有机质对广西酸性富硒土中Se(Ⅳ)吸附解吸特性的影响

为探究有机质在酸性富硒土吸附Se(Ⅳ)过程中的作用，以广西典型富硒区的赤红壤为材料，研究了酸性富硒土去除有机质后，土壤硒的赋存形态、土壤对Se(Ⅳ)的吸附解吸特征及吸附前后土壤基团的变化。结果表明：(1)去除有机质后，土壤中有机结合态硒大幅度减少，铁锰结合态硒成为土壤硒的主要赋存形态。(2)吸附试验表明，土壤对Se(Ⅳ)吸附过程以多分子层的不均质表面吸附为主，吸附过程受控于化学反应与化学吸附；去除有机质后，土壤对Se(Ⅳ)吸附量和吸附强度均显著下降。(3)解吸试验表明，吸附以难解吸的专性吸附为主；去除有机质后，解吸量与解吸率下降，固液分配系数K_d值下降。(4)傅里叶变换红外光谱表明，土壤吸附Se(Ⅳ)与静电引力、络合反应和配位体交换有关；去除有机质减少了有机官能团的数量，导致吸附量减少。综上，有机质的存在，提升了酸性富硒土对Se(Ⅳ)的吸附容量和吸附强度，是造成酸性富硒土硒有效性较低的重要因素。     

Copper retention by Danish Spodosols in relation to contents of organic matter and aluminum,iron, and manganese oxides

Abstract

The importance of various soil components on copper (Cu) retention by Spodosois was investigated. Copper sorption and extraction were conducted on samples from the B horizon from six Danish Spodosois. The investigation was conducted on untreated samples, on hydrogen peroxide‐treated samples (to remove organic matter), on oxalate‐treated samples [to remove amorphous to poorly crystalline aluminum (Al) and iron (Fe) oxides], on hydroxylamine‐treated samples [to remove manganese (Mn) oxides]. Subfractions treated with hydrogen peroxide (H₂O₂) were further treated with oxalate and citrate‐bicarbonate‐dithionite (CBD). Sorption of Cu from an initial 10^‐6 M solution after 48 hours was determined in the pH range 3 to 7 using 0.1M sodium nitrate (NaNO₃) as the background electrolyte. The pH‐dependent sorption curve (sorption edge) was shifted to a higher pH with decreasing Al oxide content in the soils, and for the treated sample after removal of organic matter and Al and Fe oxides. A negligible effect was seen after removal of the Mn oxides because of their low abundance. Extraction of sorbed Cu at pH 4 to 6 with 0.1M nitric acid (HNO₃) for 24 hours confirmed the sorption results, in inasmuch as removal of the Al (and Fe) oxides increased Cu extractability. Therefore, it was concluded that in the soils investigated, Cu retention is mainly determined by the oxalate‐extractable Al fraction with a minor contribution due to crystalline Fe oxides.     

Retention of cobalt by an oxisol in relation to the content of iron and manganese oxides

Abstract

The study aims at determining the cobalt retention properties of various soil components. Therefore, cobalt (Co) sorptions and extractions were carried out using an Oxisol sample before (untreated) and after successive removal of organic matter and active manganese (Mn) oxides (H₂O₂‐treated) and iron (Fe) oxides (H₂O₂+CBD‐treated). A synthetic goethite was included for comparison. Sorption of the four sorbents was determined over a range of Co concentrations (initially 10^‐8 M to 10^‐4 M), pH values (3 to 8) and reaction times (2 hours to 504 hours). The Co species sorbed was Co(ll), since oxygen exclusion during sorption had no effect on the amount sorbed. The pH‐dependent sorption curve (sorption edge) was shifted to lower pH at decreasing initial Co concentration and increasing reaction time. The displacements, in particular of the sorption edges corresponding to the lowest initial Co concentrations, to successively higher pH following removal of Mn oxides, organic matter and Fe oxides could be attributed to sorption onto sites of decreasing Co affinity [Mn oxides (and organic matter) > Fe oxides > kaolinite]. Extractions of sorbed Co at pH 5.5–7.5 with 2 M HCI showed that the extractability decreased with increasing sorption time and decreasing initial Co concentration. The untreated and H₂O₂‐treated soil samples retained sorbed Co at least as firmly as the synthetic goethite, whereas the H₂O₂+CBD‐treated sample (kaolinite) was clearly less effective. The results emphasized the importance of the soil Mn and Fe oxides for Co retention in soils but also the necessity of taken interior sorption sites into consideration.     

Selenite adsorption by a variety of oxides

Abstract

Selenite adsorption by a variety of oxides consisting of iron (Fe), aluminum (Al), titanium (Ti), manganes (Mn), and silicon (Si), and by two humic acids were investigated in order to grasp selenite behavior and fixation mechanisms in soil. It was found that selenite was apparently adsorbed even by the Mn oxides on which surface negative charge was dominant in normal pH range (pH <4). No selenite adsorption was observed in the silicon dioxide (SiO₂) and the two humic acids. A sequential extraction of adsorbed selenite with competitive anions showed the differences of binding force or stability of adsorbed selenite among the minerals. While the goethite fixed selenite strongly, selenite adsorbed on the Mn oxide was easily released to the liquid phase with other anions, such as phosphate. Each mineral had its inherent characteristic in ligand exchange reactions accompanied with selenite sorption. Selenite sorption by the Mn and the Ti oxides resulted in large increase of surface negative charge, while only a little increase in the Fe and Al oxides. Proton consumption with selenite sorption was extremely smaller for the Mn oxide than for the others.     

Adsorption of Heavy Metals on Mixed Fe-Al Oxides in the Absence or Presence of Organic Ligands

A study was carried out on the adsorption of Co²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions on mixed Fe-Al oxides inthe absence or presence of increasing concentrations of oxalate or tartrate. Mixed Fe-Al oxides were prepared by precipitating at pH 5.5 mixtures of Fe and Al ions at initial Fe/Al molar ratios (R) of 0, 1, 2, 4, 10 and ∞ (R0, R1, R2, R4, R10 and R∞).The oxides aged 7 days at 20 °C or 30 days at 50 °C showed different chemical composition and physico-chemical and mineralogical properties. All the mixed Fe-Al oxides showed presence of poorly crystalline materials (ferrihydrite) even after prolonged aging. The heavy metals wereselectively adsorbed on the oxides. For all the precipitates aged7 days at 20 °C, the selectivity sequence wasPb²⁺> Cu²⁺ > Zn²⁺ > Co²⁺, but the pH at which 50% ofeach cation was adsorbed (pH₅₀) was different from sample tosample. It was found that usually the greater the amounts of Fe in Fe-Al gels the lower the pH₅₀ for each metal, but the adsorption of a heavy metal was not linearly related to Fe content. The pH₅₀ usually did not change significantly when the oxides were aged 30 days at 50 °C. Competitive adsorption of Cu and Zn on ferrihydrite (R∞) showed thatCu strongly prevented Zn adsorption even at an initial Zn/Cu molar ratio of 8, whereas Cu sorption was not inhibited. In thepresence of oxalate (OX) or tartrate (TR) (organic ligand/Pb molar ratio (r_L) from 0 to 7) the quantities of Pb adsorbedon the Fe-Al oxides usually increased with increasing r_L. The adsorption increase of Pb was particularly high on the oxidesricher in Fe (R4-R∞), but a significant increase was also observed on R0-R2 samples. The adsorption of Pb on the oxides hasbeen influenced not only by the presence and concentration of organic ligands but also by the sequence of addition of Pb and tartrate on the sorbents. It has been ascertained that on each oxide the greater amounts of Pb were adsorbed when tartrate wasadded before Pb and usually according to the following sequence: Tr before Pb > Pb before Tr > Pb + Tr > Pb.     

Molybdenum Determination in Mehlich‐1 and Mehlich‐3 Soil Test Extracts and Molybdenum Adsorption in Brazilian Soils

Abstract

The extractant Mehlich‐1 is routinely used in Brazil for determination of soil nutrients, whereas Mehlich‐3 has been suggested as a promising extractor for soil fertility evaluation. Both were used for extraction of molybdenum (Mo) in Brazilian soils with Mo dosage by the KI+H₂O₂ method. The Langmuir and Freundlich isotherms were used to study soil Mo adsorption. Mehlich‐1 extracted more Mo than Mehlich‐3 in soils with high contents of organic matter, clay, and iron (Fe) oxides. Mehlich‐3 and Mehlich‐1 extractions correlated positively and significantly with amorphous Fe oxides, crystalline Fe oxides, and organic matter. Molybdenum recovering rates correlated to crystalline Fe oxides and clay contents but not to organic matter, pH, and Mo adsorption capacity. Amorphous and crystalline Fe oxides, clay, and organic matter were responsible for most of the Mo adsorption. The Langmuir isotherm described better the Mo adsorption to soil amorphous Fe oxides and organic matter than the Freundlich isotherm.     

Influence of pH on the redox chemistry of metal (hydr)oxides and organic matter in paddy soils

Purpose

The primary purpose of this study was to determine how flooding and draining cycles affect the redox chemistry of metal (hydr)oxides and organic matter in paddy soils and how the pH influences these processes. Our secondary purpose was to determine to what extent a geochemical thermodynamic equilibrium model can be used to predict the solubility of Mn and Fe during flooding and draining cycles in paddy soils.

Material and methods

We performed a carefully designed column experiment with two paddy soils with similar soil properties but contrasting pH. We monitored the redox potential (Eh) continuously and took soil solution samples regularly at four depths along the soil profile during two successive flooding and drainage cycles. To determine dominant mineral phases of Mn and Fe under equilibrium conditions, stability diagrams of Mn and Fe were constructed as a function of Eh and pH. Geochemical equilibrium model calculations were performed to identify Mn and Fe solubility-controlling minerals and to compare predicted total dissolved concentrations with their measured values.

Results and discussion

Flooding led to strong Eh gradients in the columns of both soils. In the acidic soil, pH increased with decreasing Eh and vice versa, whereas pH in the alkaline soil was buffered by CaCO₃. In the acidic soil, Mn and Fe solubility increased during flooding due to reductive dissolution of their (hydr)oxides and decreased during drainage because of re-oxidation. In the alkaline soil, Mn and Fe solubility did not increase during flooding due to Mn(II) and Fe(II) precipitation as MnCO₃, FeCO₃, and FeS. The predicted levels of soluble Mn and Fe in the acidic soil were much higher than their measured values, but predictions and measurements were rather similar in the alkaline soil. This difference is likely due to kinetically limited reductive dissolution of Mn and Fe (hydr)oxides in the acidic soil. During flooding, the solubility of dissolved organic matter increased in both soils, probably because of reductive dissolution of Fe (hydr)oxides and the observed increase in pH.

Conclusions

Under alternating flooding and draining conditions, the pH greatly affected Mn and Fe solubility via influencing either reductive dissolution or carbonate formation. Comparison between measurements and geochemical equilibrium model predictions revealed that reductive dissolution of Mn and Fe (hydr)oxides was kinetically limited in the acidic soil. Therefore, when applying such models to systems with changing redox conditions, such rate-limiting reactions should be parameterized and implemented to enable more accurate predictions of Mn and Fe solubility.     

铝同晶替代对铁(氢)氧化物表面活性的影响研究进展

铝同晶替代现象在铁(氢)氧化物中普遍存在，可改变铁(氢)氧化物的结构、表面特性和反应活性，影响土壤中元素的行为、形态和归趋。运用文献计量法分析了铝同晶替代铁(氢)氧化物的国内外研究现状，分别综述了铝同晶替代对铁(氢)氧化物晶体结构、表面电荷和界面过程的影响，从静电作用、比表面积、位点组成与密度、Fe/Al位点亲和性以及空位缺陷等方面阐明了铝同晶替代对铁(氢)氧化物表面活性和吸附行为的影响机制。在此基础上，提出了未来研究应着眼于构建铝替代量–结构–反应活性定量关系、深入探究铝同晶替代铁(氢)氧化物不同晶面上的界面机制，以及将研究对象与体系过渡到实际土壤等。     

Adsorption and Desorption of Copper in Pasture Soils

Abstract

Copper (Cu) is bound strongly to organic matter, oxides of iron (Fe) and manganese (Mn), and clay minerals in soils. To investigate the relative contribution of different soil components in the sorption of Cu, sorption was measured after the removal of various other soil components; organic matter and aluminum (Al) and Fe oxides are important in Cu adsorption. Both adsorption and desorption of Cu at various pH values were also measured by using diverse pasture soils. The differences in the sorption of Cu between the soils are attributed to the differences in the chemical characteristics of the soils. Copper sorption, as measured by the Freundlich equation sorption constants [potassium (K) and nitrogen (N)], was strongly correlated with soil properties, such as silt content, organic carbon, and soil pH. The relative importance of organic matter and oxides on Cu adsorption decreased and increased, respectively, with increasing solution Cu concentrations. In all soils, Cu sorption increased with increasing pH, but the solution Cu concentration decreased with increasing soil pH. The cumulative amounts of native and added soil Cu desorbed from two contrasting soils (Manawatu and Ngamoka) during desorption periods showed that the differences in the desorbability of Cu were a result of differences in the physico‐chemical properties of the soil matrix. This finding suggests that soil organic matter complexes of Cu added through fertilizer, resulted in decreased desorption. The proportions of added Cu desorbed during 10 desorption periods were low, ranging from 2.5% in the 24‐h to 6% in the 2‐h desorption periods. The desorption of Cu decreased with increasing soil pH. The irreversible retention of Cu might be the result of complex formation with Cu at high pH.     

Determination of High Resolution Pore Water Profiles of Trace Metals in Sediments of the Rupel River (Belgium) using Det (Diffusive Equilibrium in Thin Films) and DGT (Diffusive Gradients in Thin Films) Techniques

The techniques of diffusional equilibrium in thin films (DET) and diffusional gradients in thin films (DGT) were used to obtain high resolution pore water profiles of total dissolved and labile trace (mobilizable) metals in the sediments of the Rupel River, Belgium. DGT measures labile metal species in situ by immobilizing them on a resin gel after diffusion through a diffusive gel whereas for DET an equilibrium is established between the DET gel and the pore water. Concentrations of Pb and Zn obtained by DGT were in good agreement with the results obtained by centrifugation, and thus were well buffered by rapid equilibrium with the solid phase, whereas Fe, Mn and Cd were very tightly bound to the sediment phase and large differences were observed between the labile and the total metal concentrations. Cu, Zn, Co and Ni show intermediated behavior. Good correlations were found between the profiles of As and Fe and Mn and Co for DET as well as DGT showing a close link between the geochemical behavior of these elements. Cu, Zn, Pb and Cd are also influenced by the reductive mobilization of Fe and Mn oxides but are also closely linked to the bacterial degradation of organic matter in the surface sediments as to the precipitation of metal sulfides in the deeper layers.     

Improvement of the NH2OH-HCl-HOAc method for extracting manganese and iron oxides in Japanese Andisols and other soil types in Japan

Abstract

We evaluated the validity of Tessier’s method as applied to the extraction of manganese (Mn) and iron (Fe) oxides in Japanese Andisols and other soil types in Japan. Using the original Tessier’s extractant mixture, 0.04 mol L^?1 hydroxylamine hydrochloride in 25% acetic acid (0.04 mol L^–1 NH₂OH-HCl in 25% HOAc), we found that substantial amounts of short-range-ordered Fe oxides were not extracted from allophanic Andisol samples and that considerable amounts of total Fe oxides were not extracted from all soil types. Relatively high extraction pH and large amounts of short-range-ordered Fe oxides in the Andisol samples might be responsible for incomplete extraction. Stoichiometric calculation indicated that the concentration of NH₂OH-HCl might be insufficient for complete extraction of Fe oxides. The extracted amounts of Mn and Fe increased with increasing concentration of NH₂OH-HCl in the extractant, and most of the Mn and Fe oxides in the soil samples, including samples with as much as 5.6% Fe, were extracted with 0.6 mol L^–1 NH₂OH–HCl in 25% HOAc. As judged from the simultaneous dissolution of aluminum (Al) and silicon (Si) minerals, extraction selectivity of Fe oxides with 0.6 mol L^–1 NH₂OH-HCl in 25% HOAc was comparable to that of the original Tessier’s method and better than that of a modified Community Bureau of Reference (BCR) sequential extraction procedure or a method using an extractant consisting of a mixture of oxalate and ascorbate, especially for Andisol samples.     

Removal of 17α-Ethinylestradiol by Biogenic Manganese Oxides Produced by the Pseudomonas putida strain MnB1

Synthetic and persistent endocrine disrupting chemicals (EDCs) such as 17α-ethinylestradiol (EE2) have been frequently detected in the effluent of wastewater treatment plants and induce hazards to humans and wildlife. In this study, biogenic Mn oxides were tested for the removal of EE2, and factors affecting the reaction were also investigated. The biogenic Mn oxides produced by Pseudomonas putida strain MnB1 were nano-sized and poorly crystallized particles. A concentration of 7.9 mg l⁻¹ biogenic Mn oxides showed 87% EE2 (1 mg l⁻¹) removal efficiency in 2 h, which confirms the excellent potential of biogenic Mn oxides for removal of estrogens. EE2 removal was enhanced at high Mn oxide doses and at low pH. Co-existing heavy metals significantly inhibit EE2 removal, due to their competition for the reactive sites of biogenic Mn oxides. Humic acid (HA) also obstructed EE2 removal, but the adverse effect was alleviated as HA concentration increased, possibly due to the formation of soluble complexes with the released Mn²⁺, of which adsorption onto Mn oxides reduces surface reactive sites.     

Kinetics of Iron and Manganese Release from Contaminated Calcareous Soils

Knowledge of the release of heavy metals (HM) and their chemical speciation is necessary for characterizing HM behavior in soils. The kinetics and characteristics of iron (Fe) and manganese (Mn) release were studied in 10 contaminated calcareous soils using 0.01 M calcium chloride (CaCl₂), 0.01 M ethylenediamine tetraacetic acid (EDTA), and 0.01 M malic acid (malic acid) extractions. Iron and Mn in soil samples were fractionated before and after 2084 h kinetic release using a sequential extraction procedure. The proportion of Fe and Mn released by EDTA was greater than that with CaCl₂ and malic acid. A power model satisfactorily described Fe and Mn release from soils. In general, the mean release rate of Fe was greater than that of Mn, indicating a greater rate of Fe release from contaminated soils. It was shown that Fe and Mn distributions were similar in native soils and they were mainly found in Fe-Mn oxides and organic-matter (OM) fractions. There were changes in the proportional distribution of Fe and Mn in all soils during the 2084 h kinetic study with different extraction solutions. In general, the proportions of Fe and Mn associated with carbonate (CARB) and OM fractions tended to decrease, with corresponding increases in the Fe-Mn oxides for Mn and residual (RES) fractions for Fe during the kinetic study with all extraction solutions. The Fe and Mn solubility at the initial and final stages of release was controlled by siderite (FeCO₃), vivianite [(Fe)₃(PO₄)₂·8H₂O], MnCO₃(am), MnHPO₄, and rhodochrosite (MnCO₃) minerals in all extraction solutions. Based on a risk assessment and percentage of release of metals, there is a high potential for Mn release into the food chain from contaminated soils.     

Length of Incubation for the Estimation of the Most Probable Number of Nitrifying Bacteria in Soil

Abstract

The Zn content in 8 soil chemical fractions was determined for 21 greenhouse soils and for 8 reference (open field) soils from Kochi Prefecture, Japan to investigate the forms of spontaneously accumulated Zn in the greenhouse soils associated with heavy application of fertilizers and manures. Sequential extraction method was applied to every soil and each Zn fraction was designated as exchangeable (Ex-Zn), Pb-displaceable (Pb-Zn), acid soluble (Aci-Zn), Mn oxide-occluded (MnO-Zn), organically bound (OM-Zn), amorphous Fe oxide-occluded (AFeO-Zn), crystalline Fe oxide-occluded (CFeO-Zn), and residual (Res-Zn) fractions. The Zn content of the greenhouse soils was significantly higher than that of the reference soils in every fraction, except for the CFeO-Zn, and Res-Zn fractions. The Pb-Zn, Aci-Zn, and MnO-Zn fractions showed a difference of more than 60% in the total Zn content between the two soil groups. The amounts of Zn extracted in the Pb-Zn, Aci-Zn, MnO-Zn, AFeO-Zn, and CFeO-Zn fractions of the greenhouse soils increased con-comitantly with the accumulation of applied macro-nutrients. These results indicated that the accumulation of Zn in greenhouse soils caused by intensive fertilization had proceeded through specific adsorption onto or occlusion by the oxides and hydroxides of Fe and Mn in soils.     

Types of Crystalline Iron Oxides and Phosphate Adsorption in Variable Charge Soils

The types,contents and morphologies of crystalline Fe oxides and their relations to phosphate adsorption on the clay fractions in soils with varable charge in southern China were investigated by means of XRD,TEM,EMA and chemical analysis methods.Results indicated that the types and contents of crystalline Fe oxides varied with the soils examined.The dominant crystaline Fe oxide was hematite in the latosols and goethites in the red soils.In yellow-brown soils,the only crystalline Fe oxide was goethite.The difference between Ald and Alo came mainly from the Al Substituting for Fe in the Fe oxides.The crystal morphology of goethite appeared mainly as subrounded flat or iso-dimensional rather than acicular particles,Hematities occurred in plates of various thickness,Their MCDa/MCDc ratios in the latosols and red soils were generally above 1.5 and below 1.5,respectively.The MCD values of goethites and hematites were 15-25nm and 20-35nm,and their specific surface areas were 80-120m^2/g and 35-75m^2/g,respectively.The goethite crystals were generally smaller,Variations of the total amounts of crystalline Fe oxides in clay fractions were not related to plhophats adsorption.The types,contents and morphologies of crystalline Fe oxides in the soils remarkably affected phosphate adsorption characteristics of the soils.The phosphate adsorption of goethite was much greater than that of hematite,The higher th MCDa/MCDc ratio of hematite,the lower the phosphate adsorption.     

Distribution of Trace Elements in Soils from the Sudbury Smelting Area (Ontario,Canada)

The sequential extraction procedure proposed by the European Commission Measurement and Testing Programme, combined with Scanning Electron Microscopy and Energy Dispersive X-ray Analysis(SEM/EDS), was applied to identify and quantify the chemical andmineralogical forms of Cu, Ni, Fe, Mn, Zn, Pb, Cr and Cd presentin the topsoil from a mining and smelting area near Sudbury (Ontario, Canada). The possible mobility of the chemical forms was also assessed. The metal fractions: (1) soluble and exchangeable, (2) occluded in manganese oxides and in easily reducible iron oxides, (3) organically bound and in form of sulphides, (4) residual mainly present in the mineral lattice structures were separated. Cu and Ni were the major metallic contaminants, occurring in soils in broad ranges of concentrations: Cu 11–1890 and Ni 23–2150 mg kg^-1. Cu was uniformly distributed among allthe extracted fractions. Ni was found associated mainly withthe residual forms, accounting for 17–92%, with an averageof 64%, of the total Ni present in the soils. Fe, Mn, Zn,Pb, Cr and Cd, while occurring in most analysed samples innormal soil concentrations, were primarily held in theresidual mineral fraction (on average >50%). The solubleand exchangeable forms made a small contribution (≤8.1%)to the total content of metals extracted. At least 14% ofthe total Cd, Mn and Pb was mobilised from the reducibleforms. The oxidizable fraction assumed mean values higher than10% only for Pb and Zn. Statistical treatment of the experimental data showed significant correlations between totalmetal content of the soils, some soil properties such as pH value, clay and organic matter content, and metal concentrationsin the various fractions. SEM/EDS analysis showed Fe in form ofoxides and sulphides in soils and Cu, Ni, Mn, Zn and Cr in association with iron oxides. Numerous black carbonaceous particles and precipitates of aluminium fluoride salts, observedin the solid residue left after `total’ digestion, were found tocontain Fe, Ni and Cr.     

外源铜和镍在土壤中的化学形态及其老化研究

采用连续提取法测定了外源铜和镍进入田间土壤后的化学形态分布,比较研究了这2种重金属在3种不同类型土壤(红壤,水稻土和潮土)中随老化时间的形态转化和分布.结果表明,外源铜以残留态(40%～60%)和EDTA可提取态(40%)为主;随老化时间,EDTA可提取态、易还原锰结合态及铁铝氧化态向残留态转化;外源镍在酸性红壤中以可交换态(40%)和残留态(30%～50%)为主,在中性水稻土中以EDTA可提取态(30%)和残留态(30%～50%)为主,在碱性潮土中以铁铝氧化态(20%)和残留态(40%)为主.随老化时间,水溶态、可交换态、EDTA可提取态等向残留态转化.土壤pH较低时水溶态和可交换态含量较高,但是同时随老化时间的降低量也明显;pH较高时有利于易还原锰结合态和有机质结合态的转化.     

Dissolved organic matter sorption by mineral constituents of subsoil clay fractions

The retention of dissolved organic matter in soils is mainly attributed to interactions with the clay fraction. Yet, it is unclear to which extent certain clay‐sized soil constituents contribute to the sorption of dissolved organic matter. In order to identify the mineral constituents controlling the sorption of dissolved organic matter, we carried out experiments on bulk samples and differently pretreated clay‐size separates (untreated, organic matter oxidation with H₂O₂, and organic matter oxidation with H₂O₂ + extraction of Al and Fe oxides) from subsoil horizons of four Inceptisols and one Alfisol. The untreated clay separates of the subsoils sorbed 85 to 95% of the dissolved organic matter the whole soil sorbed. The sorption of the clay fraction increased when indigenous organic matter was oxidized by H₂O₂. Subsequent extraction of Al and Fe oxides/hydroxides caused a sharp decrease of the sorption of dissolved organic matter. This indicated that these oxides/hydroxides in the clay fraction were the main sorbents of dissolved organic matter of the investigated soils. Moreover, the coverage of these sorbents with organic matter reduced the amount of binding sites available for further sorption. The non‐expandable layer silicates, which dominated the investigated clay fractions, exhibited a weak sorption of dissolved organic matter. Whole soils and untreated clay fractions favored the sorption of ”︁hydrophobic” dissolved organic matter. The removal of oxides/hydroxides reduced the sorption of the lignin‐derived ”︁hydrophobic” dissolved organic matter onto the remaining layer silicates stronger than that of ”︁hydrophilic” dissolved organic matter.     

Source Identification and Speciation of Metals in the Topsoil of the Khli Ti Watershed, Thailand

The enrichment factor, multivariate analysis and metal speciation studies were used to identify degree, source and dispersal of metal contamination in Khli Ti watershed, Thailand. Topsoil samples were collected throughout the watershed, analyzed for total metal concentration. Sequential extraction was also carried out to determine geochemical phases of metals which were identified as exchangeable and bound to carbonates, Fe–Mn oxides, organic matter and residuals. Soil characteristics including pH, total organic carbon, redox potential, cation exchange capacity and texture were also analyzed. Principal component analysis yielded three metal groups which explained 83% of the variance. The concentrations of metals which were derived from lithogenic origin, such as Co, Cr, Fe, Ni and V were in natural background levels and were mostly bound to the residual phase. The remaining elements (i.e. Ba, Cd, Cu, Pb, Sb and Zn) were associated with the contamination from previous activities of the Pb-ore concentrator and Zn–Pb mining. Anthropogenic contamination mainly increased Pb and Zn bound to Fe–Mn oxides at the expense of residual fraction. Even though low exchangeable Pb contents in Khli Ti soils indicated low availability to plants, Pb bound to Fe–Mn oxides fraction might increase its mobility under reducing conditions.     

A NanoSIMS study on the distribution of soil organic matter,iron and manganese in a nodule from a Stagnosol

The development of Stagnosols is the consequence of perched water tables, which induce periodic oxidizing and reducing conditions. These cause the spatial distribution of iron (Fe) and manganese (Mn) between the soil matrix and ferromanganese concretions or nodules. Since oxides of these metals may interact with organic matter, we studied their spatial distribution in bulk material from the Bg horizon of a Stagnosol and in a nodule separated from the horizon. We used wet‐chemical analyses and X‐ray diffractometry together with microscopic techniques and nano‐scale secondary ion mass spectrometry (NanoSIMS), the latter allowing for a submicrometre‐scale spatial resolution. X‐ray diffractometry revealed the presence of quartz, clay minerals, micas and feldspars as the dominant minerals and indicated the presence of lepidocrocite. Relative to the bulk horizon material, the nodule was strongly enriched in organic C (by a factor of 31) and pedogenic (dithionite‐extractable) Fe and Mn (by factors of 2.2 and 62). We selected two regions on a thin section of the nodule for NanoSIMS investigations after studying the element distribution by scanning‐electron microscopy (SEM): one was located in an almost closed pore, the other one along an elongated pore. The NanoSIMS measurements allowed a clearer distinction of Fe‐ and Mn‐accumulation zones than SEM‐EDS. The evaluation of the NanoSIMS measurements by unsupervised classification revealed that zones containing silicates and Mn oxides and the transitional zones between Fe and Mn oxides were particularly enriched in soil organic matter, while, with one exception, the pure Fe‐accumulation zones did not indicate the presence of soil organic matter.