Testing a mechanistic model. II. The effects of time and temperature on the reaction of zinc with a soil

Samples of a soil were mixed with zinc nitrate solutions and incubated from 1 to 30 days at temperatures from 4 to 60°C. The solution concentration of zinc, which would not have changed on brief mixing with the soil at 25°C, was measured. Background electrolytes for this measurement of null-point concentration were both calcium and sodium nitrate. The effect of the temperature at which null-point concentration was measured was also investigated. After incubation with zinc nitrate, desorption of zinc, and sorption of further zinc, were measured. Null-point concentration of zinc decreased with increasing period of incubation, with the rate of decrease greatest at high temperatures of incubation. The effects of both temperature and time were closely described by a model which postulated an initial rapid adsorption of ZnOH⁺ ions onto heterogenous charged surfaces, followed by a diffusive penetration. Increasing the temperature of incubation increased the rate of diffusive penetration and led to low solution concentrations. In contrast, increasing the temperature at which null-points were measured increased the concentration of ZnOH⁺ ions. This was shown to be consistent with a change in position of the equilibrium of the initial, rapid, adsorption reaction. Curves for desorption of zinc were continuous with curves for sorption of further zinc, but neither desorption nor further sorption coincided with the position of the curves relating retention of previously added zinc to concentration. This result was consistent with the model and occurred because desorption must reverse diffusive penetration. However, the model under-predicted the magnitude of both desorption and sorption of further zinc. Desorption in calcium solutions was greater than in sodium solutions, even when the solution concentration of zinc approached zero. This was consistent with exchange diffusion of calcium ions for some of the penetrated zinc.     

Testing a mechanistic model. VIII. The effects of time and temperature of incubation on the sorption and subsequent desorption of selenite and selenate by a soil

Several levels of both selenite and selenate were incubated with separate samples of soil for periods of up to 30 d and at temperatures between 5 and 60°C. The concentrations of selenite or of selenate which caused neither desorption nor further sorption–that is, the null-point concentrations–were then measured at 25°C. In addition, the rate of desorption was measured after incubation at 60°C for 10 d. The ability of a mechanistic model to describe the results was tested.
There were large decreases in the null-point concentrations of selenite with both increasing period and increasing temperature of incubation. These effects were modelled as due to a relatively rapid diffusive penetration and a large activation energy for diffusion. Only a small proportion of the sorbed selenite was desorbed within 3 d but, at large solution:soil ratios, desorption appeared to be still continuing. These effects were fairly well predicted by the model applied to the sorption data. For example, the continuing desorption was ascribed to the slow reversal of the diffusive penetration. For selenate, the effects of period and temperature of incubation were much smaller. These effects were modelled as due to a slower diffusive penetration and a lower activation energy. Desorption was quicker and was more nearly complete. According to the model, a large proportion remained in the adsorbed form and was more quickly removed when desorption was induced. It is argued that the behaviour of selenite is consistent with diffusion into a crystal and the large activation energy is required to enable jumps over energy barriers. On the other hand, the lower activation energy for selenate is consistent with diffusion being limited to pores or cracks.
It is suggested that the residual value of selenite fertilizers would decrease because of the continuing reaction, but that this effect would be unimportant for selenate.     

Testing a mechanistic model. III. The effects of pH on fluoride retention by a soil

The pH of a soil was altered by incubating it with either lime or acid at 60°C for 1 day. Subsamples were then mixed with fluoride solutions in order to measure the effects of pH on fluoride retention. The results were compared with those previously obtained with phosphate on the same soil. At equal concentration of total fluorine in solution, fluoride retention was greatest at about pH 5.5 and decreased at both lower and higher pH. The decrease at low pH appeared to be caused by the formation of complexes between fluoride and aluminium in solution. As a result, only a small proportion of the total fluorine in solution was present as fluoride ions. At equal concentrations of fluoride ions, fluoride retention decreased with increasing pH. It was shown that this decrease could be explained by decreases in the electrostatic potential of the variable charge materials. The decrease in potential was steeper than that required to describe phosphate retention. This is consistent with the plane of adsorption of fluoride ions being closer to the plane of adsorption of H⁺ and OH^? ions.     

Testing a mechanistic model. IV. Describing the effects of pH on zinc retention by soils

Samples of two soils were incubated at 60°C for 24 h with several levels of either calcium carbonate or hydrochloric acid. Zinc retention was then measured on subsamples of the treated soil over 24 h at 25°C. The results were compared with published experiments (Bar-Yosef, 1979; Harter, 1983) in which zinc retention was also measured over a range of concentrations and pH values, but using different experimental conditions. Zinc retention increased as pH increased. In all cases, this effect could be described by assuming that the ZnOH⁺ ion was retained and the effect of pH was due to the increased proportion of this ion in solution. Over most of the pH range it was not necessary to assume any contribution from a decrease in the electrostatic potential of the reacting surfaces with increasing pH. This contrasts with the effects of pH on phosphate and fluoride retention and suggests that the materials that react with zinc differ from those that react with anions. The shape of plots of zinc retention against ZnOH⁺ concentration was reproduced using a model in which it was assumed that there was a range of values of electrostatic potential. A similar model had previously been used for anions. Zinc would tend to react with the most negative end of this range and phosphate with the least negative end. This further suggests that zinc and anions may react with different materials.     

应用多级反应动力学模型研究土壤甲氨基酚吸附

Metolachlor retention on a Sharkey clay soil was quantified using a kinetic batch method for different initial solution concentrations.Time-dependent adsorption was carried out by monitoring solution concentration at different reaction times.Adsorption was kinetic multireaction model which includes reverible and irreversible retention processes of the equilibrium and kinetic types,The predictive capability of the model for the dexcription of experimental results for metolachlor retention was examined and proved to be adequate。     

Influence of methanol on the dynamics of the retention and release of cyprodinil by an agricultural soil

The influence of methanol on the adsorption of the fungicide cyprodinil by a crop soil was studied by equilibrium measurements and by determining the retention-release dynamics in a continuous stirred flow tank reactor (CSTR). Equilibrium measurements showed the effective coefficient of partition of cyprodinil between soil and solution, K(dc), decreases linearly as the concentration of methanol in the solution increases until a percentage of 20% is reached. In CSTR experiments, the retention of cyprodinil was found to be almost reversible; up to a 95% of the fungicide was desorbed. The retention-release dynamics showed biphasic behavior and was partially controlled by diffusion. This behavior was reproduced by a model of diffusion into micropores identifying the soil particles as spheres and taking into account both intraparticle nonlinear adsorption and nonlinear adsorption at external surfaces. In all cases, the sorption kinetics was not the limiting step. The main effect of methanol in the retention-release dynamics ended up being based on the changes produced in the adsorption equilibrium. Methanol also increased the effective diffusion coefficient and decreased the mass transfer coefficient. The optimized Freundlich's isotherm coefficients for <5% methanol were lower than those obtained from the batch experiments.     

Anion Adsorption and Desorption Characteristics of a Piedmont Ultisol: Some Implications for the Fate of Sulfate Deposition

Batch-type experiments were used to determine the anion (fluoride, nitrate, sulfate, and phosphate) adsorption and desorption characteristics of an Ultisol from the Piedmont Province of Georgia. The results were best described using ‘initial mass’ isotherms when the initial anion concentration was less than two meq l^-1. The Langmuir adsorption maximum for sulfate (initial concentration range of 0.2–20 meq l^-1) was 24.4 meq kg^-1. Oxalate, acetate, nitrate, fluoride, and phosphate were used as exchangeable ‘counter’ anions to investigate the sulfate retention characteristics of this soil. High concentrations (> one meq l^-1) of oxalate, phosphate and fluoride were shown to release sulfate from this soil. However, little sulfate desorption resulted when lower and more environmentally realistic concentrations of competing anions were applied. Application of oxalate revealed that a minimum of 19 meq kg^-1 of sulfate has been adsorbed on this soil through natural processes. The Langmuir adsorption maximum suggests that this Ultisol, typical of many B horizons within this region, has a large capacity to adsorb additional inputs of atmospheric sulfate.     

Effects of ionic strength and nature of the cation on the desorption of fluoride from soil

The reaction between soil and added fluoride was accelerated by incubating at a high temperature. Desorption of the fluoride was then studied using solutions of chloride salts of several cations at a range of solution : soil ratios and for periods which ranged from 1 h to 4 days. Fluoride desorbed was related to the experimental variables by a regression equation. When the solution : soil ratio was small and hence only small amounts of fluoride were desorbed, decreasing the concentration of salts increased the concentration of fluoride in the solution. The concentration in the solution was lower for calcium chloride than for sodium or potassium chloride. Amongst the monovalent cations, the concentration of fluoride was highest for salts of the cations of lowest atomic number. Thus the greater the average distance between the charge conveyed to the surface by the adsorbed fluoride and the cation which balanced it, the higher the fluoride concentration in the solution. As the solution : soil ratio was increased, the differences between the cations in their effects on fluoride desorption decreased and seemed to disappear as the solution : soil ratio became very large. This contrasts with previously observed effects on phosphate. It is suggested that the difference may have arisen because appreciable desorption of fluoride occurs by exchange with hydroxyl ions rather than by escape of the fluoride ion together with its counter ion.     

The effect of time on the competition between anions for sorption

Solutions of phosphate and of selenite were gently mixed with a soil, both separately and in combination, for periods ranging from 15 min to 30 days. For both anions, the solution concentration continued to decrease throughout the period, but the decrease was more marked for phosphate. Competitive effects were smallest after brief periods of mixing, and increased with time. Phosphate was a more effective competitor for sorption than selenite, and its competitive advantage increased with time. The observed effects were closely described by a mechanistic model. According to the model, competition was largely through changes in the electric potential of the surface rather than through decreases in the number of vacant adsorption sites. This explained why competition effects were initially small. This was especially so for phosphate, which was modelled as having a marked continuing diffusive penetration of the surface. The decrease in electric potential associated with that penetration decreased the surface concentration of selenite and so decreased the rate of penetration of selenite. By the end of the experiment this was the most important aspect of the competitive effect of phosphate on selenite.     

Diffusive Mobility of Fluoride in Soil: Some Mechanistic Aspects

Abstract

The diffusive mobility of fluoride (F) in soil (Entisol heplaquent) has been investigated as a function of soil water content and F‐concentration gradient. The data on F‐diffusion coefficients were recorded by determining the F‐concentration profile in horizontally incubated and homogenized soil columns, under varied experimental conditions (volumetric soil water content (θ)=0.12?0.25 cm³ cm^?3, initial concentration of F at source ([F]^t=0)=8?32 µM/g, pH=7.6±0.5, incubation time (t)=96 and 384 h, temperature=303±1.2 K). The effects of aluminum (Al) and calcium (Ca) ions, which are known to be strong binders for F in soil, on the F mobility have also been investigated by observing F diffusion in soils amended with externally added Al and Ca. The study suggests that F diffusion in soil increases with increase in θ and [F]^t=0. Further, when the soil was amended with Ca, F diffusion was observed to decrease regularly with the increase in Ca amount. In Al‐amended soil, when Al concentration is raised, F diffusion increases, at 96 h incubation time but decreases at 384 h incubation.     

Testing a mechanistic model. VII. The effects of pH and of electrolyte on the reaction of selenite and selenate with a soil

The pH of samples of a soil was altered by mixing them either with acid or lime, and incubating the moistened samples at 60°C for a day. The sorption of selenate and of selenite was then measured using as background electrolytes, 0.01 M, 0.1 M and 1.0 M sodium chloride and also 0.01 M calcium chloride. The results were compared with previous studies with phosphate and fluoride. Selenite was sorbed more strongly than selenate, but not as strongly as phosphate or fluoride. Sorption of both selenite and selenate decreased with increasing pH. This decrease was more marked for selenate than for selenite; more marked in a sodium system than in a calcium one; and more marked with a dilute background electrolyte than a concentrated one. Under certain conditions, the steeper curves for the dilute electrolyte crossed the curves for the concentrated electrolyte giving points of zero salt effect. For selenite, these points of zero salt effect occurred near pH 6 and the greater the sorption the lower the pH for zero salt effect. For phosphate, the analogous value was near pH 5. For selenate, if a point of zero salt effect occurred, it was at such a high pH and such a low amount of sorption that it could not be measured. Thus, the larger the amount of sorption the lower the pH for the point of zero salt effect. This generalization applied both within and between different kinds of sorbates. The results were closely described by a model that had previously been applied to phosphate and fluoride. The model postulates that ions react with charged surfaces. The electric potentials of the reacting surfaces are affected by the identity and concentration of the background electrolyte and this produces the interactions between pH and electrolyte concentration. The model also postulates that there is a distribution of electric potentials. Anions react with surfaces which occur in the more positive tail of this distribution. The smaller the amount of the reaction the more positive the potential of the reacted surface and, therefore, the higher the pH required to decrease this potential to zero.     

萘在土壤上的吸附行为及温度影响的研究

采用批量平衡法研究了多环芳烃萘在塿土耕层土、塿土粘化层土、塿土古土壤、黄壤、紫色土和石灰土上的吸附行为,比较了不同吸附模型方程对实验数据的拟合情况,并探讨了温度对萘在土壤上吸附行为的影响机理及吸附热力学特征。Henry模型、Freundlich模型和deBoer-Zwikker极化模型均能较好地拟合萘的吸附等温线;45℃下的吸附等温线明显地高于25℃的吸附等温线,表现出随温度升高吸附量增大的趋势。Freundlich模型的吸附容量参数Kf和deBoer-Zwikker模型的起始吸附势ε0在两个温度下有极显著的差异;但Freundlich模型的n参数表征了土壤颗粒表面的性质,对温度变化不敏感。在25℃条件下,萘在土壤有机碳上的分配可能是吸附的主要机制,但随着温度增加,吸附机制变得复杂。土壤对萘的吸附是一吸热反应,整个吸附体系中熵增是吸附作用进行的主要驱动力。     

Testing a mechanistic model. V. The points of zero salt effect for phosphate retention, for zinc retention and for acid/alkali titration of a soil

The pH of samples of a soil was altered by adding acid or lime and incubating the moistened soil at 60°C. The effect of varying the concentration of salt on pH, retention of phosphate, and retention of zinc was then measured. At low pH, increasing the concentration of salt decreased phosphate retention; at high pH, it increased it. The pH at which the effects crossed over (that is, the point of zero salt effect on phosphate retention) was higher than the point of zero salt effect on pH. This is opposite to effects observed with uniform surfaces. These results were described by a model in which it was assumed that individual sites varied in their electrostatic potential and that phosphate was retained preferentially by sites with the highest potential. Zinc retention was decreased by high concentrations of salt. This was partly because of effects of salt in decreasing the pH of solutions in contact with soil. There was no indication of a crossing-over of effects at low pH. This suggested that the electrostatic potential of zinc-retaining sites did not vary much with pH.     

Testing a mechanistic model. XI. The effects of time and of level of application on isotopically exchangeable phosphate

A model for the reaction of ions with soil was improved to permit time trends to be followed at a given level of phosphate addition. Difference equations were also developed to describe the rate of reaction of ions with both vacant sites and occupied sites, while diffusive penetration of the surface was occurring. The model was applied to data for the effects of time and of level of application on exchangeable phosphate. Many of the observed values for isotopically exchangeable phosphate could be well-described if it was assumed that equilibration of ³²P with surface sites was very rapid and this was followed by a diffusive penetration into the adsorbing particles. However, for short periods of contact between soil and ³²P, it was necessary to also take into account the rate of the reaction between ³²P and surface sites. This reaction was largely with vacant sites. Reaction with occupied sites–that is, true exchange–was unimportant. It is suggested that the electric potential of the surface may determine whether reaction is with occupied or vacant sites. In contrast to reaction of ³²P with occupied sites, reaction with vacant sites involves a net transfer of charge. Reaction with vacant sites would be slow if the potential was large and negative. It is shown that when reaction with vacant sites is slow, the proportion of previously added ³¹P recorded as exchangeable increases with level of addition of ³¹P. This may explain published observations of slow and non-linear exchange in some soils.     

东乌珠穆沁旗草原砂土对汞的吸附特征及其影响因素

以东乌珠穆沁旗的草原砂土为研究对象,采用正交试验设计的方法,研究了土壤/溶液(固/液)、初始汞浓度、溶液pH、腐殖酸、硝酸钠浓度对东乌珠穆沁旗草原砂土吸附HgCl2的影响。结果表明:固/液越小,初始汞浓度越大,土壤吸附能力越强;土壤对汞的吸附量随溶液pH,外源腐殖酸浓度,硝酸钠浓度的增大而呈现先增大后减小的趋势,当溶液pH为7.5,外源腐殖酸浓度为0.6mg/g,硝酸钠浓度为0.08mg/L时,土壤对汞的吸附量分别达到最大。影响砂土吸附HgCl2的因素大小依次为初始汞浓度>固/液>pH>硝酸钠浓度>腐殖酸浓度。草原地区砂土对汞的吸附动力学过程与Lagergren准二级动力学模型拟合有较好的相关性,表明该吸附过程主要以化学吸附为主,且吸附过程不可逆。     

湖北恩施几种典型土壤对氟的吸附与解吸特性

采用室内试验方法,研究了恩施六种土壤氟吸附的特性。结果表明:(1)不同土壤的吸附量差异很大,表现为黄粘泥水田土>红粘壤土>泥质岩黄壤土>红砂泥水田土>中性紫色土>黑色石灰土;同一土壤的吸附量随氟离子初始浓度的增大而增大。不同土壤的解吸量在低浓度时差异不明显,高浓度时表现为黄粘泥水田土、红粘壤土、泥质岩黄壤土、红砂泥水田土>中性紫色土>黑色石灰土;同一土壤的解吸量随氟离子初始浓度的增大而增大。(2)Langmuir公式可以很好地描述土壤氟吸附的特性,Freundlich公式能够较好地描述土壤对氟的吸附。(3)去除铁、铝氧化物后土壤氟吸附量明显降低;草酸能够促进土壤对氟的吸附;共存PO43-能够抑制土壤对氟的吸附。     

Hydrous Ferric Oxide (HFO)—a Scavenger for Fluoride from Contaminated Water

Groundwaters contaminated with excess fluoride (above 1.5 mg/L) posed some risks to the public health in India. Methods available for fluoride contaminated water treatment are cogaulation–filtration using alum + lime + bleaching powder (Nalgonda technique) and adsorption using activated alumina. Use of aluminium compounds for water treatment purpose needs replacement on neurological health problem. Objective to this, the synthesis and fluoride adsorption behavior of hydrous ferric oxide (HFO) are reported here. It is seen that fluoride adsorption density varies as a function of pH, contact time, aging time, drying temperature and particle size of HFO. Highest adsorption density for fluoride is found to be at pH 4.0. Effects of competing anions in removing fluoride from solution were tested. Arsenite, arsenate, phosphate and sulfate show strong interfering effect at high anions to fluoride molar ratio in solution. Adsorption of fluoride on HFO follows the Freundlich isotherm and the Lagergren first-order kinetic model. It was also determined that HFO is a better adsorbent in removing fluoride from high fluoride groundwater than some other adsorbents. Regeneration of fluoride-rich HFO results showed that 1.0 M NaOH solution could be used up to a maximum of 75% regeneration.     

A mechanistic model for describing the sorption and desorption of phosphate by soil

A model of phosphate reaction is constructed and its output compared with observations for the sorption and desorption of phosphate by soil. The model has three components: first, the reaction between divalent phosphate ions and a variable-charge surface; second, the assumption that there is a range of values of surface properties and that these are normally distributed; third, the assumption that the initial adsorption induces a diffusion gradient towards the interior of the particle which begins a solid-state diffusion process. The model closely describes the effects on sorption of phosphate of: concentration of phosphate, pH, temperature, and time of contact. It also reproduces the effects on desorption of phosphate of: period of prior contact, period and temperature of desorption, and soil: solution ratio. The model is general and should apply to other specifically adsorbed anions and cations. It suggests that phosphate that has reacted with soil for a long period is not ‘fixed’ but has mostly penetrated into the soil particles. The phosphorus can be recovered slowly if a low enough surface activity is induced.     

Influence of carbonate on the reaction of heavy metals in soils

The reaction of Cu, Zn and Cd with soils with carbonate contents ranging from 0 to 75 mg g⁻¹ was studied before and after removal of soil carbonates with acetate buffer at pH 5. Treatment with acetate buffer caused a strong decrease in metal retention by those soils containing carbonates, although if no carbonate was originally present, the treatment caused little effect or even an increase in the amounts sorbed. Before the treatment, adsorption of increasing amounts of Cu and Zn was accompanied by a continuous increase in Ca + Mg released, and those soils containing carbonate released Ca + Mg in excess of their exchangeable amounts, due to dissolution of carbonates and/or penetration of the heavy metal into the carbonate structure. It is suggested that Cu was preferentially retained by the treated soils through precipitation of Cu oxide, and by adsorption on the soil carbonates in the case of the original samples. Zn was removed from the solution by the original carbonate soils through formation of ZnCO₃. Treated soils were likely to retain Zn by cation exchange and/or adsorption. Adsorption was probably the main process involved in retention of Cd. In all cases pH was the master variable in controlling the extent and probably the nature of the reaction.