A physicochemical framework for interpreting the biological calcification response to CO2-induced ocean acidification

A generalized physicochemical model of the response of marine organisms’ calcifying fluids to CO₂-induced ocean acidification is proposed. The model is based upon the hypothesis that some marine calcifiers induce calcification by elevating pH, and thus Ω_A, of their calcifying fluid by removing protons (H⁺). The model is explored through two end-member scenarios: one in which a fixed number of H⁺ is removed from the calcifying fluid, regardless of atmospheric pCO₂, and another in which a fixed external-internal H⁺ ratio ([H⁺]_E/[H⁺]_I) is maintained. The model is able to generate the full range of calcification response patterns observed in prior ocean acidification experiments and is consistent with the assertion that organisms’ calcification response to ocean acidification is more negative for marine calcifiers that exert weaker control over their calcifying fluid pH. The model is empirically evaluated for the temperate scleractinian coral Astrangia poculata with in situ pH microelectrode measurements of the coral’s calcifying fluid under control and acidified conditions. These measurements reveal that (1) the pH of the coral’s calcifying fluid is substantially elevated relative to its external seawater under both control and acidified conditions, (2) the coral’s [H⁺]_E/[H⁺]_I is approximately the same under control and acidified conditions, and (3) the coral removes fewer H⁺ from its calcifying fluid under acidified conditions than under control conditions. Thus, the carbonate system dynamics of A. poculata’s calcifying fluid appear to be most consistent with the fixed [H⁺]_E/[H⁺]_I end-member scenario. Similar microelectrode experiments performed on additional taxa are required to assess the model’s general applicability.     

An evaluation of acid deposition on cation leaching and weathering rates of an Andosol and a Cambisol

An evaluation of the response of an Andosol and a Cambisol to acid deposition and weathering rates was studied by using a controlled laboratory leaching experiment. Both soils where derived from mafic parent material, a Histic Andosol from Western Iceland and a Cambisol from North East Scotland. De-ionized water and water acidified with H₂SO₄ (pH 3) was leached through reconstructed soil columns to simulate 34 years of precipitation.Acidic input increased cation leaching and weathering rates in both soil types and reduced pH levels. The Andosol proved generally to have higher weathering rates, leaching potential, ion exchange and acid-buffering capacity as well as maintaining a relatively steady pH despite intense acidic input. This was due to differences in parent material and mineral composition. The Andosol was developed from basaltic volcanic tephra, which had higher dissolution rates due to its amorphous mineral structures. The Cambisol was developed from gabbro with more stable mineral structures.     

Effect of simulated acid rain on soil acidification and rare earth elements leaching loss in soils of rare earth mining area in southern Jiangxi Province of China

Acid rain has long been a great concern because of environmental and ecological problems; however, the effect of acid rain on soil acidification, loss of rare earth elements (REEs) via the leaching process, and transformation are rarely reported in rare earth mining areas. Through a simulated acid rain leaching experience, the effect of acid rain was studied on soil acidification and REEs leaching loss. The results showed that the tested soil had certain buffering capacity against nearly neutral rainwater. However, simulated acid rain of low and very low pH (pH ≤ 3.5) had a greater impact on soil acidification. After eluviating by simulated acid rain of pH 3.5 for 36 h, the pH of tailings, garden soil, paddy soil, and alluvial soil decreased by 20.41, 32.03, 13.60, 16.88, and 15.83 %, respectively, from the original values. For simulated acid rain of pH 2.5, it was 31.89, 44.76, 31.26, 29.87, and 29.15 %, respectively. After simulated acid rain eluviations of low and very low pH (pH ≤ 3.5), the order of the leaching rate of REEs in the tested soil was as follows: garden soil > tailings > paddy soil > alluvial soil. For nearly neutral rainwater (pH 4.5 simulated acid rain and pH 5.6 deionized water), the order was tailings > garden soil > paddy soil > alluvial soil. For simulated acid rain of the same pH, the leaching amounts of REEs in tailings and garden soil were higher than those in paddy soil and alluvial soil. After leaching by low and very low pH-simulated acid rain (pH ≤ 3.5), the peak value of the leaching amount of REEs in all tested soil appeared at 2 h, and then gradually reduced and reached a stable leaching state 20 h after leaching. On leaching by simulated acid rain of pH 2.5, the maximum REEs contents of leachate in tailings, garden soil, paddy soil, and alluvial soil were 156.35, 145.82, 99.88, and 85.97 mg/L, respectively. For pH 3.5 of simulated acid rain, it was 130.49, 110.49, 80.57, and 62.73 mg/L, respectively. On leaching by simulated acid rain of pH 4.5, the maximum contents of REEs in the leachate were 53.46 and 29.82 mg/L, respectively, which were observed after leaching for 6 h in tailings and garden soil that became stable 12 h after leaching. The contents of leached REEs in paddy soil and alluvial soil were always in a lower and stable state. After eluviations with deionized water of pH 5.6, the contents of leached REEs in other soils were lower, except for the slight fluctuations in tailings. The maximum content in the leachate of REEs was in the water-soluble and exchangeable fraction. When bound to carbonate fractions, REEs were not detected in the leachate. REEs bound to iron-manganese (Fe–Mn) oxides fraction and to organic matter fraction in the leachate possibly came from the tested soil or from the REEs transformation during the migration process. The content of residual fraction REEs in the leachate was very low.     

Leaching of metals from copper smelter flue dust (Mufulira,Zambian Copperbelt)

The leaching behaviour of electrostatic precipitator dust from the Mufulira Cu smelter (Copperbelt, Zambia) was studied using a 48-h pH-static leaching experiment (CEN/TS 14997). The release of metals (Cd, Co, Cu, Ni, Pb and Zn) and changes in mineralogical composition using X-ray diffraction and PHREEQC-2 modelling were investigated in the pH range of 3–7. The highest concentrations of metals were released at pH 3–4.5, which encompasses the natural pH of the dust suspension (～4.3). About 40% of the total Cu was leached at pH 3, yielding 107 g/kg. Chalcanthite (CuSO₄·5H₂O), magnetite (Fe₃O₄) and delafossite (CuFeO₂) represented the principal phases of the studied dust. In contact with water, chalcanthite was dissolved and hydrated Cu sulphates precipitated at pH 4–7. Gypsum (CaSO₄·2H₂O) and secondary Fe or Al phases were observed in the leached residues. Serious environmental impact due to leaching may occur in dust-contaminated soil systems in the vicinity of the smelting plants.     

湖南洞庭湖地区土壤酸化特征及机理研究

湖南省洞庭湖地区是中国酸雨沉降和土壤酸化严重的地区之一,酸雨沉降具有明显的季节性和地域性差异,酸雨沉降与土壤酸化具有较好的空间对应性,但土壤酸化的程度还取决于土壤中盐基离子的含量。在土壤酸化较严重的地区,垂向上,土壤酸化深度已达50cm左右,土壤酸化伴随着大量的可溶性铝的溶出。在土壤pH值与盐基离子、氮和硫含量的关系曲线上,存在着pH值突变的临界点(临界点pH值为8),在碱性范围内,硅酸盐和铝硅酸盐矿物的水解使大量盐基离子溶出,中和了因SO42-和NO3-的输入而造成的土壤酸化,但当pH值下降到小于7·25的中酸性范围时,酸性物质和碱性物质组成的土壤缓冲体系失去缓冲作用,酸性物质略有增加,就会导致土壤迅速酸化。     

Evaluation of aluminum speciation in surface waters in China and its environmental risk assessment

As the ongoing global research on acid precipitation is developing in depth, more and more attention has been paid to the ecological effects of aluminum (Al) due to its toxicity to plants and animals, which is caused by acid precipitation. As a very serious problem of terrestrial and aquatic environmental acidification occurs in China, especially in southwestern China, a systematic investigation of Al speciation in these regions is very important. In this paper, the Al speciation results of surface waters in China are reported and its ecological impacts is evaluated. More than 100 water samples were collected from about twenty provinces of China. Driscoll's Al speciation scheme combined with the modified MINQEL computer model is used for speciation of Al. This study shows that the ecological impacts of acidification are quite different between China and Western countries, because of different geographical environments and geological settings. In Western countries, acidification is mainly caused by NO₂^-. Due to low concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, the buffer capacities of soil and water are weak. Therefore, natural waters can be acidified to pH<5 very easily, resulting in a considerable mobilization of Al and worsening of the ecological environment. In China, acid precipitation is mainly in the form of sulfuric acid. In northwestern China, concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺ are high in soil and surface waters. This leads to much higher capacity and a high resistance ability to acidification. The pH values of waters in this region are high (around 7) and no serious Al toxicity is found at present. However, in northeastern and southeastern China, the soil is rich in Al (unsaturated aluminosilicates in northeastern China, saturated aluminosilicates in north and central China, aluminum-rich soil in southeastern and southwestern China). The concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺ in soil and waters are lower than those of northwestern China. Therefore the buffer capacity is limited. Numerous surface waters have already been acidified and pH values declined to 5. The impacts of Al toxicity on ecological systems in these regions are very serious, especially in Jiangxi, Hubei Provinces and Chongqing Municipality.     

酸性矿坑废水对流域酸化的影响——以贵州兴仁县典型废弃煤矿区小流域为例

人们在开采使用矿产资源的同时,堆弃大量含有硫化物的废弃矿石和废渣于周围环境中。矿山环境中因硫化矿物氧化,导致采矿产生大量的酸性矿坑排水。这种水体具有低pH值,高电导率,高硫酸根和高重金属含量的特征。酸性矿坑排水对下游水生生物及植物等具有很强的毒性,大量排放引起的环境问题受到广泛关注。为了了解酸性矿山排水对流域水体和土壤的影响,本文选择位于贵州省西南部兴仁的一个典型废弃煤矿区进行研究,通过测定矿坑排水、水库水、河水的pH值和EC,以及土壤的pH值,分析矿坑排水、地表水以及土壤pH值的空间变化情况,在此基础上对矿坑排水对流域酸化的影响进行了综合评价。调查结果表明,酸性矿坑排水和受其影响的水库水体的电导率很高,且pH值均小于3。研究区域地表水（水库水、河水）本底水化学类型为Ca2+-HCO3-型,其pH值在7左右,反映了流域内有碳酸盐岩广泛分布的自然环境特征。当受到酸性矿坑排水影响后,水化学类型转变为Ca2+-SO42-型,pH值则低于4.0。通常,酸性矿坑排水在流动过程中与河床的碳酸盐岩发生中和反应,促使水体的pH升高。野外考察发现,研究区河道中碳酸盐岩中空易碎,其CaCO3成分因长期与酸性矿山排水发生反应而被耗尽。同时,在氧化条件下,酸性矿坑排水中的铁在流动过程中生成大量的氢氧化物覆盖了沿程的河床。这种覆盖作用抑制了酸性矿山排水进一步与碳酸盐岩发生中和反应。因此,在研究区分布有广泛的碳酸盐岩情况下,受酸性矿坑排水影响的河水到下游5 km处仍保持较低的pH值。研究区的主要农作物是水稻,其灌溉水源主要是水库水。为了了解酸性矿坑排水对土壤的影响,对水库下游流域土壤pH值的空间分布进行普查,统计其出现的频率。结果表明,以受酸性矿坑排水影响的水库水作为灌溉水源的土壤,其表土的pH值较低,平均值在5.0左右。反之,土壤表土的pH值平均值在6.5左右。此外,通过对受到酸性矿坑排水影响显著的土壤进行剖面调查,发现从地表到深度90 cm的土壤的pH值均小于4.0。结合受酸性矿坑排水影响的河水pH值普遍偏低的情况可以推测流域酸化与酸性矿坑排水有密切关系。      

The dissolution kinetics of atacamite in the acid range and the stability of atacamite containing soils from Namaqualand,South Africa

The Cu hydroxy mineral, atacamite, is commonly associated with saline environments and is generally thought to dissolve rapidly in the presence of fresh water. A Cu contaminated soil from the arid Namaqualand region, South Africa, shows atacamite as the dominant Cu containing mineral. The stability of the Cu phase in this soil was determined through equilibrium and leaching studies using both deionised water (DI) and a concentrated (0.5 M) NaCl solution. Initially a high concentration of exchangeable Cu was released from the soils leached with NaCl. Continued leaching with NaCl resulted in a substantial decrease in Cu release as atacamite equilibria started to control dissolved Cu. This suggests that an initial spike of Cu laden water will leach from the soils at the onset of a large rainfall event. Further additions of water will result in a lower but sustained release of Cu from the soil. The Cu contaminated soils are exposed to acidic sulphate leachate thus the dissolution kinetics of synthetic atacamite in the acidic range (pH 5.5–4.0) was determined in both NaCl and DI solutions. The kinetic data showed that atacamite dissolution rates are significantly higher in DI than in NaCl but the rates converge at pH 4. In comparison to common acid soluble minerals, atacamite displays a moderate dissolution rate (10^−9.55–10^−7.14 mol m⁻² s⁻¹) within the acid range (pH 5.5–4.0). The atacamite dissolution reaction order with respect to pH is 1.3 and 1.6 in DI and NaCl solutions, respectively, suggesting that dissolution rates of atacamite are highly pH dependent in the acid range. The type of acid used to lower the pH had no effect on the reaction kinetics, with HNO₃ and H₂SO₄ resulting in comparable dissolution rates of atacamite at pH 4.5.     

Arsenic mobilization in alluvial soils of Punjab, North–West India under flood irrigation practices

A laboratory investigation was carried out to examine the mechanism of arsenic (As) mobilization under flooded conditions (24 and 240 h) in 18 alluvial soils of Punjab, North–West India. Total dissolved As increased from a range of 3–16 μg L^?1 (mean 9 μg L^?1) to a range of 33–1,761 μg L^?1 (mean 392 μg L^?1) with the increase in flooding period from 24 to 240 h. The amount of As mobilization varied depending upon redox potential (pe) created by flooding conditions. After 24 h of flooded conditions, pe of soil water suspension ranged from ?1.75 to 0.77 (mean ?0.24). Increasing the flooding period to 240 h, pe of soil water suspension decreased in the range of ?4.49 to ?2.74 (mean ?3.29). Pourbaix diagram identified arsenate (HAsO₄ ^2?) as predominant species in most of the alluvial soil–water suspensions under oxidized conditions, after 24 h of equilibration period, which ultimately transformed to arsenite (H₃AsO₃ ⁰) after 240 h of anaerobic condition due to more reduced status. The solid phase identified was orpiment (As₂S₃). Identification of iron and manganese species in alluvial soil water suspension by Pourbaix diagram indicated decline in both soluble Fe²⁺ and SO₄ ^2? concentration due to the formation of iron sulfide mineral phase after 240 h under anaerobic conditions. In these soils, decline in soluble Fe was also due to the precipitation of vivianite [Fe₃(PO₄)₂·8H₂O]. Elevated arsenic content and low pe value were measured in aquifers located in paddy growing fields comparative to aquifers of other sites. Large degree of variability in As concentrations was recorded in aquifers located at same sites. Thus, it is better to analyze each aquifer for their As content rather than to depends on the prediction on As content of neighbouring wells. The present investigation elucidates that flood irrigation practices in Punjab for growing paddy crop could induce the geochemical conditions favorable to mobilize arsenic from surface soils which could eventually elevate its content in the underlying shallow aquifers. Water abstracted from these aquifers by hand pumps or tube wells for drinking purposes could create hazards for local population due to loading with arsenic concentration above the safe limits. Thus, to avoid further contamination of shallow aquifers with arsenic, it is advisable to shift the flooded rice cultivation to other upland crops having lesser water requirement.     

The effect of naturally acidified irrigation water on agricultural volcanic soils. The case of Asembagus, Java, Indonesia

Acid water from the Banyuputih river (pH  3.5) is used for the irrigation of agricultural land in the Asembagus coastal area (East Java, Indonesia), with harmful consequences for rice yields. The river water has an unusual composition which is caused by seepage from the acidic Kawah Ijen crater lake into the river. This unique irrigation setting allows the study of soil acidification in situ. This paper assesses the effects of volcanogenically contaminated irrigation water on the chemical properties of the agricultural soils.The changes in soil properties were evaluated by comparing samples taken from the topsoil and sub-soil (1–3 m depth) from areas irrigated with acid water and areas irrigated with neutral water. The field survey thus resulted in four soil categories. Bulk soil composition, organic matter content, moisture content and particle size distribution were determined. Reactive phases were quantified with the selective extractions 1 M KCl, 0.1 M Na-pyrophosphate and 0.2 M acid ammonium oxalate (AAO).By comparing the four soil categories it is shown that the use of the naturally polluted irrigation water has had a large influence on the chemical composition of the topsoil. The composition of the soil solution has changed over the entire investigated soil profile. Furthermore the acid irrigation water has strongly modified the composition of the reactive phases, extracted as KCl, pyrophosphate, and AAO extractable elements, and also the bulk soil composition has been significantly modified. Overall this has resulted in the net dissolution of some elements and the net precipitation of others. The changes in the reactive phases and bulk soil composition are only apparent in the topsoil (0–20 cm) but not in the deeper soil.     

Biogeochemistry of manganese oxide coatings on pebble surfaces in the Kikukawa River System,Shizuoka, Japan

The biogeochemistry of Mn-oxide coatings formed over submerged pebble surfaces on the streambed of the Kikukawa river system has been investigated. Located in central Shizuoka, Japan, this system drains strongly acidified soils under tea plantations. Besides containing high amounts of Mn (up to 450 μg/cm²), the coatings are capable of scavenging and accumulating other elements including Ba, Zn, Ni, Co, W, Mo and Sb. When suspensions of the coating material were incubated with 0.2 mM Mn²⁺, the Mn(II) ion was microbially transformed into Mn-oxides. When the same suspensions were spread on agar plates containing acetate, yeast-extract, and 1.0 mM Mn²⁺ (AY agar medium) both Mn-oxidizing bacteria and fungi appeared, indicating the existence of a diversity of Mn-oxidizing microorganisms in the system. Plate counts using two agar culture media with varied nutrient levels indicated that the ability of these microorganisms to oxidize Mn(II) was strongly dependent on nutrient supply. The relatively nutrient-poor AY agar medium was more conducive to microbial growth than the K₁ agar medium with a higher organic nutrient content. Concentrations of Mn dissolved in the stream waters did not correlate well with the amounts of solid Mn on submerged pebbles. Thus, factors other than dissolved Mn concentration (e.g., organic nutrient supply and pH) determined the ability of microorganisms to oxidize Mn in the streambeds. A survey of dissolved Mn in streams and water draining tea plantations combined with chemical analysis of Mn in the underlying soils indicate that the soils have been strongly acidified through excessive applications of N-fertilizers. As a result, Mn was leached from the soil column into the Kikukawa river system. Biogenic Mn-oxide coatings on streambeds can therefore serve as an indicator of soil acidification and metal leaching from soils of the corresponding watershed.     

Fluoride sorption and desorption on soils located in the surroundings of an aluminium smelter in Galicia (NW Spain)

Aluminium smelters are major sources of F emission to the environment. We studied, in laboratory experiments, the sorption and desorption of fluoride on organic and mineral horizons of soils located within 2 km from one of these factories, situated in the northern coast of Galicia (NW Spain). The soils, developed from granite, are acid (pH H₂O 3.9–5.5), rich in organic matter (4–16 % C in the A horizon) and most A horizons have high Al saturation in the exchange complex. All samples showed a notable F sorption, between 1,066 and 1,589 mg kg^?1, after adding 200 mg F L^?1, which accounts for 53–80 % of F added. The sorption was slightly higher in the A horizons than in the respective organic horizons (differences of up to 194 mg kg^?1). The fluoride sorption upon addition of 200 mg F L^?1 correlated significantly (p < 0.05) with soil pH in water (r = ?0.77), iron extracted by acid ammonium oxalate (r = 0.68), aluminium plus iron extracted by acid ammonium oxalate (r = 0.63), exchange aluminium (r = 0.52) and clay percentage in soil (r = 0.76). The F sorption fitted to both Langmuir and Freundlich models. Desorbed F accounted for only 12–22 % of sorbed fluoride and correlated (p < 0.05) negatively with non-crystalline (extracted by acid ammonium oxalate) Fe (r = ?0.51) and clay content (r = ?0.74) and positively with organic matter (r = 0.69) and with the effective cation exchange capacity of the soil (r = 0.50).     

Modeling aqueous ferrous iron chemistry at low temperatures with application to Mars

Major uncertainties exist with respect to the aqueous geochemical evolution of the Martian surface. Considering the prevailing cryogenic climates and the abundance of salts and iron minerals on Mars, any attempt at comprehensive modeling of Martian aqueous chemistry should include iron chemistry and be valid at low temperatures and high solution concentrations. The objectives of this paper were to (1) estimate ferrous iron Pitzer-equation parameters and iron mineral solubility products at low temperatures (from < 0 °C to 25 °C), (2) incorporate these parameters and solubility products into the FREZCHEM model, and (3) use the model to simulate the surficial aqueous geochemical evolution of Mars.Ferrous iron Pitzer-equation parameters were derived in this work or taken from the literature. Six new iron minerals [FeCl₂·4H₂O, FeCl₂·6H₂O, FeSO₄·H₂O, FeSO₄·7H₂O, FeCO₃, and Fe(OH)₃] were added to the FREZCHEM model bringing the total solid phases to 56. Agreement between model predictions and experimental data are fair to excellent for the ferrous systems: Fe-Cl, Fe-SO₄, Fe-HCO₃, H-Fe-Cl, and H-Fe-SO₄.We quantified a conceptual model for the aqueous geochemical evolution of the Martian surface. The five stages of the conceptual model are: (1) carbonic acid weathering of primary ferromagnesian minerals to form an initial magnesium-iron-bicarbonate-rich solution; (2) evaporation and precipitation of carbonates, including siderite (FeCO₃), with evolution of the brine to a concentrated NaCl solution; (3) ferrous/ferric iron oxidation; (4) either evaporation or freezing of the brine to dryness; and (5) surface acidification.What began as a dilute Mg-Fe-HCO₃ dominated leachate representing ferromagnesian weathering evolved into an Earth-like seawater composition dominated by NaCl, and finally into a hypersaline Mg-Na-SO₄-Cl brine. Weathering appears to have taken place initially under conditions that allowed solution of ferrous iron [low O₂(g)], but later caused oxidation of iron [high O₂(g)]. Surface acidification and/or sediment burial can account for the minor amounts of Martian surface carbonates. This model rests on a large number of assumptions and is therefore speculative. Nevertheless, the model is consistent with current understanding concerning surficial salts and minerals based on Martian meteorites, Mars lander data, and remotely-sensed spectral analyses.     

Impact of nitrogenous fertiliser-induced proton release on cultivated soils with contrasting carbonate contents: A column experiment

An experimental study was carried out in order to evaluate the impact of nitrogen fertiliser-induced acidification in carbonated soils. Undisturbed soil columns containing different carbonate content were sampled in the field. Fertiliser spreading was simulated by NH₄Cl addition on top of the soil column. Soil solution composition (mainly nitrate and base cations) was studied at the soil column’s base. Nitrification occurred to a different extent depending on soil type. Higher nitrification rates were observed in calcareous soils. In all the soil types, strong correlations between leached base cation and nitrate concentrations were observed. Regression coefficients between base cations, nitrate and chloride were used to determine the dominant processes occurring following NH₄Cl spreading. In non-carbonated soils, nitrogen nitrification induced base cation leaching and soil acidification. In carbonated soils, no change of soil pH was observed. However, fertilisers induced a huge cation leaching. Carbonate mineral weathering led to the release of base cations, which replenished the soil exchangeable complex. Carbonated mineral weathering buffered acidification. Since direct weathering might have occurred without atmospheric CO₂ consumption, the use of nitrogen fertiliser on carbonated soil induces a change in the cation and carbon budgets. When the results of these experiments are extrapolated on a global scale to the surface of fertilised areas lying on carbonate, carbonated reactions with N fertilisers would imply an additional flux of 5.7 × 10¹² mol yr⁻¹ of Ca + Mg. The modifications of weathering reactions in cultivated catchments and the ability of nitrogen fertilisers to significantly modify the CO₂ budget should be included in carbon global cycle assessment.     

Sulphate adsorption capacity and pH of upland podzolic soils in Scotland: Effects of parent material,texture and precipitation chemistry

A regional survey of podzol B horizons has been carried out to investigate the effects of parent material and soil texture on sulphate (SO₄²⁻) adsorption capacity and pH in Scottish soils. Sulphate adsorption was measured on equilibration of the soils with 10 and 100 mg 1⁻¹ SO₄²⁻ solutions. The results showed that soil texture and soil parent material had a significant effect on SO₄²⁻ adsorption. Significant correlations were found between sulphur (S) deposition loads and SO₄²⁻ adsorption, and between precipitation pH and soil pH, but not between total hydrogen ion (H⁺) load and soil pH, even on sensitive soils. Relationships between the chemical composition of atmospheric deposition and soil pH could be marginally improved if the possible amelioration of acidification by base cation inputs, especially on sensitive soils, was taken into account.     

Exploring aberrant bivalve shell ultrastructure and geochemistry as proxies for past sea water acidification

Throughout much of Earth's history, marine carbonates have represented one of the most important geological archives of environmental change. Several pivotal events during the Phanerozoic, such as mass extinctions or hyperthermal events have recently been associated with ocean acidification. Nevertheless, well‐defined geological proxies for past ocean acidification events are, at best, scarce. Here, experimental work explores the response of bivalve shell ultrastructure and isotope geochemistry (δ¹³C, δ¹⁸O and δ²⁶Mg) to stressful environments, in particular to sea water acidification. In this study, the common blue mussel, Mytilus edulis, was cultured (from early juvenile stages to one year of age) at four pH regimes (pH_NBS 7·2 to pH 8·0). Shell growth rate and ultrastructure of mainly the calcitic portion of the shells were compared between experimental treatments. Specimens exposed to low‐pH environments show patches of disordered calcitic fibre orientation in otherwise well‐structured shells. Furthermore, the electron backscattered diffraction analyses reveal that, under acidified conditions, the c‐axis of the calcite prisms exhibits a bimodal or multi‐modal distribution pattern. Similar shell disorder patterns have been reported from mytilids kept under naturally acidified sea water conditions. In contrast, this study found no evidence that different pH regimes affect shell carbon, oxygen or magnesium isotope ratios. Based on these observations, it is proposed that: (i) stressful environments, in this case low sea water pH, predictably affect bivalve biomineralization patterns; and (ii) these findings bear potential as a novel (petrographic) proxy for ancient sea water acidification. An assessment of the applicability of these data to well‐preserved fossil shell material from selected time intervals requires additional work.     

The distribution and elevated solubility of lead,arsenic and cesium in contaminated paddy soil enhanced with the electrokinetic field

The objectives of this study were to investigate fractionation, solubility and potential bioavailability of Pb, As and Cs in Mississippi River Delta paddy soil under an electrokinetic field (EKF). Effects of EKF on soil pH changes and solid-phase distributions of metal(loid)s were examined. Results showed that fractionation of Pb, As and Cs was largely determined by the nature of elements, loading levels and EKF treatment. Native Pb in the soil was mostly in the amorphous iron oxide, organic matter and residual fractions, native As in the amorphous iron oxide, easily reducible oxide and residue fractions while native Cs in the residue fraction. Added Pb, As and Cs showed distinguished solid-phase distributions: Pb dominantly in the organic matter fraction; As in the amorphous iron oxide fraction, and Cs in the residue with a significant water-soluble plus exchangeable fraction. EKF treatment is effective on lowering soil pH to 1.5 near the anode due to water electrolysis releasing proton which is beneficial for dissolution of metal(loid)s, increasing their overall solubility. The acidification in the anode soil efficiently increased the water-soluble Pb and the exchangeable Cs, implying enhanced solubility and elevated their overall potential bioavailability in the anode region while lower solubility in the cathode area. The building up of water-soluble As in the anode region may be from electromigration of As anion from the cathode. This study shows significant enhancement of redistribution, elevated solubility and overall bioavailability of Pb, As and Cs in Mississippi Delta paddy soil under the EKF.     

辽河流域土壤酸化与作物籽实镉生物效应的地球化学预警

土壤酸化及其引发的生态安全问题是公众广泛关注、全球研究的热点。辽河流域多目标区域地球化学调查结果显示：当土壤中的w(CaO+K2O)≥3.86%时,土壤对酸性沉降物具有缓冲能力;当w(CaO+K₂O)<3.86%时,则土壤对酸性沉降物的缓冲能力显著下降。利用土壤酸化缓冲能力的地球化学预测模型,对全流域土壤酸化的缓冲能力进行了预测,指出辽河流域东部,即营口-鞍山-辽阳-沈阳-抚顺-铁岭-开源的广大区域内既是土壤酸化的脆弱区,也是作物籽实Cd超标的预警区,辽河流域土壤酸化区域将进一步扩大。     

Rare earth element behavior and Pb,Sr, Nd isotope systematics in a heavy metal contaminated soil

The aim of this study is to characterize the processes and phases which control migration and retention of rare earth elements (REE) in a heavy metal contaminated soil. In addition to concentration data, we used Pb, Sr and Nd isotopic compositions in order to distinguish between natural and anthropogenic trace metals and to characterize the phases leached away during the sequential extraction procedure.The samples were sequentially extracted in 3 steps with 1 N acetic acid (HAc), 1 N HCl and 1 N HNO₃. The Pb isotope data showed that anthropogenic Pb had mainly been retained in the uppermost 10 cm by the organic matter of the topsoil. The⁸⁷Sr/⁸⁶Sr ratios of the HAc extracts are almost constant and indicate that soil carbonate is derived from regionally outcropping carbonate-rich sediments. Most HCl and HNO₃ extracts have more radiogenic Sr isotopic compositions, but it is unclear whether this reflects a growing influence of anthropogenic or silicate-derived Sr.The depth distribution of the REE is mainly controlled by two different parameters: soil pH for the HAc extractable REE and FeMn oxides for the REE in the HCl and HNO₃ extracts. A part of the HNO₃ extractable REE was also bound to the organic matter of the topsoil. The REE concentrations in the HAc extractable phase increase with depth and increasing soil pH, which indicates that they are derived from the surface and hence are of anthropogenic origin. This is confirmed by¹⁴³Nd/¹⁴⁴Nd isotope ratios which show a mixing between a natural end-member at the top and an anthropogenic end-member at the base of the profile. We assume that the anthropogenic REE were transported in dissolved form as carbonate complexes and then precipitated during downward migration as soil pH increased.     

Recharge in northern clime calcareous sandy soils: soil water chemical and carbon-14 evolution

Chemical analyses were performed on soil water extracted from two cores taken from a sandy calcareous soil near Delhi, Ontario. Calcite saturation is attained within the unsaturated zone over short distances and short periods of time, whereas dolomite undersaturation persists to the groundwater table. The progressive dissolution of dolomite by soil water, within the unsaturated zone, after calcite saturation is reached results in calcite supersaturation.Deposition of iron and manganese oxyhydroxide phases occurs at the carbonate leached/unleached zone boundary. This is a result of soil water neutralization due to carbonate dissolution during infiltration but may also reflect the increased rate of oxidation of dissolved ferrous and manganous ions at higher pH's. The role of bacteria in this process has not been investigated.The depth of the carbonate leached/unleached zone boundary in a calcareous soil has important implications for ¹⁴C groundwater dating. The depth of this interface at the study site (?2 m) does not appear to limit ¹⁴C diffusion from the root zone to the depth at which carbonate dissolution occurs. Thus, soil water achieves open system isotopic equilibrium with the soil CO₂ gas phase. It is calculated that in soils with similar physical properties to the study soil but with depths of leaching of 5 m or more, complete ¹⁴C isotopic equilibration of soil water with soil gas would not occur. Soil water, under these conditions would recharge to the groundwater exhibiting some degree of closed system ¹⁴C isotopic evolution.