Solubility and Stability of Calcium Arsenates at 25∘C

The solubility and stability of calcium arsenates at 25 ^°C was determined by both precipitation and dissolution experiments. Ca₃(AsO₄)₂? 3H₂O(c), Ca₃(AsO₄)₂? 2¹/₄H₂O(c), Ca₅(AsO₄)₃(OH)(c) and Ca₄(OH)₂(AsO₄)₂? 4H₂O(c) were identified in our experiment over a wide range of pH and for Ca/As molar ratios between 1.25 and 4.0. The solids precipitated at pH = 3 ～ 7 and Ca/As = 1.5 were phase-pure and well-crystallized Ca₃(AsO₄)₂? xH₂O(c) and had relatively larger grain size than those formed at pH > 7. Based on the analytical results and using the computer program PHREEQC, the solubility products for Ca₃(AsO₄)₂? 3H₂O(c), Ca₃(AsO₄)₂? 2¹/₄H₂O(c), Ca₅(AsO₄)₃(OH)(c) and Ca₄(OH)₂(AsO₄)₂? 4H₂O(c) were calculated as K _sp of 10^{? 21.14}(10^{? 20.01} ～ 10^{? 22.02}), 10^{? 21.40}(10^{? 20.08} ～ 10^{? 21.98}), 10^{? 40.12}(10^{? 37.53} ～ 10^{? 42.72}) and 10^{? 27.49}(10^{? 26.10} ～ 10^{? 28.91}), respectively. Correspondingly, the free energies of forming (Δ G _f ^o ) of these calcium arsenates were calculated to be ?3787.87 kJ/mol, ?3611.50 kJ/mol, ?5096.47 kJ/mol and ?4928.86 kJ/mol.     

Geochemical Behavior of Arsenic in Kelly Lake,Ontario

The objective of this study was to investigate the geochemical behavior of As in Kelly Lake, Ontario. A total of 65 water samples were collected from August 1998 to October 1999. Concentrations of As (total, dissolved and particulate) were determined in each water sample. Thermodynamic models suggest that As interactions with Fe hydroxides could limit As concentrations in Kelly Lake water samples (by adsorption and forming arsenate minerals). It was concluded that Fe(II) arsenates (and not Fe(III) arsenate) formation/dissolution could control As concentrations in Kelly Lake. The activity of As (in terms of HAsO₄ ^2-) in the water samples was calculated from the monitoring data. The calculated activities were superimposed on the thermodynamic model of As minerals. All the calculated activity observations clustered around the Fe₃(AsO₄)₂.8H₂O(c)/Fe₄Fe₂(OH)₁₂. SO₄(c).These observation suggest that the solubility of As in Kelly Lake is controlled by the equilibrium of Fe₃(AsO₄)₂.8H₂O(c)/Fe₄Fe₂(OH)₁₂.SO₄(c), as suggested by thermodynamic solubility isotherms. The As concentration data were limited and spatial trend could not be verified in Kelly Lake water samples, although a decreasing trend in As concentrations at all depths was noticed from June to October 1999.     

lead contamination of GROUNDWATER in an INDUSTRIAL complex

The objectives of this study were to investigate Pb contamination of a shallow groundwater aquifer underneath an industrial complex and Pb chemical forms that may be found in the study area. Concentrations of Pb in the groundwater samples ranged between 0.04 to 1570.12 µg L^-1 (10^-9.4 to 10^-5.1 mole L^-1), with an average of 88.2 µg L^-1 (10^-6.4 mole L^-1). The results of analysis of variance showed that sampling locations had significantly (p <0.05) affected Pb concentrations in the groundwater samples. Contour map of Pb concentrations and Pb/Cl ratios indicated that Pb contamination of groundwater could be associated to the seepage of irrigation water, corrosion of buried metallic structure (largely of iron), and leachate from the industrial dust pile. Thermodynamic modeling approach was used to speciate total Pb concentrations in the groundwater samples. The results of these computations revealed that, in the very dilute groundwater (salinity < 1 parts per thousand, i.e., ppt), Pb²⁺ and Pb-OH complexes were the most significant chemical forms. Groundwater salinities between 1 to 5 ppt, Pb²⁺, Pb-OH, Pb-Cl, Pb-HCO₃, and Pb-SO₄ were present in appreciable concentrations, depending on their respective anionic concentrations. Over 80% of the total Pb in water was present in Pb-Cl complexes in groundwater samples with salinities >40 ppt. A comparison of the calculated activities of Pb²⁺ and the thermodynamic solubility isotherms of Pb minerals suggested that mineral Pb(OH)₂(c) was too soluble to precipitate in these water samples. The distribution trend in Pb²⁺ activities do not support equilibrium with Pb₃(PO₄)₂(c). Formation of PbCO₃(c) and Pb₂CO₃Cl₂(c) in the groundwater samples appeared to be the most logical option. From the foregoing, it was concluded that Pb-CO₃ minerals can limit Pb solubility in saline groundwaters.     

Arsenic Chemistry in Soils: An Overview of Thermodynamic Predictions and Field Observations

Published information, both theoretical and experimental, on As chemical behavior in soils is reviewed. Because of many emission sources, As is ubiquitous. Thermodynamic calculations revealed that As(V) species (HAsO ₄ ^2- >H₂AsO ₄ ^- at pH 7) are more abundant in soil solutions that are oxidized more than pe+pH>9. Arsenic is expected to be in As(III) form (HAsO ₂ ⁰ =H₃AsO ₃ ⁰ >AsO ₂ ^- =H₂AsO ₃ ^- at pH 7) in relatively anoxic soil solutions with pe+pH<7. Adsorption on soil colloids is an important As scavenging mechanism. The adsorption capacity and behavior of these colloids (clay, oxides or hydroxides surfaces of Al, Fe and Mn, calcium carbonates, and/or organic matter) are dependent on ever-changing factors, such as hydration, soil pH, specific adsorption, changes in cation coordination, isomorphous replacement, crystallinity, etc. Because of the altering tendencies of soil colloids properties, adsorption of As has become a complex, empirical, ambiguous, and often a self contradicting process in soils. In general, Fe oxides/hydroxides are the most commonly involved in the adsorption of As in both acidic and alkaline soils. The surfaces of Al oxides/hydroxides and clay may play a role in As adsorption, but only in acidic soils. The carbonate minerals are expected to adsorb As in calcareous soils. The role of Mn oxides and biogenic particles in the As adsorption in soils appears to be limited to acidic soils. Kinetically, As adsorption may reach over 90% completion in terms of hours. Precipitation of a solid phase is another mechanism of As removal from soil solutions. Thermodynamic calculations showed that in the acidic oxic and suboxic soils, Fe-arsenate (Fe₃(AsO₄)₄)₂) may control As solubility, whereas in the anoxic soils, sulfides of As(III) may control the concentrations of the dissolved As in soil solutions. In alkaline acidic oxic and suboxic soils, precipitation of both Fe- and Ca-arsenate may limit As concentrations in soil solutions. Field observations suggest that direct precipitation of discrete As solid phases may not occur, except in contaminated soils. Chemisorption of As oxyanions on soil colloid surfaces, especially those of Fe oxide/hydroxides and carbonates, is believed to a common mechanisms for As solid phase formation in soils. It is suggested that As oxyanions gradually concentrate on colloid surfaces to a level high enough to precipitate a discrete or mixed As solid phase. Arsenic volatilization is another As scavenging mechanism operating in soils. Many soil organisms are capable of converting arsenate and arsenite to several reduced forms, largely methylated arsines which are volatile. These organisms may generate different or similar biochemical products. Methylation and volatilization of As can be affected by several biotic (such as type of organisms, ability of organism for methylation, etc.) and abiotic factors (soil pH, temperature, redox conditions, methyl donor, presence of other ions, etc.) factors. Information on the rate of As biotransformations in soils is limited. In comparison to the biologically assisted volatilization, the chemical volatilization of As in soils is negligible.     

Effects of Land Use on Hydrochemistry and Contamination of Karst Groundwater from Nandong Underground River System, China

The Nandong Underground River System (NURS) is located in Southeast Yunnan Province, China. Groundwater in NURS plays a critical role in socio-economical development of the region. However, with the rapid increase of population in recent years, groundwater quality has degraded greatly. In this study, the analysis of 36 groundwater samples collected from springs in both rain and dry seasons shows significant spatial disparities and slight seasonal variations of major element concentrations in the groundwater. In addition, results from factor analysis indicate that NO ₃ ^? , Cl^?, SO ₄ ^2? , Na⁺, K⁺, and EC in the groundwater are mainly from the sources related to human activities while Ca²⁺, Mg²⁺, HCO ₃ ^? , and pH are primarily controlled by water–rock interactions in karst system with Ca²⁺ and HCO ₃ ^? somewhat from anthropogenic inputs. With the increased anthropogenic contaminations, the groundwater chemistry changes widely from Ca-HCO₃ or Ca (Mg)-HCO₃ type to Ca-Cl (+NO₃) or Ca (Mg)-Cl (+NO₃), and Ca-Cl (+NO₃+SO₄) or Ca (Mg)-Cl (+NO₃+SO₄) type. Concentrations of NO ₃ ^? , Cl^?, SO ₄ ^2? , Na⁺, and K⁺ generally show an indistinct grouping with respect to land use types, with very high concentrations observed in the groundwater from residential and agricultural areas. This suggests that those ions are mainly derived from sewage effluents and fertilizers. No specific land use control on the Mg²⁺ ion distribution is observed, suggesting Mg²⁺ is originated from natural dissolution of carbonate rocks. The distribution of Ca²⁺ and HCO ₃ ^? does not show any distinct land use control either, except for the samples from residential zones, suggesting the Ca²⁺ and HCO ₃ ^- mainly come from both natural dissolution of carbonate rocks and sewage effluents.     

Soluble components of sediments and their relation with dissolved arsenic in aquifers from the Hetao Basin,Inner Mongolia

Purpose

High groundwater arsenic (As) and salinity have been detected in aquifers of the Hetao Basin in Mongolia which have caused serious public health concerns. The objective of this study was to characterize the distributions of the soluble components in sediment in different lithologies and depths and to assess the relationship between soluble As in sediments and dissolved As in groundwater.

Materials and methods

One hundred and one sediment samples and 13 groundwater samples were collected from four boreholes at varied depths. In addition to total chemicals and mineralogical phases of sediments, the soluble components (including major ions and As, Fe, and Mn) in sediments and dissolved chemicals in groundwater were analyzed.

Results and discussion

Clay or silty clay had relatively higher EC values (189–805 μS cm^?1) than aquifer sands (approximately 92–261 μS cm^?1). The major soluble components were Na⁺, Ca²⁺, HCO₃ ^?, and SO₄ ^2?, which were more variable in clay samples than fine sand samples. Soluble As concentrations ranged between 2 and 950 μS cm^?1, and high contents generally occurred in clay sediments with high contents of soluble Fe and Mn. A comparison of chemicals between soluble components in sediments and dissolved species in groundwaters at matched depths showed that chemicals were preferentially partitioned into sediments at the mountain front and deep aquifers (>60 m), while partitioned into groundwater in the shallow aquifers (<60 m) of the flat plain. Arsenic was preferentially partitioned into groundwater in aquifers with relatively low dissolved SO₄ ^2?.

Conclusions

Groundwater components were mostly sourced from corresponding sediments. In clay sediments, As was desorbed from the surface sites along with other soluble components. Under reducing conditions, reduction of Fe oxides with high surface sites for As adsorption led to a weak association of As with other phases (such as carbonates), and therefore resulted in high dissolved As concentrations and low As partition between sediments and groundwater in deep aquifers.

     

Inorganic anion sorption and interactions with phosphate sorption by hydrous ferric oxide gel

The sorption of inorganic anions by hydrous ferric oxide gel (Fe gel) from 10 ^?1 M NaClO₄ at pH 6.5 decreased in the order: orthophosphate (H₂PO₄)>arsenate (H₂AsO₄) = selenite (HseO3) > silicate (H₄SiO₄) > molybdate (MoO²₄^?) > sulphate (SO²₄^?) > selenate(SeO²₄^?)>chloride (Cl^?) = nitrate (NO^?₃). When each anion was added to Fe gel with an equimolar quantity of H₂PO^?₄, there was no detectable effect of SO²₄^?, SeO²₄^?, Cl^?, and NO^?₃ on the amount of H₂PO^?₄ sorbed. Other anions depressed H₂PO₄ sorption in the order H₂AsO₄ >HseO₃ > H₄SiO₄ > MoO²₄. At the lowest level of anion addition (500 mmol kg ^?1), H₂PO₄ sorption was depressed by no more than 6% of the sorption level in the absence of a competing anion. In contrast, at the highest level of anion addition (1700 mmol kg-¹ of each), H₂AsO₄ decreased H₂PO₄ sorption by 44%. The sorption of SO₄^? was completely eliminated when this anion was added with equimolar amounts of H₂PO₄. The ability of anions to compete with H₂PO₄ for sorption sites could not be explained solely by the results obtained for the sorption of each anion alone. Thus, H₂AsO₄ was more competitive than H₂PO₄ when added together, even though more H₂PO₄ than H₂AsO₄ was sorbed when each anion was added alone. Although H₂PO₄ was sorbed in larger amounts, there is no evidence to suggest that H₂PO₄. H₂AsO₄, and HseO₃ were sorbed on different sites.     

Sorption of As(V) Species from Aqueous Systems

Arsenic is of increasing environmental concern due to risk to plants, animal and human health. In aqueous systems arsenic is dominated by the As^V oxyanions H₂AsO₄ ^- and HAsO₄ ^2- under oxidizing conditions. The possibility to remove arsenic from aqueous solutions, using sorption processes, was studied with both inorganic and organic-based sorbents. Both of tested inorganic sorbents, calcined synthetic hydrotalcite and calcined natural boehmite, were acceptable for removal of As^V compounds from aqueous systems at laboratory temperature (20 °C) and neutral pH due to their crystal structure changes. They were able to remove more than 70% of As^V compounds from aqueous solution at low sorbent-solution ratios (1 g L^-1 and 2.6 g L^-1, respectively) and relatively high concentration of AsO₄ ^3- ions in the initial solution (about 2.10^-3 mol L^-1). Humic acid-type sorbents (i.e. pure humic acid and oxihumolite) efficiences remined low even at increased sorbent-solutionratios (about 20 g L^-1) and significantly lower concentrations of As in the initial solution. At higher pH values (about 9), the sorption process slightly improved due to solubility of humic substances in alkaline solutions. The sorption increment did not exceed 50% of the initial As content. These results were confirmed by infrared spectroscopy. Both the original calcined and the sorbed inorganic sorbent samples show significant As-O vibrations, while in spectra of original and sorbed oxihumolite no significant As-O vibrations were observed, due to negligible content of sorbed As compounts.     

Leaching of nutrients and major ions from an arable field with an unfertilized fallow as infield buffer zone

Abstract

Leaching of nutrients and major ions from a tile-drained arable field was evaluated over a 25-year period (1980–2005). The soil, classified as a Gleyic Cambisol, received moderate applications of fertilizers. During later years the soil was more frequently under ley, and since 2002, an area of ravine (29% of the total field) has been managed as a permanent fallow with annual cutting without removing the grass material. A decrease in the concentration of nitrate nitrogen (NO₃?N) in the drainwater was estimated in 1980–2005, based on non-parametric tests on concentrations flow-normalized with a robust fitting curve procedure. The average concentration of soluble reactive phosphorus after pre-filtration (RP_f) of 0.030 mg l^?l corresponded to a calculated degree of phosphorus saturation (DPS) of 6% in acid extract of ammonium lactate from the topsoil. Between 1995 and 2005, concentrations of suspended solids (SS), RP and total organic carbon (TOC) decreased significantly in the drainage water from the entire field. Throughout the entire period, there was a negative net accumulation of the major ions to the soil. The order of decrease in relative terms was calcium (Ca²⁺)>magnesium (Mg²⁺)>sulphate (SO₄ ^2?)>chloride (Cl^?)>potassium (K⁺). In addition, the soil phosphorus (P) balance was negative. A significant reduction in ion concentrations in the drainwater, including sodium (Na⁺) and hydrogen carbonate HCO₃ ^?, was estimated. The order of reduction in relative terms was: SO₄ ^2?>Mg²⁺>Ca²⁺>HCO₃ ^?>Cl^?>NO₃ ^?>Na⁺>HPO₄. Altogether these trends were equal to approximately 0.1 mmol_c l^?1. yr^?1 of positively and negatively charged ions. Based on measured concentrations, decreasing trends in SO₄ ^2? and Ca²⁺ were also observed in the shallow groundwater (3.6 m below the soil surface), while K⁺, Na⁺ and HCO₃ ^? tended to increase. Trends of cations in deeper groundwater (5.8 m below the soil surface) were in some cases the opposite of the trends in the drainwater.     

Short-Term Effects of Arsenate-Induced Toxicity on Growth, Chlorophyll and Carotenoid Contents, and Total Content of Phenolic Compounds of Azolla filiculoides

This study evaluated the toxic effects of arsenic (As) on the growth, total antioxidant activity, total content of phenolic compounds, and content of photosynthetic pigments of Azolla filiculoides. The aquatic fern was propagated and exposed to Yoshida nutrient solution contaminated with sodium arsenate (Na₂HAsO₄??7H₂O) at six concentrations (5, 10, 20, 30, 60, and 120???g?As?mL^?1), including the control without As contamination. Azolla cultures were kept under environmental chamber conditions?26??C, 12?h photoperiod and 80% HR for 96?h. Increased As concentrations (>30???g?mL^?1) significantly diminished growth of A. filiculoides and the total content of chlorophyll and total phenolic compounds, but significantly enhanced of total carotenoid?+?xanthophylls content. The concentrations of 5 and 10???g?As?mL^?1 significantly stimulated the growth of A. filiculoides. This aquatic fern tolerates As concentrations lower than 30???g?mL^?1, and its maximum As accumulation (28???g?g^?1 dry weight) was achieved when exposed to 60???g As mL^?1, but showing clear symptoms of As toxicity.     

Biotic and abiotic processes controlling water chemistry during snowmelt at rabbit ears pass,Rocky Mountains,Colorado, U.S.A.

The chemical composition of snowmelt, groundwater, and streamwater was monitored during the spring of 1991 and 1992 in a 200-ha subalpine catchment on the western flank of the Rocky Mountains near Steamboat Springs, Colorado. Most of the snowmelt occurred during a one-month period annually that began in mid-May 1991 and mid-April 1992. The average water quality characteristics of individual sampling sites (meltwater, streamwater, and groundwater) were similar in 1991 and 1992. The major ions in meltwater were differentially eluted from the snowpack, and meltwater was dominated by Ca²⁺, SO ₄ ^2? , and NO ₃ ^? . Groundwater and streamwater were dominated by weathering products, including Ca²⁺, HCO ₃ ^? (measured as alkalinity), and SiO₂, and their concentrations decreased as snowmelt progressed. One well had extremely high NO ₃ ^? . concentrations, which were balanced by Ca²⁺ concentrations. For this well, hydrogen ion was hypothesized to be generated from nitrification in overlying soils, and subsequently exchanged with other cations, particularly Ca²⁺. Solute concentrations in streamwater also decreased as snowmelt progressed. Variations in groundwater levels and solute concentrations indicate that most of the meltwater traveled through the surficial materials. A mass balance for 1992 indicated that the watershed retained H⁺, NH ₄ ⁺ , NO ₃ ^? , SO ₄ ^2? and Cl^? and was the primary source of base cations and other weathering products. Proportionally more SO ₄ ^2? was deposited with the unusually high summer rainfall in 1992 compared to that released from snowmelt, whereas NO ₃ ^? was higher in snowmelt and Cl^? was the same. The sum of snowmelt and rainfall could account for greater than 90% of the H⁺ and NH ₄ ⁺ retained by the watershed and greater than 50% of the NO ₃ ^? .     

Specific features of the chemical composition of groundwater in floodplain soils of the Selenga River delta (Lake Baikal region)

The chemical composition of groundwater has been studied at several test plots in the Selenga River delta. The fresh groundwater containing calcium bicarbonates favors the formation of nonsaline soils in the delta and, hence, contributes to preservation of fresh water in Lake Baikal. The role of groundwater as a source of dissolved organic matter, iron compounds, phosphorus, and other elements is discussed. It is shown that the depth and chemical composition of the groundwater in particular areas depend on the character of the mesotopography, the drainage of the area, and the soil properties. After the flood period, the concentrations of Ca²⁺, HCO ₃ ^? , Fe³⁺, and SO ₄ ^2? in the groundwater increase with the rise in the soil temperatures. In the dry periods, the concentrations of Na⁺ and Cl^? ions increase, whereas the concentration of C_total decreases.     

Short‐Term Effect of Lime,Phosphorus, and Iron Amendments on Water‐Extractable Lead and Arsenic in Orchard Soils

Abstract

Lead arsenate was extensively used to control insects in apple and plum orchards in the 1900s. Continuous use of lead arsenate resulted in elevated soil levels of lead (Pb) and arsenic (As). There are concerns that As and Pb will become solubilized upon a change in land use. In situ chemical stabilization practices, such as the use of phosphate‐phosphorus (P), have been investigated as a possible method for reducing the solubility, mobility, and potential toxicity of Pb and As in these soils. The objective of this study was to determine the effectiveness of calcium carbonate (lime), P, and iron (Fe) amendments in reducing the solubility of As and Pb in lead‐arsenate‐treated soils over time. Under controlled conditions, two orchard soils, Thurmont loam (Hapludults) and Burch loam (Haploxerolls), were amended with reagent‐grade calcium carbonate (CaCO₃), iron hydroxide [Fe(OH)₃], and potassium phosphate (KH₂PO₄) and incubated for 16 weeks at 26°C. The experimental results suggested that the inorganic P increased competitive sorption between H₂PO₄ ^? and dihydrogen arsenate (H₂AsO₄ ^?), resulting in greater desorption of As in both Thurmont and Burch soils. Therefore, addition of lime, potassium phosphate, and Fe to lead‐arsenate‐contaminated soils could increase the risk of loss of soluble As and Pb from surface soil and potentially increase these metal species in runoff and movement to groundwater.     

Evaluation of the Nutrients Variability in Sap of Different Petiole Samples in Tomato Plant

The aim of this trial was to study the variability of anions and cations concentrations in the different petiole samples: young leaves (YL), mature leaves (ML) and aged leaves (AL). The experimental design consisted of four blocks and four plants per block being each plant one replication. In each plant, petiole samples were collected at 135 days after transplanting (DAT) to determine chloride, nitrate-nitrogen, phosphate-phosphorus, sulfate-sulfur, sodium, potassium, calcium, and magnesium (Cl^?, NO₃^–N, H₂PO₄^–P, SO₄^2–S, Na^+, K⁺, Ca²⁺ and Mg²⁺) concentrations. Our results showed that the selection of sample petiole in the tomato crop did not modify the Ca²⁺, Cl^?, SO₄^2–S and Na⁺ concentrations, while NO₃^–N, K⁺, Mg²⁺ and H₂PO₄^–P concentrations showed a great variability due to the selection of the sample petiole, therefore it is necessary to be careful with the sample selection.     

Selenium Behavior in Open Bulk Precipitation, Soil Solution and Groundwater in Alluvial Fan Area in Tsukui, Central Japan

A monitoring study was carried out in an alluvial fan area in Tsukui, Central Japan during the study period of 1999–2003, in order to explain selenium (Se) behaviors in ecosystem combined with air, soil and groundwater. Monthly Se concentrations in open bulk precipitation (rainfall+aerosol, gaseous deposition and etc.), soil solution (collected by porous ceramic-cup) and groundwater ranged from 0.1 to 1.4 μg L^?1 (volume-weighted average: 0.34 μg L^?1), 0.21 to 1.0 μg L^?1 (0.48 μg L^?1) and 1.6 to 2.4 μg L^?1 (2.2 μg L^?1), respectively. Se concentration in open bulk precipitation was negatively correlated with the rainfall amount. Se concentration in soil solution significantly increased with DOC concentration in soil solution. Besides, despite atmospheric Se input and rainfall to the grassland study area, Se concentration in soil solution and groundwater received no significant effect from the rainfall amount, pH, Se, DOC, SO₄ ^2?, NO₃ ^? and EC in rainfall. Even though Se concentrations in groundwater were significantly correlated with soil solution volume, Se, DOC and NO₃ ^? and groundwater level, the result of multiple regression analyses (MRA) indicated that the groundwater Se was negatively influenced by groundwater level, which depended on groundwater recharge. Se was transported into the groundwater through the groundwater recharge that largely increased in this alluvial fan study area after heavy rain.     

Immobilization of trace metals by phosphates in contaminated soil near lead/zinc mine tailings evaluated by sequential extraction and TCLP

Purpose

The combined contamination of Pb, Zn, Cu, Cd, and As in the soils near lead/zinc mine waste posed a potential threat to the surrounding environment. Mitigation methods are needed to reduce the environmental risk. The aims of this paper were to evaluate the feasibility and efficiency of different forms of phosphates in remediating combined contamination caused by multi-metals and arsenic near the lead/zinc mining tailings.

Materials and methods

The tested soil was taken from a clayey illitic thermic typic epiaquepts soil (depth of 0–20 cm) near a lead and zinc mine tailing, located in Shaoxing, Zhejiang Province, China. Four pure chemical reagents, K₂HPO₄, Ca(H₂PO₄)₂.2H₂O, Ca₃(PO₄)₂, and Ca₅(PO₄)₃OH, were added to the soil in solution form as the trace metal stabilizing additives at a P application rate equivalent to 2,283 mg P/kg-soil and P/Pb molar ratio of 1. Shifts in trace metal speciation were determined using sequential extraction procedures and relative toxicities were evaluated using the standard EPA toxicity characteristic leaching procedure (TCLP).

Results and discussion

The addition of K₂HPO₄, Ca₃(PO₄)₂, Ca₅(PO₄)₃OH, and Ca(H₂PO₄)₂.2H₂O in the contaminated soil at the phosphorus application rate 2,283 mg P/kg-soil reduced Pb concentrations in TCLP extraction solution from 90.47 to 56.05, 83.80, 67.78, and 86.32 mg/kg (38.0, 7.36, 25.1, and 4.59% reduction), respectively. Sequential extraction analysis showed that phosphate treatments caused the transformation of easily available trace metal species to more stable forms. However, TCLP As in the soil increased from its initial value of 0.23 to 2.1, 0.70, 0.67, and 0.77 mg/kg, respectively, for the four treatments. The TCLP leachable As concentration of the K₂HPO₄-treated soil was about nine times of that from the untreated soil.

Conclusions

The addition of K₂HPO₄, Ca₃(PO₄)₂, Ca₅(PO₄)₃OH, and Ca(H₂PO₄)₂.2H₂O were effective in reducing water soluble and exchangeable Pb, Zn, Cu, and Cd, and minimizing TCLP-extractable Pb, Zn, and Cu. The sequential extraction test and the TCLP indicate that Ca₅(PO₄)₃OH treatment has a higher potential in immobilizing Pb, Zn, Cu, and Cd, though a slight enhancement of As mobility, comparing with other phosphate treatments.     

Calcium-Ammonium Selectivity of Two Benchmark Soils from Botswana as Assessed by Competing Semi-empirical Ion Exchange Equations

The effectiveness of lime-ammonium-nitrate (LAN) as a nitrogen (N) fertilizer in weathered soils depends on the respective selectivity for ammonium (NH₄) and calcium (Ca) by the soils. The study assessed Ca²⁺/NH₄⁺ exchange selectivity of two benchmark soils from Botswana and examined the soil fertility management implications. Surface horizons (0–20 cm) of Pellustert and Haplustalf were equilibrated with 50 ml stock solution containing variable concentrations of Ca²⁺ and NH₄⁺. The Ca²⁺/NH₄⁺ exchange data were fitted into the Vanselow (K_V), Gaines and Thomas (K_GT), Davies (K_D), and the regular solution (K_RS) equations. The selectivity coefficients for the Ca²⁺/NH₄⁺ exchange reactions varied widely with the soil exchanger composition except for the relatively stable K_RS. The selectivity coefficients indicated strong preference for NH₄⁺ to Ca²⁺. The thermodynamic exchange constant, K_ex, was 5.75 ± 1.24 in the Pellustert, indicating preferential adsorption of NH₄⁺, but not in the Haplustalf with K_ex = 0.92 ± 0.27. The free energy for Ca²⁺/NH₄⁺ exchange (ΔG°_ex) was negative (?4.26 ± 0.59 kJ mol^?1) in the Pellustert but slightly positive in the Haplustalf (0.34 ± 0.87 kJ mol^?1). In conclusion, the soil-NH₄ complex was more stable than soil-Ca complex in the Pellustert, indicating LAN as a N fertilizer would have greater potential effectiveness in the Pellustert than in the Haplustalf.     

PHYTOTOXICITY OF SALTS IN COMPOSTED SEWAGE SLUDGE AND CORRELATION WITH SODIUM CHLORIDE,CALCIUM NITRATE,AND MAGNESIUM NITRATE

The phytotoxicity of salts in composted sewage sludge (CSS) was evaluated. Concentrations of sodium (Na⁺), chloride (Cl^?1), calcium (Ca²⁺), and magnesium (Mg²⁺) were present at levels that would induce salt stress in plants. Nutrient imbalances were also found that would adversely affect the use of CSS as a growth medium. To further understand the phytotoxic nature of these salts, sodium chloride (NaCl), calcium nitrate [Ca(NO₃)₂] and magnesium nitrate [Mg(NO₃)₂] solutions were used to simulate the composition of salts found in CSS in an investigation of radish (Raphanus sativus L.) seed germination. High concentrations of Ca²⁺ (92.1 mmol.L^?1) and Mg²⁺ (27.4 mmol.L^?1) inhibited seed germination to an equal extent as did Na⁺ (40.6 mmol.L^?1). The lower concentration of Ca²⁺ (10 mmol.L^?1), however, significantly relieved the stress caused by NaCl. These results indicated that the composition and total amount of Na⁺, Cl^?1, Ca²⁺, and Mg²⁺ in CSS should be carefully monitored before it is used as a soil amendment or growth medium.     

QUARTZ PORPHYRY TREATMENT ALTERS IRRIGATION WATER CHEMISTRY,AFFECTING HYDROPONIC VEGETABLE PRODUCTION

This study focused on using quartz porphyry (QP) as a water treatment to improve hydroponic production of komatsuna (Brassica rapa L. nothovar; Japanese mustard spinach). We compared the chemistries of the control and QP-treated nutrient solutions and found that magnesium (Mg²⁺) and calcium (Ca²⁺) concentrations increased linearly up to day 21 following sowing in both conditions, then declined slightly. The QP treatment reduced sodium (Na⁺) and chloride (Cl^?) concentrations for the whole cultivation period. In both the control and QP-treated solutions, nitrate (NO^? ₃) and sulfate (SO^2? ₄) showed the same trend to a daily increase. In spite of these similarities, however, komatsuna production was better with the QP-treated nutrient solution compared to control. Treatment with QP during cultivation in August–September reduced the harmful effects of Na⁺, chloride (Cl^?), nitrite (NO^? ₂), and SO^2? ₄ by reducing concentrations of these ions, possibly leading to decreased salinity and toxicity effects in the plants. Mineral concentrations during October–November differed from those of August/September, resulting in variation among the different growth parameters for komatsuna.     

A comparison of the convection-dispersion equation and transfer function model for predicting chloride leaching through an undisturbed, structured clay soil

The transfer function mode) (TFM) and convection-dispersion equation (CDE) were compared for predicting Cl ^? transport through a calcareous pelosol during steady, nearsaturated water flow. Large, undisturbed soil cores were used at constant irrigation intensities (q₀) between 0.3 and 3 cm h^?1, with a step-change in Cl^? concentration. The assumption of a lognormal distribution of travel times–characterized by the mean (μ) and variance (σ²)–permitted the flux-averaged breakthrough curves (BTCs) to be modelled very accurately by the TFM. The BTCs could be modelled equally well by the CDE when both the mean pore water velocity (v) and dispersion coefficient (D) were optimized simultaneously by the method of least squares, but not when v was put equal to q₀/_v, where _V was the mean volumetric water content. The best estimate of v was consistently > q₀/_v, which suggested that not all the pore water was effective in chloride transport. An operationally defined transport volume (θ_st) was calculated from the mean () or median (τ_m) travel times derived from the TFM. Chloride exclusion was not solely responsible for θ_st() being <_V: immobile water also contributed. The positive skewness of the travel time distributions meant that θ_st(τ_m) < θ_st(), indicating the effectiveness of macropore flow in solute transport. Dαv^1.42 (from the CDE), and σ²αv (from the TFM), confirmed that Cl^? dispersion increased as flow velocity increased. Flux-averaged concentrations were used to calculate the volume-averaged resident concentrations. They matched the measured Cl^? concentrations most closely when there was a gradual decrease in measured Cl ^? concentration with depth, but not when Cl ^? decreased sharply below c. 10 cm. Calculations assuming that all the water was effective in chloride transport gave less accurate results. Comparison of the measured and predicted concentrations of solute demonstrated that this must be a critical part of the evaluation of any model of solute transport.