Remediation of arsenic-contaminated paddy soil: Effects of elemental sulfur and gypsum fertilizer application.

Heavy metal(loid) contamination represents an immense challenge in sustainable agriculture. Arsenic, in particular, poses a great risk to the quality of agricultural products (e.g., rice grain). The sulfur amendment is recommended as an effective practice to remediate heavy metal(loid)-polluted soil, given its function in enhancing crop production and alleviating heavy metal(loid) accumulation in the plant. This study aims to investigate the roles of sulfur fertilizer on arsenic accumulation in rice and to explore the key mechanisms. In this study, Elemental sulfur (ES) and gypsum sulfur (GS) were chosen as sulfur fertilizers, with different application rates (0, 0.15, and 0.30 g S kg-1 soil). The results showed that ES and GS treatment significantly increased rice grain yield by 46.6-59.7% and significantly reduced the rice grain arsenic content by more than 39.1%. The sulfur treatment decreased soil pe + pH values. ES treatment increased the availability of arsenic in the bulk soil, whereas GS showed little effect. Sulfur application promoted the formation of iron and manganese plaques, which could suppress the migration of arsenic from soil to rice root. In addition, the sulfur treatment decreased the arsenic that migrating from rice roots to grains by 33.3-66.7%. This study indicates that sulfur application could increase arsenic availability in paddy soil; however, it can inhibit arsenic accumulation in rice grains via increasing the root plaques content and inhibiting the translocation of arsenic from roots to grains.

Removal and tolerance mechanism of Pb by a filamentous fungus: A case study.

Currently, Pb pollution has become a severe environmental problem and filamentous fungi hold a promising potential for the treatment of Pb-containing wastewater. The present study showed that the strain Pleurotus ostreatus ISS-1 had a strong ability to tolerate Pb at high concentration and reached a removal rate of 53.7% in liquid media. Pb was removed by extracellular biosorption, intracellular bioaccumulation by mycelia, or precipitation with extracellular oxalic acids. On the cellular level, Pb was mainly distributed in the cell wall, followed by vacuoles and organelles. Fourier transform infrared spectroscopy (FTIR) analysis indicated that hydroxyl, amides, carboxyl, and sulfhydryl groups provided binding sites for Pb. Furthermore, Pb was found on the cell surface in the form of PbS and PbCO3 through X-ray diffraction (XRD). Intracellular chelates such as thiol compounds and oxalic acid, as well as extracellular oxalic acid, might play an important role in the tolerance of Pb. In addition, isobaric tags for relative and absolute quantitation (iTRAQ) analysis showed that ATP-binding cassette (ABC) transporter, cytochrome P450, peroxisome, and the calcium signaling pathway might participate in both accumulation and detoxification of Pb. These results have successfully provided a basis for further developing Pb polluted water treatment technology by fungi.

Effect of elemental sulfur and gypsum application on the bioavailability and redistribution of cadmium during rice growth.

Recently, concerns over heavy metal contamination of soil have grown. The application of sulfur has been recommended to enhance crop productivity and increase soil cadmium (Cd) immobilization. In this study, a pool experiment was conducted to investigate the effects of two sulfur sources and multiple treatment levels on rice growth and Cd accumulation. The two sulfur forms were elemental sulfur (S0) and gypsum, both of which were applied at 0, 0.15, and 0.30 g S kg-1 soil, for a total of five treatments. The results showed that both S0 and gypsum significantly increased rice biomass compared to the control (CK), and rice yield was increased 2.8-4.8 folds. The effect size was greater for gypsum than S0. The application of S0 reduced the rice grain Cd concentration from 0.61 mg kg-1 (CK) to 0.41-0.46 mg kg-1, while gypsum reduced the Cd concentration to 0.24-0.43 mg kg-1. The lower gypsum application level achieved the greatest reduction in rice grain Cd accumulation. This study further demonstrated that the application of S0 and gypsum led to a decrease in the labile Cd percentage and an increase in the stable Cd percentage. In bulk soil, iron and manganese oxide-bound Cd increased by 6.4-7.3% and 0.7-2.0% for the S0 and gypsum treatments, respectively. In the rhizosphere, residual Cd increased by >0.6%. Furthermore, this study found that sulfur application reduced Cd transfer from root to shoot, and significantly decreased rice grain Cd accumulation. These findings indicate that sulfur application to paddy soils can promote rice productivity and effectively remediate soil Cd contamination, with a greater effect by gypsum than S0.

Effects of sulfur application on cadmium bioaccumulation in tobacco and its possible mechanisms of rhizospheric microorganisms.

A potting experiment was conducted to investigate the effect of sulfur application on the bioaccumulation by tobacco and its mechanisms of rhizospheric microorganisms. Cadmium content in tobacco was analyzed using atomic absorption spectrometer, while bacterial community and related gene in soil were analyzed via high-throughput sequencing and quantitative PCR techniques, respectively. The obtained results indicated that tobacco had the ability to accumulate cadmium under no sulfur application conditions, with cadmium contents of 35.4, 23.6, and 26.3 mg kg-1 in leaves, stems, and roots, respectively. Under high-sulfur treatment, these values increased to 66.4, 46.1, and 42.6 mg kg-1, respectively, probably due to the increase of the available cadmium content (from 1.1 to 3.3 mg kg-1) in the soil through a decrease of the soil pH value, which was contributed by the sulfur oxidation reaction. dsrA and soxB genes might play an important role in sulfur oxidation, and Thiobacillus sp. was the dominant bacterial genus during the sulfur oxidation process. In addition, sulfur application exerted little effect on the diversity and structure of the soil bacterial community. The combined results indicate that sulfur application is an effective and safe method for Cd phytoextraction by tobacco.

[Transformation and Migration of Sulfur Speciation in the Rhizosphere and Bulk Soil of Paddy Soil].

A pool culture experiment was carried out to investigate the influence of different forms of sulfur fertilizers (sulfur and gypsum) on the transformation and migration of sulfur speciation in the rhizosphere and bulk soil of unpolluted and polluted paddy soils.The results showed that the redox potential (Eh) was about 93-283 mV and 83-254 mV, respectively, the soil solution pH was 7.5-8.4 and 7.7-8.4, respectively, and pe+pH was 9.1-13.2 and 9.1-12.5, respectively, in the bulk and bulk soil.Solution Eh values in Rhizosphere soil were generally higher than those in bulk soil, and solution pH in the former was generally lower than that in the latter.The different forms of inorganic sulfur followed the order of water-soluble sulfur (41%-81% of total inorganic sulfur, similarly hereinafter)>>sulfur adsorption (9%-34%)>hydrochloric acid soluble sulfur (8%-24%)>hydrochloric acid volatile sulfide (2%-8%) in the rhizosphere.In tillering and earing flowering,the concentrations of water-soluble and absorbed sulfur by application of gypsum were significantly higher than those using elemental sulfur.And its content in unpolluted paddy soil was significantly higher than that in polluted paddy soil. In the bulk soil,the forms of inorganic sulfur followed the order of water-soluble sulfur (40%-69%)>>hydrochloric acid soluble sulfur (18%-41%)>hydrochloric acid volatile sulfide (6%-16%)>adsorbed sulfur (0.7%-7.5%).The mass fractions of inorganic sulfur, organic sulphur and the total sulfur in the rhizosphere soil were in the range of 223-738 mg·kg-1, 574-1647 mg·kg-1 and 825-2287 mg·kg-1,respectively,and the corresponding fractions were in the range of 68-128 mg·kg-1, 108-391 mg·kg-1 and 200-477 mg·kg-1 in the bulk soil,respectively.Inorganic sulfur and organic sulfur of total sulfur in the rhizosphere were 20%-40% and 60%-80%, respectively, and those in the bulk were 18%-46% and 54%-82%, respectively.Total sulfur. organic sulfur and water-soluble sulfur and adsorbed sulfur and hydrochloric acid soluble sulfur in the rhizosphere were 3-11 times, 3-5 times, 5-7 times, 12-20 times, and 2-3 times of those in the bulk soil, respectively, whereas the hydrochloric acid volatile sulfur in the rhizosphere was lower than that in the bulk soil.

[Adsorption Characteristics of Arsenite on Goethite by Flow Stirring Method].

Adsorption characteristics of arsenite on goethite under the effects of the solution pH, concentration and temperature were investigated using a flow-stirring dynamic device. The results showed that the adsorption process of arsenic could be divided into rapid and slow reactions under different conditions.The maximum of arsenite adsorption fitted by the first order equation remarkably decreased with increase in the solution pH, for example, 246.9 mg·kg-1 at pH 3.0 and 99.8 mg·kg-1 at pH 7.0, respectively. The rate constant(k')of the apparent adsorption increased gradually along with the increase of solution pH, and so the half reaction time(t1/2)was smaller, and the equilibrium time of arsenic adsorption was shorter. At the same time, the b values of diffusion rate constant were reduced gradually. With the increase of arsenic concentration, the amounts of arsenic adsorption and the k' values increased gradually. The maximum amount of adsorption of arsenic was 96.5 mg·kg-1 and 249.1 mg·kg-1 when the arsenic concentration was 0.10 mg·L-1 and 1.00 mg·L-1, respectively. Adsorption constant(Kf)by the Freundlich equation decreased gradually with the extension of the reaction time and its ability of adsorption was gradually weakened. Distribution factor(RL)by Langmuir equation was between 0-1, and the adsorption of arsenic on goethite was accounted for the preferential adsorption. With the increase of temperature, the maximum amount of adsorption of arsenic was increased, for example, 241.1 mg·kg-1 at 298 K and 315.6 mg·kg-1 at 313 K, respectively. And the k' values of the apparent adsorption rate constant gradually rose in the meantime. The false thermodynamic constants were calculated using the b values of the diffusion rate by the parabolic diffusion equation. The reaction activation energy(Ea*)of Arsenic adsorption was 14.60 kJ·mol-1. The change of arsenic diffusion activation enthalpy(ΔHθ)decreased with the increase of temperature, and ΔHθ was positive in values and on behalf of the endothermic process. So the rising temperature was beneficial to the diffusion of arsenic. ΔGθ of activation free energy was increased as the temperatures rose, and helpful to accelerate the diffusion process. Entropy of activation(ΔSθ)was negative in all cases, suggesting that the system could improve its degree of order.

[Influence of Sulfur on the Formation of Fe-Mn Plaque on Root and Uptake of Cd by Rice (Oryza sativa L.)].

A pool culture experiment using exogenous Cd-polluted paddy soils was carried out to investigate the influence of different forms of sulfur fertilizers (sulfur and gypsum) on the formation of Fe-Mn plaque on rice root and the uptake of Cd by rice. The results showed that the redox potential ( Eh) was about--200-100 mV, the pH was 6.9-7.9 and the pe + pH was 4-10 in different growth periods of rice. The mass fractions of Fe and Mn plaque on rice root were 5000-13,000 mg · kg(-1) and 170-580 mg · kg(-1), respectively. The high sulfur treatment led to the formation of more Fe plaque than the low sulfur treatment, and the mass fractions of Fe plaque in both treatments were 9400 mg · kg(-1) and 8600 mg · kg(-1) respectively in the boot stage. Contents of Mn plaque, significantly different in the tiller stage by elemental sulfur treatment and gypsum treatment, were 600 mg · kg(-1) and 400 mg · kg(-1), respectively. The elemental S treatment led to the formation of more Mn plaque on rice root than the gypsum treatment. The excessive intake of Fe2+ might be prevented by the formation of the plaque which had little significant influence on the uptake of Mn2+. The mass fractions of Cd adsorbed by rice roots surface plaque were 78.8-131.1 mg · kg(-1) in tiller stage, 16.6-21.1 mg · kg(-1) in boot stage, and 3.0-9.2 mg · kg(-1) in mature stage. The high sulfur treatment led to higher adsorption of Cd by the plaque than the low sulfur treatment in the tiller stage and the boot stage, while opposite result was observed in the mature stage. The values of Cd on the plaque measured by ACA could not truly show its actual contents adsorbed. The mass fractions of Cd in the different parts of rice followed the order of roots > stem leaf > grain. The sulfur fertilizers applied significantly reduced the mass fractions of Cd uptake in different parts of rice, and the elemental sulfur treatment had better effects than the gypsum treatment before the mature stage in roots and stem leaf, and the gypsum sulfur treatment was better for grain. A certain amount of sulfur fertilizers could effectively prevent the migration of Cd from roots to stem and grain. The transfer coefficients of Cd from roots to stem leaf in the elemental sulfur treatment and the gypsum treatment were 0.13 and 0.25 in the boot stage, respectively, and the difference was significant. Elemental sulfur was more effective to prevent the Cd migration from roots to stem leaf, and the gypsum treatment was more active from roots to grain.

[Influence of sulfur on the bioavailability of arsenic uptake by rice (Oryza. sativa L. ) and its speciation in soil ].

Pot experiments using exogenous arsenic-polluted paddy soils were carried out to investigate the influence of different forms of sulfur fertilizers (sulfur and gypsum) on As uptake by rice and its chemical speciation. Soil solution pH value ranged 7. 38-7. 45 in different growth period of rice, and the pH value of AsS0 and AsS1 treatments was higher than that of AsS2 treatment. Variation of Eh value in soil solution was about 200 mV and the Eh of AsS0 was higher than those of AsS1 and AsS2 treatments. From dry matter weight of root and stem and grain of rice, S-fertilizer applied by sulfur and gypsum could improve the amounts of dry matter in rice, while the effects of sulfur treatments and gypsum treatments were not significant. Concentrations of Fe and Mn in iron-manganese plaque on rice roots were 10-30 g.kg-1 and 0.1-1.3 g.kg-1, respectively. Contents of Fe-Mn plaque were mainly different in the tiller stage. Elemental S treatment could more greatly promote the formation of Fe-Mn plaque of rice root than gypsum treatment. Concentrations of As adsorbed by rice roots surface plaque were 583-719 mg.kg-' in tiller stage, 466-621 mg.kg-1 in boot stage, and 310-384 mg kg-1 in flower and matur stage. And it was consistent with the thickness of Fe-Mn plaque on rice root surface. Concentrations of As uptake in roots and stem and leaf and grain were significantly reduced by the application of S fertilizer, and it may be related to the amount of As adsorbed by Fe-Mn plaque at boot stage. According to chemical speciation of soil arsenic, As of non-specific and specific adsorption was most active, and their amounts of As adsorbed in AsS, treatment were significantly lower by 2.85 mg kg-~' than that in AsS2 treatment in tiller stage, and was 0.77 mg.kg- higher than that in AsS2 treatment in the flower stage. Perhaps soil arsenic was easily dissolved in the soil solution and the bioavailability of AsS, treatment was better than that of AsS, treatment.

[Kinetic characteristics of Cd2+ desorption in minerals and soils under simulated acid rain].

The kinetic characteristics of Cd2+ desorption in minerals and soils under simulated acid rain were studied by using the flow-stirred method. It showed that Cd2+ desorption could be described by first-order kinetics. Percents of desorption amounts of Cd2+ calculated were 70%-100% in red soil and goethite, and 25%-50% in latosols and kaolinite. Parabolic diffusion could describe Cd2+ desorption kinetics in latosols and not suitable for red soil and goethite and kaolinite. Cd2+ desorption, regarded as a heterogeneous diffusion in minerals and soils, could be fitted by Elovich equation more than Parabolic diffusion and two-constant equation. Cd2+ desorption could be divided into fast reaction and slow reaction. Except for latosols, fast reaction would be over during 60 min and be close to quasi-equilibrium. Adsorption forms of Cd2+ in soil surface could be exchangeable and specific. Fast reaction was relative to easily desorbed Cd2+. The affinity of edge hydroxyl to Cd2+ would lead to the difference of Cd2+ desorption rate and amounts. Increase of pH value in effluent indicated H+ consumption in the processes of Cd2+ desorption.