The diffusion of dissolved silica in dilute aqueous solution

The diffusion coefficient of dissolved silica at 25.5 ± .5°C was determined as a function of concentration using a non-steady-state method whereby agar-gelled solutions containing dissolved silica from 0.09 to 1.50 mM (pH = 5.5) were placed in contact with distilled water in glass cells. Diffusion coefficients were obtained by measuring the dissolved silica content of the distilled water after a given length of time. The measured diffusion coefficients decreased as a function of increasing dissolved silica concentration, which is thought to reflect an increase in dimeric silica according to the equilibrium: 2Si(OH)₄ = Si₂O(OH)₆ + H₂O. The tracer diffusion coefficients for Si(OH)₄ and Si₂O(OH)₆ and an association constant for the above reaction were determined by fitting the following equation to the experimental data: D_obs = αD_monomer + (1 − α)D_dimer where α is the fraction of total dissolved silica which is Si(OH)₄. The best fit yielded tracer D's for Si(OH)₄ and Si₂O(OH)₆ of 2.2 and 1.0 (in units of 10⁻⁵ cm² sec⁻¹), respectively, and an association constant of 330.     

Removal of dissolved boron and silicon during estuarine mixing of sea and river waters

The behaviour of dissolved boron and silicon during mixing of sea and river waters has been studied in two surveys of the estuary of the River Alde in Suffolk, England. Removal of approximately 25–30 per cent was found for both elements. This appears to be the first report of estuarine removal of dissolved boron. The extent of removal of silicon in the Alde is somewhat higher than that found in other estuaries.     

Interaction between dissolved silica and calcium carbonate: 1. Spontaneous precipitation of calcium carbonate in the presence of dissolved silica

The kinetics of spontaneous precipitation of CaCO₃ from aqueous solution in the presence of dissolved silica was investigated by recording pH as a function of time. The presence of dissolved silica, at concentrations below saturation with respect to the amorphous phase, decreases induction time for CaCO₃ nucleation, but does not affect CaCO₃ polymorphism. For a “pure” system without silica, the surface free energy, σ, determined from classical nucleation theory is 42 mJ m⁻². This agrees well with values reported in the literature for vaterite and indicates some degree of heterogeneous nucleation, which can occur because of the relatively low degree of supersaturation used for the experiments. In the presence of 1 and 2 mM silica, σ is 37 and 34 mJ m⁻², indicating an increasing degree of heterogeneous nucleation as the amount of polymeric silica increases. The ratio of Ca²⁺ to CO₃²⁻ activity was a governing parameter for determining which CaCO₃ polymorph precipitated. At high Ca²⁺ to CO₃²⁻ activity ratios, almost all initial solid was vaterite, whereas at low ratios, a mixture of vaterite and calcite was observed. In solutions with low Ca²⁺ to CO₃²⁻ activity ratios, the presence of silica at concentrations above saturation with respect to amorphous silica led to formation of only calcite and strongly influenced the crystalline structure and morphology of the precipitates. At high Ca²⁺ to CO₃²⁻ ratios, system behaviour did not differ from that without silica.     

The behaviour of dissolved organic material,iron and manganese in estuarine mixing

Fractionation by ultra-filtration of the dissolved organic material (DOM) in the River Beaulieu, with typical concentrations of dissolved organic carbon (DOC) of 7–8 mg C/l, showed it to be mainly in the nominal molecular weight range of 10³–10⁵, with 16–23% of the total DOC in the fraction > 10⁵. The molecular weight distribution of DOM in the more alkaline River Test (average DOC, 2 mg C/l) was similar. In the River Beaulieu water, containing 136–314 βg Fe/l in ‘dissolved’ forms, 90% or more of this Fe was in the nominal molecular weight fraction > 10⁵. Experiments showed that DOM of nominal molecular weight <10⁵ could stabilize Fe(III) in ‘dissolved’ forms. The concentrations of ‘dissolved’ Fe in the river water probably reflect the presence of colloidal Fe stabilized by organic material and this process may influence the apparent molecular weight of the DOM. Dissolved. Mn (100–136 βg/l) in the River Beaulieu was mainly in true solution, probably as Mn(II), with some 30% in forms of molecular weight greater than ca 10⁴.During mi xing in the Beaulieu Estuary, DOC and dissolved Mn behave essentially conservatively. This contrasts with the removal of a large fraction of the dissolved Fe (Holliday and LISS, 1976, Est. Coastal Mar. Sci. 4, 349–353). Concentrations of lattice-held Fe and Mn in suspended particulate material were essentially uniform in the estuary, at 3.2 and 0.012%, respectively, whereas the non-lattice held fractions decreased markedly with increase in salinity. For Mn the decrease was linear and could be most simply accounted for by the physical mixing of riverborne and marine participates, although the possibility that some desorption occurs is not excluded. The non-linear decrease in the concentration of non-lattice held Fe in particulates reflected the more complex situation in which physical mixing is accompanied by removal of material from the ‘dissolved’ fraction.     

Metals removal during estuarine mixing of Arvand River water with the Persian Gulf water

In the present study, the removal of dissolved and colloidal Cd, Co, Cu, Ni and Zn in Arvand River water during estuarine mixing with the Persian Gulf water is investigated. The flocculation process was investigated for a series of mixtures with salinities ranging from 0.48 to 30.3^. The flocculation rates were indicative of the non-conservative behavior of studied metals during estuarine mixing. Rapid flocculation in the low salinity regimes was observed. The order of the final flocculation rate of metals in the river water was as follows: Co (91.2%)> Cd (86.9%)> Zn (83%)> Cu (75.2%)> Ni (74.3%). Salinity, pH, EC and dissolved oxygen do not govern the flocculation of metals during estuarine mixing. The results of the present investigation show that estuarine processes can be considered as an effective mechanism in self purification of colloidal metals that are anthropogenically introduced into the fresh water ecosystem.     

The removal kinetics of dissolved organic matter and the optical clarity of groundwater

Concentrations of dissolved organic matter (DOM) and ultraviolet/visible light absorbance decrease systematically as groundwater moves through the unsaturated zones overlying aquifers and along flowpaths within aquifers. These changes occur over distances of tens of meters (m) implying rapid removal kinetics of the chromophoric DOM that imparts color to groundwater. A one-compartment input-output model was used to derive a differential equation describing the removal of DOM from the dissolved phase due to the combined effects of biodegradation and sorption. The general solution to the equation was parameterized using a 2-year record of dissolved organic carbon (DOC) concentration changes in groundwater at a long-term observation well. Estimated rates of DOC loss were rapid and ranged from 0.093 to 0.21 micromoles per liter per day (μM d^?1), and rate constants for DOC removal ranged from 0.0021 to 0.011 per day (d^?1). Applying these removal rate constants to an advective-dispersion model illustrates substantial depletion of DOC over flow-path distances of 200 m or less and in timeframes of 2 years or less. These results explain the low to moderate DOC concentrations (20–75 μM; 0.26–1 mg L^?1) and ultraviolet absorption coefficient values (a ₂₅₄?<?5 m^?1) observed in groundwater produced from 59 wells tapping eight different aquifer systems of the United States. The nearly uniform optical clarity of groundwater, therefore, results from similarly rapid DOM-removal kinetics exhibited by geologically and hydrologically dissimilar aquifers.     

激光光谱技术在溶解无机碳碳同位素分析中的应用

天然水溶解无机碳（DIC）碳同位素组成（δ13CDIC）分析是研究碳元素循环及相关生物地球化学过程的重要手段之一。近年来,激光光谱技术的发展为碳同位素比值测定提供了一种新的方法。文中阐述了一种总有机碳仪—激光光谱同位素仪联用在线测定水中DIC含量及δ13CDIC值的技术方法。该方法具有较高的测试精度,DIC含量测试结果相对标准偏差能控制在1%以内,δ13CDIC值精度优于±0.1‰（1σ）。不同类型岩溶水δ13CDIC值的测试结果与质谱仪法结果接近,差值总体≤0.3‰,表明该测试技术具有较高的准确度。由于吸收光谱信号与目标气体浓度有关,较低的CO2浓度会影响激光光谱仪的稳定性,在测试时需要根据DIC浓度控制样品进样量,最好采用多标样法来校准仪器测量值。激光光谱技术因其具有低成本、测试快速可靠,且仪器小巧便携等特点,在岩溶水溶解无机碳碳同位素分析中具有较大应用前景。     

桂林潮田河溶解无机碳来源与昼夜动态变化

生物地球化学过程是地球关键带的三大过程之一。在全球变化与岩溶碳循环研究领域,了解生物地球化学过程、影响因素与机制,对解决岩溶作用时间尺度与碳汇稳定性问题具有至关重要的作用。以广西桂林漓江支流潮田河为例,通过高分辨率在线监测与高频率自动化取样分析,研究了溶解无机碳（DIC）及其同位素、电导率与其他离子含量的昼夜变化规律,并分析了其影响因素。pH、溶解氧、电导率、Ca²⁺和HCO₃^-离子含量均呈现显著的昼夜动态变化,反映水生植物和藻类光合作用及钙沉积的影响。鸟岭桥与两河沟两个监测点的电导率、Ca²⁺和HCO₃^-离子含量具有白天下降、夜间上升的变化规律,鸟岭桥白天下降幅度分别为10%、11.5%~12.5%和10%~12%。48小时监测期间鸟岭桥光合作用DIC平均转化速率为0.64×10^-5mmol/L ·s,日转化HCO₃^-的量约为1484kg/d（相当于292kg C/d）,占总DIC输入量的4%;平均钙沉积速率为0.38×10^-5mmol/L ·s,较小的速率可能与水体中较高的溶解有机碳含量产生的抑制作用有关。Ca²⁺与HCO₃^-离子含量夜间的快速上升主要是上游来自岩溶含水层的地下水直接补给造成的。在年际尺度上潮田河DIC中75%~80%来自于岩溶地下水。

     

内陆水体溶解无机碳同位素研究进展及展望

碳是陆地生态系统中最重要的生命元素,能够在各个圈层之间相互转换和运移。碳循环作为地球各圈层相互连接的纽带,被认为是地表系统最重要的元素地球化学循环。碳在水体中主要以溶解性有机碳（dissolved organic carbon,简称DOC）和溶解性无机碳（dissolved inorganic carbon,简称DIC）形式存在。内陆水体（包括河水、地下水、湖水和水库等）尽管只占陆地表面1%,但它们在碳循环过程中发挥了重要作用。其DIC浓度(CDIC)和DIC同位素组成(δ13CDIC)可以为碳循环研究提供线索,因此成为近年来国际研究的热点之一。本文系统总结了全球内陆水体DIC同位素的研究进展。DIC同位素具有广泛的地质应用,不同碳源具有独特的同位素特征。因此,DIC同位素被用于定性和定量地示踪碳源,指示碳循环过程以及确定河水补给端元。     

Processes of supply and removal of dissolved silicon in the oceans

A critical assessment has been made of the processes of supply and removal of dissolved silicon in the ocean. The only sources of importance appear to be continental drainage (supplying 4.3 × 10¹⁴ g SiO₂/yr), Antarctic weathering and migration from sediment pore waters. The magnitudes of the last two processes are uncertain but there is evidence that they may add significantly to the river input. The total input appears to lie uncertainly within the range of 5.12 × 10₁₄ g SiO₂/yr.Estimates of removal in estuarine mixing processes (less than 1 × 10₁₄ g SiO₂/yr) and in pelagic siliceous oozes (less than 2 × 10₁₄ g SiO₂/yr) suggest that deposition by these processes may not balance the input. Other removal processes could include biological deposition in coastal waters, but the hypothesis that some removal in the sea occurs by inorganic processes, such as reverse weathering reactions, cannot be discounted.     

咸化水体中DIC的同位素组成及环境过程分析

系统采集了天津地区地表水样品,对其溶解无机碳的稳定碳同位素比值(δ13CDIC)及相关化学组成进行了全面测定,以期对咸化并且受到污染水体中的溶解无机碳的来源和迁移转化特征进行探讨.研究结果显示,天津地表水中溶解无机碳的δ13CDIC值主要分布于0.27‰～ -14.9‰之间,显示了相对较宽的分布范围.同时,天津地表水体...     

Enhanced removal of dissolved metal ions in radioactive effluents by flocculation

Decontamination of radioactive effluents of low or intermediate level of radioactivity generated from different nuclear industries is done through the chemical precipitation route. The precipitates thus formed are of very fine sizes thereby requiring flocculation for faster settlement. The presence of polyacrylamide-based polymer not only enhances settling velocity but also increases removal of dissolved non-radioactive as well as radioactive metal ions from the liquid. About 99.5% of Cu²⁺ and Fe³⁺ ions are removed due to the presence of cationic Rishabh 611. Flocculation by either cationic or anionic flocculant has shown improvement in removal of radioactive strontium while turbidity of the liquid is reduced to a very low value (0.8 NTU). Decontamination factors (DF) of the effluents from different plants are improved by 3–5 times.     

峰丛洼地自然封育过程岩溶水溶解无机碳的变化——以桂林丫吉试验场为例

以桂林丫吉试验场为例,检验植被恢复对岩溶碳汇的影响作用。上世纪80年代,丫吉试验场峰丛洼地的植被受砍伐干扰,地表以草地居多,此后施行自然封山育林,植被经二十多年的封育后,现在变成为灌木林,土壤中植物根系呼吸和微生物异养呼吸增强,土壤空气中CO2含量上升,岩溶作用增强,泉水输出的无机碳通量增加。通过建立系统的水化学混合模型,将泉水的无机碳通量分解为原水输出和降雨补给引起的新水输出。在一次典型的强降雨过程,S31岩溶泉原水输出量占总量的65%~83%,而降雨补给引起的新水无机碳输出通量较少,且不能反映生态环境的变化,因此应以原水中溶解无机碳的浓度衡量整个生态环境对岩溶作用的影响。长期的观测表明由于植被的改变导致原水输出的无机碳含量提高了27%,而原水在岩溶泉的年流量中所占的比例高达72%~84%,其中由植被演化造成岩溶碳汇增加的幅度为19%~23%。      

单生卵囊藻对DIC的利用及其对CaCO3沉积影响的研究

以生长在钙华池中的单生卵囊藻（Oocystis solitaria Wittr）为研究对象,利用pH漂移方法,探讨了封闭系统中单生卵囊藻在岩溶水和非岩溶水环境下对溶解无机碳（DIC）利用及其对水体Ca^2＋沉积影响的差异。结果表明,单生卵囊藻在低CO2浓度时,通过胞外碳酸酐酶的催化,以HCO3^-作为无机碳源进行光合作用。在岩溶水环境下单生卵囊藻DIC利用能力要高于非岩溶水环境（4．78倍）,而在此过程中对水体中Ca^2＋沉积的影响也更高（2．13倍）。在岩溶水（非岩溶水）环境下,有42．6％（8．9％）的Ca^2＋通过物理化学效应以CaCO3形式沉积,其余Ca^2＋可能被藻体生长而吸收利用．．     

Fertilization and pH effects on processes and mechanisms controlling dissolved inorganic phosphorus in soils

We used of a set of mechanistic adsorption models (1-pK TPM, ion exchange and Nica-Donnan) within the framework of the component additive (CA) approach in an attempt to determine the effect of repeated massive application of inorganic P fertilizer on the processes and mechanisms controlling the concentration of dissolved inorganic phosphorus (DIP) in soils. We studied the surface layer of a Luvisol with markedly different total concentrations of inorganic P as the result of different P fertilizer history (i.e. massive or no application for 40 years). Soil pH was made to vary from acid to alkaline. Soil solutions were extracted with water and CaCl₂ (0.01 M). The occurrence of montmorillonite led us to determine the binding properties of P and Ca ions for this clay mineral.Satisfactory results were obtained using generic values for model parameters and soil-specific ones, which were either determined directly by measurements or estimated from the literature. We showed that adsorption largely controlled the variations of DIP concentration and that, because of kinetic constrains, only little Ca-phosphates may be precipitated under alkaline conditions, particularly in the P fertilized treatment. The mineral-P pool initially present in both P treatments did not dissolve significantly during the course of the experiments. The adsorption of Ca ions onto soil minerals also promoted adsorption of P ions through electrostatic interactions. The intensity of the mechanism was high under neutral to alkaline conditions. Changes in DIP concentration as a function of these environmental variables can be related to changes in the contribution of the various soil minerals to P adsorption. The extra P adsorbed in the fertilized treatment compared with the control treatment was mainly adsorbed onto illite. This clay mineral was the major P-fixing constituent from neutral to alkaline pH conditions, because the repulsion interactions between deprotonated hydroxyl surface sites and P ions were sufficiently counterbalanced by Ca ions. The drastic increase of DIP observed at acid pH was due to the effect of the lower concentration of surface sites of Fe oxides and kaolinite.In addition to confirming the validity of our approach to model DIP concentrations in soils, the present investigation showed that adsorption was the predominant geochemical process even in the P fertilized soil, and that Ca ions can have an important promoting effect on P adsorption. However the influence of the dissolution of the mineral-P pool under field conditions remained questionable.     

On the estuarine mixing of dissolved substances in relation to colloid stability and surface properties

The estuarine mixing of dissolved Fe, Cu, Ni, Si and surface-active organic matter has been investigated in the Taieri Estuary, New Zealand, simultaneously with measurements of the electrokinetic charge on colloidal particles. Dissolved Fe showed almost quantitative removal from solution characteristic of the coagulation of iron-containing colloids by seawater electrolytes. Surface active organic matter behaved conservatively, indicating that a relatively constant fraction of estuarine organic matter is surface active, but that organic species associated with iron during removal are a minor fraction. Results for Cu, Ni and Si were scattered but offered no evidence for gross removal during estuarine mixing. The negative charge on suspended colloids was not reversed by adsorption of seawater cations, but remained uniformly negative throughout the salinity range, decreasing sharply in magnitude during the first few %. salinity.     

Behavior of stable isotopes of dissolved oxygen,dissolved inorganic carbon and nitrate in groundwater at a former wood treatment facility containing hydrocarbon contamination

The speciation and mobility of a variety of chemical species in groundwater are strongly influenced by redox and pH conditions. Dissolved O₂ (DO) and dissolved inorganic C (DIC) concentrations are significant controls of these conditions, respectively. It is not always clear what the major processes are that influence changes in the concentration of DO and DIC across a groundwater flowpath. The combined use of the stable isotope compositions of DO (δ¹⁸O–DO) and DIC (δ¹³C–DIC) has the potential to help investigators discriminate between sources and sinks of DO and DIC in groundwater systems.     

农业活动对表层岩溶泉水化学特征及溶解无机碳损失量的影响

以重庆市南川区不同农业活动强度下的3个表层岩溶泉为例,通过分析泉点水化学变化特征来估算不同泉点的DIC损失量（△DIC）和探讨农业活动对岩溶碳汇的影响。研究结果表明:柏树湾泉水温变幅较小,电导率、pH值相对兰花沟泉、后沟泉较低。由于农业活动的影响,兰花沟泉、后沟泉Ca2+ 浓度偏高,而HCO3-浓度偏低。NO3-、SO42-浓度也因农业活动干扰,表现出后沟泉、兰花沟泉远大于柏树湾泉。随农业活动强度的增加,△DIC也逐渐增大,表现为:柏树湾泉（1.64 mmol/L）＜兰花沟泉（4.28 mmol/L）＜后沟泉（4.36 mmol/L）。△DIC与(SO42-+NO3-)呈正相关,表明农业活动越强烈,DIC损失量越大,岩溶碳汇的损失也随之增大。