Determination of salt-transport model parameters for leaching of saturated superficially salted soils

Analytical solutions were given for the problem of dissolution and leaching of salts in waterlogged regions with deep and shallow water tables under surface salinization and with account for equilibrium sorption (characterized by a linear exchange isotherm) and the problem of nonequilibrium irreversible sorption (characterized by biological transformation in the soil solution, which follows the first-order kinetics). A method was developed for determining the hydrochemical parameters (mixing step and dissolution rate coefficient of solidphase salts) from the average salt contents of water-saturated soils of given thickness before and after leaching determined under laboratory and field conditions.     

干旱区长期膜下滴灌农田耕层土壤盐分变化

新疆地处干旱地区,水资源短缺,膜下滴灌作为有效的节水灌溉技术在新疆农业生产中广泛使用。但新疆同时是盐碱化普遍发生的地区,采用滴灌技术后,传统灌溉的灌溉洗盐过程随之消失,且该地区灌溉水矿化度较高,对土壤盐分有一定的补充作用,因此滴灌技术虽然节水增产效果明显,但长期滴灌可能导致土壤盐分积累,降低土壤质量。通过监测一个长期膜下滴灌、面积约为224 km2的农场15a农田土壤盐分含量,对长期膜下滴灌土壤盐分含量的变化进行研究,研究结果可为膜下滴灌土壤盐分管理提供科学依据。以位于新疆北部玛纳斯河流域的新疆生产建设兵团第八师147团农田为研究对象,通过收集该团场自1996年至2010年记录的农田耕层盐分和养分指标的分析数据,利用统计学知识及地理信息系统技术,对不同滴灌年限下土壤耕层盐分变异特征及土壤耕层盐分与其他土壤指标的相关性进行研究。研究表明:(1)在滴灌的第一个3年周期内,土壤耕层盐分平均含量从3.13 g kg-1降低至3.00 g kg-1,降幅达4.2%,但经12年滴灌年限,土壤耕层盐分从3.13 g kg-1升高至4.81 g kg-1,升高了53.7%;(2)在五个监测周期(15 a)内,土壤盐分含量明显上升,研究区土壤耕层盐分含量集中分布在4~10 g kg-1,土壤耕层多为中度盐化土和重度盐化土;(3)长期膜下滴灌导致土壤耕层盐分发生重新分配,全团范围内农田盐分含量分布接近正态分布。本研究条件下,长期膜下滴灌会造成土壤耕层盐分积累,滴灌水携带的盐分对土壤盐分有一定的补充作用;采用2~3年进行一次大水洗盐以及培肥土壤、提高土壤有机质含量对防止滴灌土壤积盐具有重要作用。     

基于COMSOL软件的绿洲盐渍化土壤中多离子耦合运移模型构建

了解盐分离子在土壤中的迁移规律可以为盐渍土的综合治理以及高效利用提供科学依据,在COMSOL多孔介质和地下水流模块模拟非饱和土壤水流的基础上,自定义偏微分方程组构建盐渍化土壤SO42–、Ca2+、Na+、Cl–、Mg2+耦合运移模型,考虑阳离子交换过程以及硫酸钙的沉淀溶解反应,并通过新疆绿洲膜下滴灌田间试验对模型进行检验,对比不同活度系数估算方法对模拟结果的影响。结果表明,各离子模拟值与实测值吻合较好,平均相对误差介于9.15%~28.57%,决定系数介于0.41~0.88,该模型能够较好地反映土壤中盐分离子的动态变化规律;在膜下滴灌条件下,膜下0~40 cm土层的盐分离子有不同程度的淋洗,Cl–和Na+的淋洗效果好于Ca2+和SO42–;活度系数的估算对模拟结果的准确性有重要影响,尤其是盐分含量较高时,采用通用的函数关系可能会带来较大的模拟误差。     

非饱和土壤热质运移的数学模型

A system atic study of coupled heat and mass transfer in unsaturated soils under complex boundary conditions was carried out and a mathematical model of heat and mass transfer in unsaturated soils was established by non-equilibrium thermodynamic theory. The gradient of volumetric moisture content, the gradient of temperature, the salt mass concentration and vapor pressure were the primary driving forces influencing the process of heat and mass transfer in unsaturated soils. Based on the thermodynam ic analysis and the mass and energy conservation principles, a set of mass and energy equations were developed. The initial and boundary conditions of soil column for one dimension were also given out.     

Cation treatment and drying‐temperature effects on nonylphenol and phenanthrene sorption to a sandy soil

The objective of this study was to investigate the effects of mono‐ and polyvalent cations on sorption of the two hydrophobic compounds nonylphenol (NP) and phenanthrene (Phe). To this end, exchange sites of a sandy soil were saturated with either Na⁺, Ca²⁺, or Al³⁺ and excess salts were removed by washing. The samples were then sterilized and either stored moist, dried at room temperature, or at 20°C, 60°C, or 105°C in a vented oven. Saturation with Na⁺ led to an increase of dissolved organic C (DOC) concentration in the soil water extracts, whereas the polyvalent cations Ca²⁺ and Al³⁺ decreased it. The ¹H‐NMR relaxometry analyses showed that Al³⁺ restricted the mobility of water molecules that are confined within the SOM structure to a higher extent than Ca²⁺ or Na⁺. According to contact‐angle (CA) analyses, cation treatment did not significantly change the wetting properties of the samples. Batch sorption–desorption experiments showed no clear salt‐treatment effects on the sorption and desorption equilibria or kinetics of NP and Phe. Instead, the sorption coefficients and sorption hysteresis of NP and Phe increased in dry soil. With increasing drying temperature the CA of the soils and the sorption of both xenobiotics increased significantly. We conclude that structural modifications of SOM due to incorporation of polyvalent cations into the interphase structure do not modify the sorption characteristics of the soil for hydrophobic compounds. Instead, increasing hydrophobization of organic soil constituents due to heat treatment significantly increased the accessible sorption sites for nonpolar organic compounds in this soil.     

水洗处理对秸秆生物质炭吸附解吸Cd2+和Pb2+的特性影响

水洗处理在不影响生物质炭性质的前提下,可以去除附着在其表面的热解副产物,从而保证对重金属离子的去除能力。以小麦和玉米秸秆为原料,比较两种秸秆类生物质炭对溶液Cd2+和Pb2+的吸附解吸特点及其水溶性盐分含量的影响。结果表明,小麦和玉米秸秆生物质炭对Cd2+和Pb2+的吸附过程均更好地符合准二级动力学方程和Langmuir方程。小麦秸秆生物质炭对Cd2+和Pb2+的最大吸附量达12.82 mg g?1和9.91 mg g?1,为玉米秸秆吸附量的1.31 ~ 1.76倍和1.06 ~ 1.53倍。洗脱水溶性盐分可以降低生物质炭对Cd2+和Pb2+的吸附,水洗后小麦秸秆和玉米秸秆生物质炭对Cd2+的最大吸附量分别降低42.36%和60.13%,对Pb2+的最大吸附量分别降低29.47%和62.72%。水洗处理提高了两种秸秆生物质炭对Cd2+和Pb2+的解吸率,其中小麦秸秆生物质炭提高幅度较大,由原来对Cd2+的解吸率为1.84% ~ 13.05%提高到7.88% ~ 20.19%,对Pb2+的解吸率为1.57% ~ 11.82%提高到6.34% ~ 16.94%。因此,可溶性盐分在秸秆生物质炭吸附Cd2+和Pb2+的过程中具有重要作用,该研究结果将为制备高效修复重金属污染土壤的生物质材料提供技术支撑。     

重庆北碚区土壤水热耦合模型研究

以南方的土壤和气候条件为背景,在裸露土壤的非恒温模型一维非饱和水流运动方程后增加根系吸水函数项得到作物地的土壤一维非饱和水流运动方程,与土壤中非稳态热流方程构成作物地的土壤水热耦合运移模型,联合初始边界条件,采用隐式差分法进行数值求解.结果表明：该水热运移耦合模型模拟值与实测值基本吻合,其拟合精度随土层深度增加而提高.     

利用放射性I131和S35研究松沙土土体和地下水盐分的运动

盐碱土表土盐分积累的来源,不仅与地下水埋深及其矿化度有关,而且与土体盐分再分配有关。本试验是利用放射性I¹³¹(NaI¹³¹)和S³⁵(Na₂S³⁵O₄)的示踪方法,探讨在含盐地下水正常补给情况下,土体和地下水盐分运行的一些规律,有助于全面了解表土积盐的过程。     

Describing the effect of time on sorption of phosphate by iron and aluminium hydroxides

Iron and aluminium hydroxides were precipitated both in the presence and absence of kaolinite. The reaction between phosphate and these hydroxides was measured for periods which ranged from 5 min to 72 h. The effect of time on phosphate sorption was examined by plotting the sorption data according to different, simple, kinetic equations such as the first order, second order, the parabolic diffusion equation, the Elovich equation and the modified Freundlich equation. The effect of time on sorption was also examined by the mechanistic model recently developed by Barrow (1983b) for the sorption and desorption of phosphate by soils. The sorption of phosphate by iron and aluminium hydroxides increased with time and the reaction continued through the period of observation without reaching a true equilibrium. Curvilinear relationships were obtained when the data were plotted according to the simple kinetic equations. These simple kinetic equations fail to describe the effect of time on sorption partly because the mechanism is different from that postulated and partly because they do not consider electrostatic effects when phosphate ions react with a charged surface. The mechanistic model of Barrow (1983b), which takes this effect into account, described effect of both concentration and time on phosphate sorption. According to this model, the increase in phosphate sorption with time was caused by a redistribution of adsorbed phosphate into the interior of the particles of iron and aluminium hydroxides by solid-state diffusion.     

Mathematical simulation of water and salt transfer in geosystems of solonetzic soils in the Northern Caspian region

The Dzhanybek two-dimensional radial-axial mathematical model was developed for water and salt transfer in geosystems of solonetzic complexes of the Northern Caspian region; the model is capable of considering the geochemical links and revealing the features of migration processes between the conjugated elements of the microcatena. The simulation results suggested that the stabilization of salinization-desalinization processes occurs under stable weather conditions within approximately 100 years. When the weather conditions changed (the total moisture pool of the area increased from 1978), the simulation results indicated a tendency toward salinization of dark-colored soils in microdepressions and removal of salts in the upper 1-m thick soil layer on microhighs and microslopes. Predictions for 2040 showed that a deep accumulation of salts in microdepressions and desalinization of soils of microhighs and microslopes will occur under the current weather conditions. Thus, the changes in the halogeochemical capacity of geosystems of solonetzic complexes primarily depend on the climatic conditions, although the capacity value remains almost constant with increasing total water reserves; the changes occur only between the conjugated soils of solonetzic complexes, which is of great importance for predicting the soil-geochemical status of the entire landscape.     

Experimental studies and physically substantiated model of carbon dioxide emission from the exposed cultural layer of Velikii Novgorod

The results of quantitative assessment and modeling of carbon dioxide emission from urban pedolithosediments (cultural layer) in the central part of Velikii Novgorod are discussed. At the first stages after the exposure of the cultural layer to the surface in archaeological excavations, very high CO₂ emission values reaching 10–15 g C/(m² h) have been determined. These values exceed the normal equilibrium emission from the soil surface by two orders of magnitude. However, they should not be interpreted as indications of the high biological activity of the buried urban sediments. A model based on physical processes shows that the measured emission values can be reliably explained by degassing of the soil water and desorption of gases from the urban sediments. This model suggests the diffusion mechanism of the transfer of carbon dioxide from the cultural layer into the atmosphere; in addition, it includes the equations to describe nonequilibrium interphase interactions (sorption–desorption and dissolution–degassing of CO₂) with the first-order kinetics. With the use of statistically reliable data on physical parameters—the effective diffusion coefficient as dependent on the aeration porosity, the effective solubility, the Henry constant for the CO₂ sorption, and the kinetic constants of the CO₂ desorption and degassing of the soil solution—this model reproduces the experimental data on the dynamics of CO₂ emission from the surface of the exposed cultural layer obtained by the static chamber method.     

Modern state of soil salinity in solonetzic soil complexes at the Dzhanybek Research Station in the North Caspian Region

Variations in the salinity of virgin soils of solonetzic soil complexes at the Dzhanybek Research Station are characterized on the basis of field materials obtained by the author in 2002–2004. The soil salinity is characterized with respect to the depth of the upper boundary of salt-bearing horizons, the total amount of salts, the content of toxic salts, and the chemical composition of salts. Changes in the soil salinity under dry farming conditions are estimated for the following soil management practices: (1) agroforest amelioration with additional moistening of virgin soils owing to snow retention by the adjacent shelterbelts and forest plantations, (2) intense grazing, (3) soil fallowing after normal tillage to a depth of 20–25 cm, and (4) soil fallowing after deep tillage to a depth of 40–50 cm. It is shown that normal tillage and considerable grazing pressure do not affect the salinity of the studied soils. No definite effect of the shelterbelts on the salt status of the adjacent virgin soils of the solonetzic complex has been revealed. Deep soil tillage strongly affects the salt status of solonetzes: the content of toxic salts in the upper meter considerably decreases. Virgin solonetzes are usually moderately or strongly saline soils, whereas deeply tilled solonetzes are slightly or moderately saline soils.     

MECHANISMS OF PHOSPHATE SORPTION BY SOILS AND HYDROUS FERRIC OXIDE GEL

Data for the sorption of added inorganic phosphate (P) by contrasting soils and iron oxide gel were resolved by a method of successive approximation of the Langmuir sorption constants. Three distinct Langmuir equations described the overall isotherm irrespective of the experimental conditions used. Free energies of sorption (ΔG) for a particular region were similar for each soil and for iron oxide gel under all experimental conditions. In contrast, the sorption maximum for each region was influenced by the sorbent and experimental conditions. Because of the pronounced similarities in their P sorption characteristics iron oxide gel was used as a model surface for P sorption by soils. Three mechanisms for P sorption by iron oxide gel and by soils are proposed: (i) chemisorption at protonated surface sites, (ii) chemisorption by replacement of surface hydroxyls, and (iii) a more-physical sorption of P as a potential-determining ion.     

数值模拟盐渍土盐分的季节变化

本文用数值模拟方法对土壤的盐分动态进行了周年的模拟计算.计算中考虑了盐分的沉淀与溶解,地下水位随时间变化等情况.计算结果表明:(1)数值模拟方法能反映季风区土壤盐分的季节变化;(2)雨季的集中降雨更有利于土壤脱盐;(3)地下水位的变化与气象因素的变化相配合影响着土壤水盐运动.     

Effects of equilibrating salt solutions on phosphate sorption by soils

Abstract

Several equilibrating salt solutions have been used in the studies of P sorption by soils and sediments. This study was conducted to evaluate the effects of 10 salt solutions on estimation of P sorption by soils. Results obtained showed that, when the equilibrating solution was made to contain 0.01M with respect to CaCl₂, Ca(NO₃)₂, CaSO₄, MgCl₂, KCl, LiCl, Nacl, or KHCO₃, the amount of P sorbed by soil always exceeded the amount sorbed from the soil‐water system. In comparison with the amount of P sorbed from water, 0.01M NaHCO₃ reduced P sorption by soils. Use of THAM buffer (0.05M pH 7.0) to control the pH increased P sorption by some soils and decreased P sorption by others, relative to that sorbed from the soil‐water system. The results indicated that inclusion of salts in the equilibrating solution for P‐sorption studies should be avoided, especially in studies related to water quality.     

SOME NEW EQUATIONS TO DESCRIBE PHOSPHATE SORPTION BY SOILS

Phosphate sorption was studied on surface and subsurface soils sampled from the old field experiments at Askov, Denmark. Two empirical three-parameter equations, an extended Freundlich and an extended Langmuir equation are proposed and compared with the Langmuir, the Freundlich, the ‘double’ Langmuir, the Gunary, and the Fitter-Sutton equations. In the proposed equations the affinity parameter of the Langmuir equation and the exponent of the Freundlich equation are replaced by the term Bc ^?D the value of which decreases with increasing phosphate concentration, c. On average the Freundlich equation thus modified yielded the closest fit to the sorption data. This was followed by the Fitter-Sutton, the modified Langmuir, the ‘double’ Langmuir, the Gunary, the Freundlich, and finally the Langmuir equation. Out of the three equations that yielded the closest fit to the sorption data the correlation between the parameters within the equations, furthermore, was least for the proposed extended Freundlich equation. Therefore, this equation may be generally suitable for describing phosphate sorption by soils.     

季节性冻融土壤盐分离子组成与冻结层盐分运移规律研究

该文对冻融过程中土壤盐分离子组成及冻结层盐分运移规律进行研究,在内蒙古河套灌区永联试验站开展了冻融期土壤水盐及其离子成分监测试验,分析了冻融期地温、冻结层深度、地下水埋深与水质、土壤含水率、土壤盐分及离子组成的变化规律,通过离子相关性分析确定了土壤盐分运移的主控离子成分和盐分类型,进一步利用二元水盐体系相图探讨了冻融期主控盐分的运移规律。结果表明:冻融期地温梯度变化主要发生在0～1.0 m范围土层中,地下水埋深在冻融期变化趋势为快速增大-缓慢增大-减少,地下水矿化度均值在融化期显著降低;研究区地下水中变异性最大的离子为Na~+、Cl~–和SO_4~(2–),土壤盐分运移和扩散是地下水矿化度变化的主要原因;土壤中Na~+、Cl~–与SO_4~(2–)与含盐量相关系数高于0.9,冻融期土壤盐分浓度变化的主控盐分类型为氯化钠和硫酸钠;冻结层积盐或者脱盐取决于土壤盐分梯度和不同盐分的共饱和点,研究区最大氯化钠浓度(质量分数1.55%)和最大硫酸钠浓度(2.01%)均低于各自的共饱和点,当冻结前土壤溶液浓度梯度为正(从上到下浓度增大)时,冻结层易积盐,反之冻结层主要表现为脱盐。研究对阐明冻融期冻结层盐分累积规律的成因具有重要意义。     

A MODEL FOR PREDICTING THE REDISTRIBUTION OF SALTS APPLIED TO FALLOW SOILS AFTER EXCESS RAINFALL OR EVAPORATION

A model is described for predicting the movement of soluble unadsorbed anions (such as nitrate or chloride) in fallow freely drained soil under field conditions. The model includes routines for estimating both the downward leaching of salts (after excess rainfall or irrigation) and the capillary movement of anions to the soil surface (after evaporation). The profile is divided into layers each of which is characterized by a maximum and minimum water content (the field capacity and evaporation limit respectively). Daily amounts of rainfall and evaporation are applied to the surface and the redistribution of water and salts is calculated (on a layer-to-layer basis) from the initial water and salt contents of each layer by adding or subtracting water to or from the moisture content until the stated maximum or minimum value is reached. Salt transfer is calculated from the amount af water movement on a proportional basis. The predictions of the model have been tested using field data for the redistribution of nitrate and chloride in a sandy loam soil. Good agreement was obtained for chloride, but the results for nitrate showed some deviations probably due to the effects of denitrification.