissociation constants for the first hydroxyl group vary significantly with temperature. Increasing ionic strength was found to result in increased values for the second dissociation constant, whereas the effect on the first dissociation constant was less clear.

Adsorption and Transport of Ciprofloxacin in Quartz Sand at Different pH and Ionic Strength

Effects of pH and ionic strength on ciprofloxacin adsorption in quartz sand were studied through a batch equilibrium adsorption experiment in this paper. The experimental data were fitted by empirical formulas from Langmuir and Freundlich adsorption isothermal curves, and the transport experiments in quartz sand at different pH and ionic strength were conducted to investigate the transport characteristics of ciprofloxacin. It was found that with the increase of pH value or ionic strength, adsorption capacity of ciprofloxacin decreased, so that it could move easier. The results indicated that low pH or ionic strength was conductive to the adsorption of ciprofloxacin in quartz sand. Meanwhile, a higher initial concentration or stronger ionic strength could result in a smaller linear distribution coefficient of ciprofloxacin, which meant a low adsorption capacity. According to the fitting results, the adsorption of ciprofloxacin in quartz sand could be described well by both Langmuir and Freundlich equations, of which Freundlich equation had a better efficacy.

Transport of Phenol in BS-12 Modified Lou Soil in the Column Experiment: Effect of Concentrations, pH and Ionic Strength

Lou soil was modified by amphoteric surfactant in column experiment which was conducted. This study attempts to explore the transport of phenol to unmodified and modified soil and inquire into the effect of concentrations, pH, and ionic strength on phenol migration in the soil column. The parameters and breakthrough curves (BTCs) of transport were fitted by using the CXTFIT (version 2.1) model. The result of Cl?’s BTCs for non-reactive by determining the equilibrium model (EM) showed the retardation factor which was smaller than 1. The result of phenol’s BTCs by determining non-equilibrium model (NEM) was shown that the R-value increased while modification ratio increased, and the R was in order of CK (1.337) < 50BS-12 (4.085) < 100BS-12 (7.048). The lower to higher concentration of phenol didn’t affect to CK and 100BS-12 was able to block higher concentration. The effect of pH on transport to CK and 100BS-12 didn’t react and the average pore water velocity was decreased at pH = 4. The decreasing ionic strength of phenol migration on CK and 100BS-12 had effect and the average pore water velocity and retardation factor also decreased. The residual retention in soil was in order of CK < 50BS-12 < 100BS-12, and 100BS-12 could hold the amount of phenol than CK 7.21 times. Thus, amphoteric modified lou soil definitely blocks phenol migration and controls groundwater pollution.

Titrimetric Study of the Solubility and Dissociation of Benzoic Acid in Water: Effect of Ionic Strength and Temperature

The apparent acid dissociation constant (Kc) of benzoic acid in water has been determined ti-trimetrically under ionic strength values between 0.00 and 0.50 mol·L﹣1 at a range of temperatures between 16°C and 41°C. The thermodynamic dissociation constant (as pKa) of benzoic acid was determined as 4.176 at 25°C. No regular correlation between pKa of benzoic acid and the temperature in the range was used. The values of pKa are inversely proportional to temperatures between 16°C and 30°C. In this range of temperature, the values of thermodynamic quantities () for the dissociation process of benzoic acid in water were calculated by using Van’t Hoff plot. For this case the dissociation was not favoured through entropy and enthalpy changes. The values of pKa are directly proportional to temperatures between 30°C and 41°C.

Aqueous-phase formation of N-containing secondary organic compounds affected by the ionic strength

The reaction of carbonyl-to-imine/hemiaminal conversion in the atmospheric aqueous phase is a critical pathway to produce the light-absorbing N-containing secondary organic compounds(SOC).The formation mechanism of these compounds has been wildly investigated in bulk solutions with a low ionic strength.However,the ionic strength in the aqueous phase of the polluted atmosphere may be higher.It is still unclear whether and to what extent the inorganic ions can affect the SOC formation.Here we prepared the bulk solution with certain ionic strength,in which glyoxal and ammonium were mixed to mimic the aqueous-phase reaction.Molecular characterization by High-resolution Mass Spectrometry was performed to identify the N-containing products,and the light absorption of the mixtures was measured by ultraviolet-visible spectroscopy.Thirty-nine N-containing compounds were identified and divided into four categories(N-heterocyclic chromophores,high-molecular-weight compounds with N-heterocycle,aliphatic imines/hemiaminals,and the unclassified).It was observed that the longer reaction time and higher ionic strength led to the formation of more N-heterocyclic chromophores and the increasing of the lightabsorbance of the mixture.The added inorganic ions were proposed to make the aqueous phase somewhat viscous so that the molecules were prone to undergo consecutive and intramolecular reactions to form the heterocycles.In general,this study revealed that the enhanced ionic strength and prolonged reaction time had the promotion effect on the lightabsorbing SOC formation.It implies that the aldehyde-derived aqueous-phase SOC would contribute more light-absorbing particulate matter in the industrial or populated area where inorganic ions are abundant.

Probing the roles of pH and ionic strength on electrostatic binding of tetracycline by dissolved organic matters: Reevaluation of modified fitting model

The binding performance of dissolved organic matters (DOM) plays a critical role in the migration,diffusion and removal of various residual pollutants in the natural water environment. In the currentstudy, four typical DOMs (including bovine serum proteins BSA (proteins), sodium alginate SAA (polysaccharides), humic acid HA and fulvic acid FA (humus)) are selected to investigate the binding roles inzwitterionic tetracycline (TET) antibiotic under various ionic strength (IS ¼ 0.001e0.1 M) and pH (5.0e9.0). The dialysis equilibration technique was employed to determine the binding concentrations ofTET, and the influence of IS and pH on binding performance was evaluated via UVevis spectroscopy, totalorganic carbon (TOC), and Excitation-Emission-Matrix spectra (EEM), zeta potentials and molecule sizedistribution analysis. Our results suggested that carboxyl and phenolic hydroxyl were identified as themain contributors to TET binding based on the fourier transform infrared spectroscopy (FTIR) analysis,and the binding capability of four DOMs followed as HA > FA » BSA > SAA. The biggest binding concentrations of TET by 10 mg C/L HA, FA, BSA and SAA were 0.863 mM, 0.487 mM, 0.084 mM and 0.086 mM,respectively. The higher binding capability of HA and FA is mainly attributed to their richer functionalgroups, lower zeta potential (HA/FA ¼ 15.92/-13.54 mV) and the bigger molecular size (HA/FA ¼ 24668/27750 nm). IS significantly inhibits the binding interaction by compressing the molecular structure andthe surface electric double layer, while pH had a weak effect. By combining the Donnan model and themultiple linear regression analysis, a modified Karickhoff model was established to effectively predict thebinding performance of DOM under different IS (0.001e0.1 M) and pH (5.0e9.0) conditions, and the R2 oflinear fitting between experiment-measured logKDOC and model-calculated logKOC were 0.94 for HA and0.91 for FA. This finding provides a theoretical basis for characterizing and predicting the binding performance of various DOMs to residual micropollutants in the natural water environment.

Effect of saline water ionic strength on phosphorus recovery from synthetic swine wastewater

Declining worldwide phosphate rock reserves has driven a growing interest in exploration of alternative phosphate supplies. This study involved phosphorus recovery from swine wastewater through precipitation of struvite, a valuable slow-release fertiliser. The economic feasibility of this process is highly dependent on the cost of magnesium source. Two different magnesium sources were used for phosphorus recovery: pure magnesium chloride and nanofiltration(NF) saline water retentate. The paper focuses on the impact of ionic strength on phosphorus recovery performance that has not been reported elsewhere. Experimental design with five numerical variables(Mg/P molar ratio, pH, PO_(4)^(3-)-P, NH_(4)^(+)-N, and Ca^(2+) levels) and one categorical variable(type of magnesium source) was used to evaluate the effect of ionic strength on phosphorus removal and struvite purity. The experimental data were analysed using analysis of variance(ANOVA) and principal component analysis(PCA). Results indicated that a magnesium source obtained from NF retentate was as effective as MgCl_(2) for struvite precipitation. It was also revealed that ionic strength had a more positive effect on struvite purity than on phosphorus removal. Within the range of parameters studied in this research, high ionic strength, high p H and wastewater with high phosphate, high ammonium and low calcium contents were found to be the most favourable conditions for struvite precipitation. Findings from this study will be beneficial to determine the feasibility of using high ionic strength saline water, such as NF seawater retentate, as a magnesium source for phosphorus recovery from wastewater that is rich in ammoniumnitrogen and phosphate.

The Effect of Ionic Strength on the Electrochemiluminescence Generation by Tris(2,2′-bipyridyl)ruthenium(II)/Tri-n-propylamine

Electrochemiluminescence(ECL)is a powerful transduction technique used in biosensing and in vitro diagnosis,while the mechanism of ECL generation is complicated and affected by various factors.Herein the effect of ionic strength on ECL generation by the classical tris(2,2′-bipyridyl)ruthenium(II)[Ru(bpy)_(3)^(2+)]/tri-n-propylamine(TPrA)system was investigated.It is clear that the ECL intensity decreases significantly with the increase of ionic strength,most likely arising from the reduced deprotonation rate of TPrA^(+)·.We further combined microtube electrode(MTE)with ECL microscopy to unravel the evolution of ECL layer with the variation of ionic strength.At a low concentration of Ru(bpy)_(3)^(2+),the thickness of ECL layer(TEL)nearly kept unchanged with the ionic strength,indicating the surface-confined ECL generation is dominated by the oxidative-reduction route.While at a high concentration of Ru(bpy)_(3)^(2+),ECL generation is dominated by the catalytic route and TEL increases remarkably with the increase of ionic strength,because of the extended diffusion length of Ru(bpy)_(3)^(3+) at a reduced concentration of TPrA·.

Sorption of uranyl ions on TiO_2:Effects of pH,contact time,ionic strength,temperature and HA

Sorption of U(VI) onto TiO_2 as functions of pH, ionic strength, contact time, soil humic acid(SHA), solid-to-liquid ratio and temperature was studied under ambient conditions using batch and spectroscopic approaches. The sorption of U(VI) on TiO_2 was significantly dependent on pH and ionic strength. The presence of SHA slightly enhanced the sorption of U(VI) on TiO_2 below pH 4.0, while it inhibited U(VI) sorption in the higher pH range. U(VI)sorption on TiO_2 was favored at high temperatures, and the sorption process was estimated to be endothermic and spontaneous. Reduction of U(VI) to lower valent species was confirmed by X-ray photo-electron spectroscopy analysis. It is very interesting to find that U(VI) sorption on TiO_2 was promoted in solutions with higher back-ground electrolyte concentrations. In the presence of U(VI), higher back-ground electrolyte made more TiO_2 particles aggregate through(001) facets, leading more(101) facets to be exposed. Therefore, the reduction of U(VI) was enhanced by the exposed(101) facets and more U(VI) removal was observed.

Composite Model of Interracial Stepwise Ion/Coordination Particle Exchange-Electrostatic Exchange——Ionic Strength Effect on the Interaction Between Minor Metal Ion and Solid Particle in Seawater

It is the first time in experiment that we have found that the ionic strength effects on E(%)-pH curves of the systems with clay minerals as solid particles and those systems with hydrous oxides as solid particles, are 'right-left shift phenomenon' and 'left monodirectional shift phenomenon', respectively.The corresponding ionic strength effects on isotherms are 'downward-upward shift phenomenon' and 'upward monodirectional shift phenomenon', respectively.All the four phenomena are consistent with each other.In order to explain the above phenomena, the composite model of interracial ion/coordination particle exchange-electrostatic exchange is put forward to make a unified explanation and quantitative treatment.From the structures of the two kinds of materials——clay minerals and hydrous oxides, we know that the former has two kinds of exchange actions, i.e. when one increases,the other decreases, which makes the position of curves shift didirectionally,while the latter only has interfacial stepwise ion/coordin ation particle exchange,the curves of which shift therefore monodirectionally.The calculated results of the model accord with the experimental.

高庙子膨润土胶体释放影响因素试验研究

高放废物处置库中缓冲回填材料膨润土与地下水相互作用时会释放出胶体,而胶体会直接影响放射性核素的迁移行为,因此,膨润土遇水释放胶体研究是处置库系统安全评价的重要内容之一。本文以高庙子膨润土为研究对象,通过室内试验,获得了离子种类、离子强度、pH、温度与浊度的关系曲线,并对这些因素影响胶体释放的机理进行了分析。结果表明:阳离子种类抑制膨润土释放胶体排序依次为Li^(+)<Na^(+)<K^(+)<NH_(4)^(+)<Zn^(2+)<Mg^(2+)<H+<Ca^(2+);阴离子中除OH-对胶体释放有显著促进作用外,其他阴离子基本对胶体释放影响不大;溶液离子强度越大,抑制膨润土胶体释放的能力越强;膨润土在中性条件下更容易释放胶体;在20~50℃之间,升高温度有利于膨润土胶体的释放。

离子强度对乳清蛋白-透明质酸乳液凝胶结构和流变性能的影响

文章旨在探究不同浓度Na^(+)、Ca^(2+)对于乳清分离蛋白-透明质酸乳液凝胶结构和流变性能的影响。制备乳清分离蛋白-透明质酸复合物溶液,向其中分别加入不同浓度(25、50、100、200 mmol/L)的NaCl、CaCl2制成乳液凝胶;利用粒径电位、内源荧光测定溶液性质。通过测定乳液凝胶质构、持水性、流变性,激光共聚焦显微镜观察乳液凝胶网络结构。结果表明,加入离子使乳液凝胶具有更好的质构、流变性能、持水性和更均匀更紧凑的网络结构,提高了乳液凝胶稳定性。Na^(+)浓度为50 mmol/L时,溶液电位绝对值最高,凝胶质构、流变性能、持水性较好,此时乳液凝胶有良好的网络结构。Ca^(2+)浓度为50 mmol/L时,乳液凝胶质构性能最好、保水能力最强,有良好的乳液凝胶网络结构。为乳清蛋白基乳液凝胶的凝胶特性调控提供了技术支撑。

高强度离子导电凝胶研究进展

近年来,柔性可穿戴电子产品在人机交互、传感器等方面应用越来越广泛。离子导电凝胶具有高导电性、生物相容性、自愈合、良好的机械性能以及对可见光透明度高等特点,力学性能对离子导电凝胶的应用影响较大,通过添加纳米填料、无机盐和有机盐、聚电解质可以提升离子导电凝胶的力学性能。综述了纳米复合离子导电凝胶、盐离子导电凝胶、聚电解质离子导电凝胶的研究进展,对离子导电凝胶在传感器、柔性皮肤、生物医药等领域中的应用以及面临的困难和挑战进行了探讨,并展望了其未来发展方向。