Use of Power Plant Ash to Remove and Solidify Heavy Metals from a Metal-finishing Wastewater

This laboratory-scale study investigated initially the potential of heavy metal removal from a metal-finishing wastewater using fly and bottom ash from a power plant as coagulants. It was found that the maximum heavy metal content in the ash–sludge mix was obtained at a fly ash-to-bottom ash ratio of 1.5:1 and a stirring time of 3 h, which resulted in heavy metal removal (i.e., Cr, Ni, Cu, Zn, Cd, and Pb) in excess of 99%, with effluent concentrations below the corresponding regulatory standards of Thailand. Furthermore, the feasibility of using fly ash as an admixture to stabilize and solidify the ash–sludge mix generated previously was explored. Results indicated that the stabilization/solidification process can achieve a high level of heavy metal removal efficiency from the ash–sludge mix. The optimum ratio regarding chromium leaching was found to be 1:0.75:0.75 (cement:fly ash:ash–sludge). In addition, the compressive strength and the chromium leaching concentration of the solidified sludge were within acceptable levels for secure landfill disposal and/or use as a construction material.     

Changes in soil physical properties of forest floor horizons due to long-term deposition of lignite fly ash

Background, aim, and scope

From the beginning of the twentieth century until the 1990s, energy in Upper Lusatia, Saxony in Eastern Germany was produced at power plants that burnt lignite coals. As a result, alkaline fly ash and aerosols from the combustion of brown coal have accumulated in adjacent areas that are partly under forestry. We ask the question, “how have these atmospheric depositions of fly ash influenced the soil physical properties (bulk density, particle density, saturated hydraulic conductivity, pore size distribution, and water repellency) of forest floor horizons?”

Materials and methods

The experimental sites represented typical soil types and stands of the sylviculturally used areas in the region of Upper Lusatia. Three forest sites were located close to the emission sources, where high amounts of fly ashes accumulated, and three control sites were without fly ash enrichment. Pore size distribution, saturated hydraulic conductivity, and bulk density were examined with undisturbed samples (metal cylinder 100 cm³). Disturbed samples were used for the characterization of particle density, texture, and water repellency (Wilhelmy plate method). Additionally, the carbon content was determined. Scanning electron microscopy was used to show fly ash enrichment.

Results

The enrichment of mineral fly ash particles could be proven for sites close to the emission source. Using scanning electron microscopy, spherical fly ash particles could be identified. Total quantities of persistent fly ash enrichment amounted to approximately 150-280 Mg ha^–1. The enrichment of fly ash affected the soil-physical characteristics. Close to the emission source (sandy fly ashes), particle density, air capacity, and saturated hydraulic conductivity were significantly increased, whereas the plant available water was significantly reduced. With increasing distance from the emission source (silty fly ashes or no ash enrichment), air capacity and saturated hydraulic conductivity were reduced, while an increase of plant available water was observed. Furthermore, the forest floor horizons close to the emission source were characterized by significantly reduced water repellency due to the dominance of hydrophilic mineral fly ash particles.

Discussion

Fly ash deposition in Upper Lusatia must be considered as relevant for properties of forest soils. Mean particle density was significantly higher at sites with fly ash accumulation. This indicates the admixture of mineral particles. While bulk densities were not noticeably influenced, the increase of particle density and the dominance of sandy to coarse silty particles close to the emission sources cause an increase in total porosity, air capacity, and a relative reduction of plant available water. Hollows in spherical fly ash particles might contribute to the meso- and macropores. Due to the admixture of hydrophilic fly ash, the enriched forest floor horizons feature a distinct increase in potential wettability, which coincides with a higher pore and, hence, nutrient and contaminant accessibility. In combination with a higher saturated hydraulic conductivity, an increase in translocation of dissolved substances can be expected especially in the course of acidification, which causes an additional mobilization of nutrients and contaminants.

Conclusions

With this study, we could prove the impact of fly ash enrichment on physical soil properties of forest floor horizons. Via SEM, we detected fly ash particles. The amounts of persistent fly ash accumulation could modify particle density, thickness, bulk density, and carbon content. To characterize hydraulic properties, we investigated the pore size distribution, the saturated hydraulic conductivity, and a water repellency parameter. Thereby, we detected a distinct increase of coarse pores and an accompanying extremely high saturated hydraulic conductivity. The water repellency parameter indicated a significant decrease of hydrophobicity of fly-ash-enriched forest floor horizons.

Recommendations and perspectives

Fly ash enrichment in forest floor horizons not only causes distinct chemical modifications but also alters soil physical properties, which must be considered in further hydrological investigations, as they may influence seepage of water and contaminant translocation within the soil and into groundwater.

     

Comparing chemical-enhanced washing and waste-based stabilisation approach for soil remediation

Purpose

This study aimed to compare the effectiveness of chemical-enhanced soil washing (with chelating agents, humic substances and inorganic acids) and soil stabilisation by inorganic industrial by-products (coal fly ash, acid mine drainage sludge and zero-valent iron) and organic resource (lignite) for timber treatment site remediation.

Materials and methods

Both remediation options were assessed in terms of extraction/leaching kinetics and residual leachability (toxicity characteristic leaching procedure, TCLP) of the major risk drivers, i.e. Cu and As.

Results and discussion

In chemical-enhanced soil washing, chelating agents only minimised the Cu leachability. Humic substances were ineffective while inorganic acids reduced the As leachability to the detriment of the soil quality. For the waste-stabilised soil, the short-term leaching potential (72 h) and long-term TCLP leachability (9 months) revealed that Fe-/Al-/Ca-rich AMD sludge and coal fly ash sequestered As through adsorption and (co-)precipitation, while carbonaceous lignite stabilised Cu with oxygen-containing functional groups. The short-term and long-term leaching of Cu and As into the soil solution was negligible in the presence of the waste materials. However, the waste-stabilised soil did not maintain sufficient Cu stability in the TCLP tests, in which acetate buffer induced significant mineral dissolution of the waste materials.

Conclusions

These results suggest that chelant-enhanced washing (significant reduction of Cu leachability) may be augmented with subsequent stabilisation with inorganic waste materials (effective control of As leachability), thus minimising the environmental risks of both Cu (heavy metal) and As (metalloid) while preserving the reuse value of the soil. Additional tests under field-relevant conditions are required to provide a holistic performance evaluation.     

Characters of soil algae during primary succession on copper mine dumps

Purpose

Algae play an important role in degraded areas during the initial stages of soil formation by improving its physico-chemical properties, reducing the erosion of soil, and thus favoring the settlement of vascular plants. This study investigates the characters of soil algal communities on copper tailing dumps and discusses the contribution of soil algae to the primary succession progress of young mine tailings ecosystems.

Materials and methods

Five representative potential successional series (bare land, algae crust, mixed algal–moss crust, moss crust, and vegetated site) on copper tailing dumps and a nearby reference site were selected. The soil algae were identified using growth slide method, dilution plate method, and by direct microscopic observation of the soil suspensions. All experiments were carried in an incubation chamber at a temperature of 25 °C and with a 16 h/8 h light–dark cycle at a light intensity of 3,000 lux.

Results and discussion

A total of 120 algal species were recorded. Cyanophyta (blue-green algae) were the most diverse taxonomic group, followed by Bacillariophyta (diatoms) and Chlorophyta (green algae), although diatoms were absolutely absent in bare sites. Diversity of soil algae was highest in vegetated site, whereas it was lowest in bare sites. Total algal abundance ranged between 0.15?×?10³ cells/g to 46.8?×?10³ cells/g dry soil, with the lowest abundance in the youngest site and the highest abundance in the mixed algal–moss crust site. Correlation analysis showed that the growth of soil algae was inhibited by high Cu, Zn, and Fe concentrations and low nutrient content and that the green algae were more sensitive to nutrient content than blue-green algae.

Conclusions

Our results suggest that blue-green algae were most diverse, followed by diatoms and green algae. Species and abundance of soil algae in the tailings increased with the early succession process because of the decrease in heavy metal content and the improvement of nutrient conditions. The growth of soil algae created conditions for the settlement and growth of higher plants, but the appearance of moss and vascular plants inhibited the growth of soil algae.     

Reduction in CO2 emission from normal and saline soils amended with coal fly ash

Purpose

Fly ash can reduce CO₂ emission from soils via biochemical (i.e., inhibition of microbial activity) and physicochemical (i.e., carbonation) mechanisms. This study investigated the effects of fly ash amendment on biochemical and physicochemical reduction in CO₂ emission from normal and saline soils.

Materials and methods

The physicochemical mechanisms of reduction in CO₂ emission by fly ash were estimated in a batch experiment with carbonate solution as a CO₂ source by the scanning electron microscope (SEM) and inductively coupled plasma analyses. Biochemical mechanisms of reduction in CO₂ emission by fly ash were investigated in a 3-day laboratory incubation experiment with normal and saline soils in the absence and presence of fly ash. Finally, the effects of fly ash amendment at a variety rate from 2 to 15?% (w/w) on CO₂ emission from normal and saline soils in the presence of additional organic carbon source (glucose) were investigated through a 15-day laboratory incubation study.

Results and discussion

In the batch experiment with carbonate solution, both the SEM image of fly ash and changes in soluble Ca and Mg concentrations during reaction with carbonate suggested that the formation of CaCO₃ and MgCO₃ via carbonation was the principal physicochemical mechanism of carbonate removal by fly ash. In the 3-day incubation study conducted to examine biochemical mechanisms of reduction in CO₂ emission by fly ash, microbial respiration of saline soil was inhibited (P?<?0.05) by fly ash due to high pH, salinity, and boron concentration of fly ash; meanwhile, for normal soil, there was no inhibitory effect of fly ash on microbial respiration. In the 15-day incubation with glucose, fly ash application at a variety rates from 2 to 15?% (w/w) reduced CO₂ emission by 3.6 to 21.4?% for normal and by 19.8 to 30.3?% for saline soil compared to the control without fly ash. For saline soil, the reduction in CO₂ emission was attributed primarily to inhibition of microbial respiration by fly ash; however, for normal soil in which suppression of microbial respiration by fly ash was not apparent, carbonation was believed to play an important role in reduction of CO₂ emission.

Conclusions

Therefore, fly ash may be helpful in reducing CO₂ emission from normal soils via carbonation. For saline soil, however, fly ash needs to be carefully considered as a soil amendment to reduce CO₂ emission as it can inhibit soil microbial activities and thus degrade soil quality.     

Characteristics of synthetic soil aggregates produced by mixing acidic “Kunigami Mahji” soil with coal fly ash and their utilization as a medium for crop growth

Abstract

This study was carried out to examine the characteristics and potential utilization of synthetic soil aggregates (SSA) produced by mixing acidic “Kunigami Mahji” soil in Okinawa, Japan, with waste materials, such as coal fly ash, used paper and starch, as media for crop growth. A series of different SSA were produced by incorporating various percentages (i.e. 0, 20, 40, 60, 80 and 100%) of coal fly ash into the “Kunigami Mahji” soil with used paper and starch. The particle density and bulk density of the original “Kunigami Mahji” soil were 2.67 and 1.23 g cm^?3, respectively. The increased percentages of added coal fly ash, used paper and starch significantly decreased the particle and bulk densities of SSA compared with the original “Kunigami Mahji” soil because of the low particle and bulk densities of the coal fly ash (2.10 and 0.96 g cm^?3, respectively). The SSA particle density varied between 2.39 and 2.14 g cm^?3, and bulk density varied between 0.72 and 0.81 g cm^?3, depending on the additional percentages of coal ash from 20–100%. Maximum water-holding capacity and saturated hydraulic conductivity were increased with the formation of SSA with coal fly ash, used paper and starch binder compared with the original “Kunigami Mahji” soil. The saturated hydraulic conductivity values of the SSA increased because of their low bulk density compared with the original soil. The addition of coal fly ash, used paper and starch to the acidic (pH = 4.62) “Kunigami Mahji” soil to form SSA increased the pH (6.70–9.96), electrical conductivity, exchangeable cation concentration and cation exchange capacity. The addition of coal fly ash up to 60% increased the aggregate strength. The growth and yield of komatsuna and soybean crops with SSA as a crop growth medium was assessed. Both crops showed the highest growth and yield when grown with SSA containing 20% of coal fly ash. Synthetic soil aggregates containing more than 20% of coal fly ash reduced plant growth and yield. Therefore, SSA produced from “Kunigami Mahji” soil with 20% of coal fly ash, used paper and starch can be successfully used as a medium for crop growth.     

Changes in soil morphology of Podzols affected by alkaline fly ash blown out from the dumping site of an electric power plant

Purpose

The impacts of fly ash on the chemistry of forest floors were previously described in literature, while impacts on soil properties were less recognised. Soil investigations were focussed mainly on increases of pH and base saturations in surface horizons. The purpose of this study was to describe the influence of alkaline fly ash blown out from the dumping site of a lignite-fired power plant on pH changes of ectohumus horizons of Podzols and the morphology of deeper horizons.

Materials and methods

We investigated the soil profiles of Podzols derived from loose quartz sand and developed under pine forest surrounding the dumping site of the power plant Be?chatów, central Poland. In the vicinity of the fly ash dumping site, five Podzol profiles located at a distance of 50 m from the dumping site were investigated, as well as soil profiles located along the transect set at distances of 50, 300, 800 and 2000 m from the dumping site. Control profiles were located at a distance of 7.3 km from the dumping site. Soil morphology was described in the field and the following properties were determined: soil texture, hydrolytic acidity, exchangeable cations, total organic carbon and total nitrogen content.

Results and discussion

The pH values of Podzol ectohumus horizons located close to the dumping site ranged from 6.01 to 7.34 compared to a range of 3.08–3.72 in the control. Ectohumus horizon located 300 m from the dumping site showed a pH range of 4.13–4.26, while at a distance of 800 m, the pH values did not differ from those of the control site. The upper part of the eluvial soil horizons located close to the dumping site had been transformed into transitional AE horizons in which humic substances translocated from ectohumus horizons were accumulated. Moreover, the organic carbon content of this horizon increased compared to the carbon content of the illuvial Bs horizon located below it. Under the influence of alkalisation of upper horizons, the illuvial Bhs horizons vanished and were transformed into Bs horizons.

Conclusions

Changes in soils affected by fly ashes are connected with alkalinisation of ectohumus horizons. Podzolisation processes can be reduced or even completely stopped regarding the distance from the dumping site. Eluvial Podzol horizons located close to the dumping site may be transformed into AE horizons in which humic substances translocated from ectohumus horizons are accumulated. Due to transformation and translocation of organic components, Bhs horizons can be transformed into Bs horizons.

     

Adsorption of Dioxin by Bag Filter + Powdered Activated Carbon

A novel bag filter + powdered activated carbon technique is here proposed to address the low utilization rate of powdered activated carbon and the low dioxin removal rate associated with the conventional activated carbon injection + bag filter technique, better known as the fly ash + activated carbon + bag technique. In this method, dibenzofuran serves as a dioxin simulant. The effect of the adsorption temperature and dibenzofuran inlet concentration on the adsorption performance of activated carbon was studied using a filter cloth adsorption device with an inner diameter of 25 mm, and the adsorption performances of fly ash, activated carbon, and fly ash +5% activated carbon were compared. The results showed that activated carbon exhibited a higher adsorption efficiency and remained highly efficient longer than fly ash +5% activated carbon. When the dibenzofuran inlet concentration was 0.0956 g/m³ (about one million times the concentration of dioxin in the flue gas of incinerated waste), the duration of the high-efficiency (>90%) adsorption of the powdered activated carbon (thickness 1.2 mm) on the filter cloth was over 7 h. These results prove that the replacement of fly ash + activated carbon + filter bag with powdered activated carbon + bag filter can significantly improve the removal efficiency of the dioxin in waste incineration flue gas and the utilization rate of activated carbon.     

The potential feasibility for soil improvement, based on the properties of biochars pyrolyzed from different feedstocks

Purpose

Biochars have been considered as useful soil amendments due to their beneficial properties in improving soil fertility, carbon (C) sequestration, and soil decontamination. In our study, a series of biochars produced from different types of feedstocks at two pyrolysis temperatures (300 and 500 °C) were characterized to evaluate their different potentials as soil amendments.

Materials and methods

Ten types of feedstocks were used to prepare biochars at the pyrolysis temperatures of 300 and 500 °C, for 2 h. Chemical and physical analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier Transform Infrared (FTIR) analyses were conducted to determine differences in biochar properties. Then, soil incubation studies were used to investigate the relationships between these biochar properties and their different ameliorant values in soil.

Results and discussion

The pH, ash, total C, total potassium, total phosphorus, total base cation concentrations, surface areas, and total pore volumes of biochars produced at 500 °C were higher than at 300 °C, while the reverse applied for yields, total oxygen and total hydrogen, and average pore widths and particle sizes. Cluster analysis suggested that biochars derived from similar feedstock types belonged in the same category. The SEM, XRD, and FTIR analyses of typical biochars from the different categories suggested both variations and similarities in their characteristics. In addition, the results from soil incubation experiments were consistent with the conclusions made from biochar characteristics analysis.

Conclusions

Biochars derived from swine manures, fruit peels, and leaves with high pH and macro-nutrients appeared appropriate to increase soil pH and soil nutrient availability; whereas, biochars from wetland plant residues with high C concentrations and Brunauer–Emmett–Teller were better for soil C sequestration and contaminant adsorption.     

Soil structure amelioration with quicklime and irrigation experiments in earth graves

Purpose

Air supply and soil moisture have significant impact on the decay time necessary for complete decomposition of an interred body. Concerning the general structure and hydraulic as well as pneumatic conditions, in many cases, a permeable refilled soil material surrounded by the undisturbed and less permeable soil outside the grave results in water ponding, less aerated conditions, and lower redox potential values within the grave. This reduces the decomposition speed or even leads to preservation of the entire body.

Materials and methods

In order to ascertain soil structural processes and hydraulic properties in an earth grave within the first year after burial, a monitoring of soil redox and matric potentials was realized in newly refilled artificial (empty) graves. We surveyed four variations: undisturbed reference soil, soil backfill in artificial grave, soil backfill in artificial grave amended with 20 kg CaO m^?3, and grave base and walls strewed with CaO. In the fourth artificial grave (soil backfill only), irrigation experiments were conducted in order to simulate the effects of grave maintenance on soil water budget. Pore size distribution, air conductivity, and saturated hydraulic conductivity were measured on soil core samples from the variations. The monitoring was realized with redox sensors and tensiometers in 50- and 130-cm depth in all four variations.

Results and discussion

Soil structure disruption increased soil porosity but also favored saturation of the soil in context with precipitation events. Compared with the graves without amendment, the addition of quicklime resulted in higher air capacity and air permeability, saturated hydraulic conductivity, and a better-aerated (higher redox potentials) and less water-saturated soil. Non-recurring irrigation with 2.2, 4.4, and 8.9 mm did not affect the soil moisture in the 50- and 130-cm depth. Repeated irrigation with 8.9 mm on consecutive days led to persistent water saturation in the soil, especially in the 130-cm depth.

Conclusions

The disturbed soil structure in the cover layer of an earth grave is sensitive to settlement and, together with a tendency to the development of stagnic conditions, this can have negative impact on soil aeration in the grave. Addition of quicklime to the soil enhances crack development in the base and walls of the grave, stabilizes the soil fragments in the backfill, and prevents intensive settlement processes. This reduces water ponding and leads to a better aeration of the soil. Irrigation of earth graves should be reduced to a minimum.

     

Possible utilization of ash from meat and bone meal and dried distillers grains gasification as a phosphorus fertilizer: crop growth response and changes in soil chemical properties

Purpose

Gasification process is regarded as a viable option for disposal of various types of organic waste. Its application to meat and bone meal (MBM) and dried distillers grains (DDG) is a new concept that is recently receiving more attention as possible strategy for safe recycling of these materials to benefit from their energy and nutrient value. Besides, this process produces ash that is rich in phosphorus (P) and may be suitable for utilization as a P fertilizer. Therefore, the objective of this study was to evaluate the effectiveness of these specific types of ashes as P fertilizer via evaluating their direct effect on canola growth, P uptake, apparent P recovery and changes in some selected soil chemical properties after their addition to a P-deficient soil.

Materials and methods

A growth chamber experiment was set up to meet the study objective. The experimental treatments included meat and bone meal ash (MBMA) and dried distillers grains ash (DDGA) applied at three rates (25, 50, or 100 kg P ha^?1) in comparison to a mineral (mono-calcium phosphate) fertilizer (MP) applied at the same rates in addition to non-P-treated soil (control).

Results and discussion

After a growth period of 5 weeks, the DDGA was the most effective ash type and provided biomass yield, P uptake, and apparent P recovery better or similar to that of MP, indicating high availability of its P. The MBMA had a limited effect on measured crop variables, suggesting that a significant portion of this ash P is present in insoluble form and is not as readily available for plant uptake. This was also indicated by its lesser effect on enhancing extractable available P remaining in soil after harvest in comparison to MP or DDGA. Application of all ash material caused a slight but significant change in soil content of inorganic N as well as soil pH and EC; however, this change was more evident with DDGA treatments.

Conclusions

Ash derived from gasified DDG was the most effective P fertilizer and was comparable to mineral fertilizer. The results of this study demonstrated that the effectiveness of organic material ash as a P fertilizer is controlled by the type of gasified feedstock. The positive results obtained from this study should stimulate further research on utilization of these ashes as a source of P for different crops in different soil types, especially repeated application under field conditions.     

Remediation of organochlorine pesticides (OCPs) contaminated soil by successive hydroxypropyl-β-cyclodextrin and peanut oil enhanced soil washing–nutrient addition: a laboratory evaluation

Purpose

Problems associated with organochlorine pesticides (OCPs)-contaminated sites have received wide attention. To address the associated environmental concerns, innovative ex situ techniques are urgently needed.

Materials and methods

As regards long-term contamination by OCPs in Wujiang region, China, we investigated the feasibility of a cleanup strategy that employed hydroxypropyl-β-cyclodextrin (HPCD) and peanut oil to enhance ex situ soil washing for extracting OCPs, followed by the addition of supplemental nutrients to the residual soil.

Results and discussion

Elevated temperature (50 °C) in combination with ultrasonication (35 kHz, 30 min) at 50 g?L^?1 HPCD and 10 % peanut oil were effective in extracting, and therefore washing, the OCPs in soil. Ninety-three percent of total OCPs, 98 % of dichlorodiphenyltrichloroethanes, 93 % of chlordane as well as 85 % of Mirex were removed from soil after three successive washing cycles. Treating the residual soil with nutrients addition for 12 weeks led to significant increases (p?<?0.05) in the average well color development obtained by the BIOLOG Eco plate assay, Shannon–Weaver index, Simpson index, and EC₅₀ ecotoxicological evaluation compared with the controls. This implied that this cleanup strategy at least partially restored the microbiological functioning of the OCPs-contaminated soil and has the advantage of being an environmental-friendly technology.

Conclusions

The ex situ cleanup strategy through HPCD and peanut oil enhanced soil washing followed by nutrients addition could be effective in remediation of OCPs-contaminated soil.     

Mechanisms of phosphate retention by calcite: effects of magnesium and pH

Purpose

Sorption and precipitation of phosphate are important processes in controlling fate of phosphorus (P) in P-fertilized soils, especially those affected by magnesium (Mg) ions.

Materials and methods

The interaction between Mg(II) (0.42 and 8.33 mM) ions and phosphate (0.32 and 6.45 mM) at the calcite–water interface were investigated with various pH values from 6.0 to 12.0, using a combination of sorption envelopes, Fourier transform infrared spectroscopy, and X-ray diffraction.

Results and discussion

Amorphous calcium phosphate, dibasic calcium phosphate dihydrate, and hydroxyapatite are formed at high phosphate concentration (6.45 mM) and high pH (>8.0). The presence of low Mg(II) ion level (0.42 mM) had little effect on phosphate sorption. When Mg(II) ions increased to 8.33 mM, phosphate retention was inhibited in the weak acid condition since incorporation of Mg(II) ions kinetically hinders precipitation resulting in greater solubility of calcium phosphate while high pH favors Mg adsorption to provide more =Mg sites and OH functional groups on the surface of calcite, which enhanced the formation of Mg–P phases. The likely mechanism is attributed to the different surface terminations of calcite sorbed by phosphate at pH?<?8.0 and pH?>?8.0 in the presence of Mg(II) ions.

Conclusions

Our experimental results suggested that soil pH, initial concentration of phosphate, and the presence of Mg(II) ions and calcite play an important role to affect the fate of phosphate in P-fertilized soils.     

Experimental Study on Adsorption of Hexavalent Chromium with Microwave-Assisted Alkali Modified Fly Ash

Coal fly ash, a kind of solid waste generated from coal-fired power stations, can be used as an absorbent for Cr(VI) from aqueous solution. The adsorption characteristics of microwave-assisted alkali modified fly ash to hexavalent chromium had been investigated. Many experimental conditions including pH, contact time, temperature, and fly ash dosage have significant impacts on adsorption performance of modified fly ash. The optimum conditions for the preparation of modified fly ash were as follows: microwave power 600 W, microwave temperature 60 °C, and microwave time 10 min. Batch experiments on adsorption isothermal characteristics of the fly ash modified under the optimal conditions were carried out. The results show that the adsorption of hexavalent chromium ions by modified fly ash is in accordance with the Freundlich and Langmuir isotherm adsorption models, which indicates that the adsorption process is monomolecular adsorption. Environmental-benign utilization of fly ash as low-cost adsorbents in wastewater treatment would bring long-term economic and environmental benefits.     

Short-term effects of prescribed burning on phosphorus availability in a suburban native forest of subtropical Australia

Purpose

Prescribed burning can alter nutrient availability to plants. Plant growth in tropical and subtropical forests is frequently phosphorus (P) limited. Soil P availability is influenced by a combination of multiple factors including soil chemical and biological properties. The aims of this study were to investigate the short-term effects of prescribed burning on soil P status and to evaluate the key drivers responsible for the variation in soil P fractions.

Materials and methods

Soil samples were collected at a depth of 0–10 cm at two sites in a suburban native forest. One site (the burnt site) was burned on 11 August 2011. The other site (the control site) was not burned but served as a reference. Sampling was conducted at four times: before burning, 12 days after burning (T1), 1 week after T1 (T2), and 1 month after T2 (T3). Soil pH, P fractions, microbial biomass carbon (C) and P, and activities of acid and alkaline phosphatase were measured.

Results and discussion

Total P was relatively low at both sites compared with other subtropical forests. Microbial biomass P accounted for approximately 10 % of soil total P at the two sites, suggesting that the turnover of microbial biomass is critical for soil P availability. Soil properties at the control site remained unchanged over the time. Soil organic forms of P at the burnt site were decreased by the prescribed burning, and the greatest reduction was found in moderately labile organic P (e.g., NaOH-extractable fractions). Soil inorganic forms of P, however, were not correspondingly increased by the prescribed burning. Microbial biomass P was closely related to the shifts in P fractions. These effects were only detected immediately after the fire.

Conclusions

Microbial biomass could serve as a sink of P in P-impoverished soils and play an important role in soil P transformation. Our results indicate that microbial biomass is an important factor that governs P status after prescribed burning. The rapid recovery of microbial biomass P could be beneficial to the P requirement for plant regrowth after prescribed burning.     

Fractionation of Cd and Zn in Cd-contaminated soils amended by sugarcane waste products from an ethanol production plant

Purpose

Sugarcane waste products (boiler ash, filter cake, and vinasse) from an ethanol production plant were used as soil amendments by adding 3 % (w/w) in single and/or in combination, with a research focus towards stabilization of cadmium (Cd) and zinc (Zn) in contaminated soils. The objective of this laboratory study was to evaluate the effects of adding these sugarcane waste products on bioavailability of Cd and Zn over time (aging) in Cd- and Zn-contaminated agricultural soils of Thailand.

Materials and methods

Two agricultural contaminated soils of low (<3 mg kg^?1) and high (10–15 mg kg^?1) Cd concentrations were collected from Tak Province, Northwest Thailand. Fourteen treatments were sampled at 2-week intervals for 84 days for metal bioavailability using BCR extraction procedures (proposed by The Standards, Measurements and Testing Programme of the European Union, SM&T) that determined exchangeable (BCR1), reducible (BCR2), oxidizable (BCR3), and residual (BCR4) fractions, and total concentration was determined using aqua regia digestion and microwave digestion.

Results and discussion

Cd was potentially bioavailable, predominantly in exchangeable (BCR1) and reducible (BCR2) fractions, while the higher contribution of Zn was more prevalent in refractory fractions (BCR2 and BCR4). Aging had an influence on fractionation of Cd and Zn, most notably in the first two fractions (BCR1 and BCR2) of BCR sequential extraction, which resulted in reduction of exchangeable Cd during the first few weeks of incubation (T?=?0 to 28 days). At the end of pot experiment, the exchangeable Cd fraction in the low Cd (LCdS) soil was reduced from 2.3 to 4.7 % and 9.4 to 39.9 % in low and high Cd (HCdS)-contaminated soils, respectively, as compared to nonamended soils.

Conclusions

The observed reduction in exchangeable Cd (BCR1) in the amended soils at the 3 % (w/w) application rate, the low total metal concentrations, and the significant amount of essential plant nutrients (N, P, and K) within these waste products highlight the benefits of amending metal-rich soils with them.     

Impact of manure application on forms and quantities of phosphorus in a Chinese Cambisol under different land use

Purpose

It is critical to understand the effect of manure application on the availability of phosphorus (P) and the potential environmental contamination by runoff and leaching. However, previous studies generally focused on cultivated soil layer in single cropping systems. The aim of this study was to evaluate the effect of manure application on soil P forms and quantities to the 200 cm depth in a Chinese alkaline Cambisol in different cropping systems and the potential environmental implications.

Materials and methods

The sampling site, Shunyi District, is located in the peri-urban area of Beijing in the North China Plain, where large quantities of manure generated from intensive animal operations have been applied to agricultural fields. A field survey was carried out before sampling to identify soil sampling sites with long-term manure application and an adjacent area receiving no manure used for the same crop production. Soil samples from three cropping systems (vegetables, cereals, and trees) were vertically collected to a depth of 200 cm with the following depth increments: 0–20, 20–60, 60–90, 90–120, 120–160 and 160–200 cm. Soil samples were analyzed for plant-available P (Olsen P) and various P fractions by sequential P fractionation. Degree of P saturation (DPS) was also determined.

Results and discussion

Soil calcium bound P was the most abundant P fraction, followed by the residual P. Organic P only accounted for less than 5 % of total P in most of the soils. Manure application increased the levels of inorganic P (Pi), with higher proportions of Pi in labile forms than stable forms. After manure application for 8–15 year, available P (Olsen P) and DPS values of the 0–20 cm layer in all sites exceeded the threshold for Olsen P (60 mg?kg^?1) and DPS (30 %) and the risk of P loss by runoff is expected to significantly increase. The DPS values were generally lower than 30 % below 20 cm, indicating a minimal risk of P loss via leaching from deeper soil.

Conclusions

The results indicated that in typical peri-urban areas of the North China Plain, the on-going practice of manure application not only increased the size of each of the labile and non-labile P pools, but also caused a shift in the relative sizes of the different pools, regardless of the cropping systems. However, contrary to what was expected, soil P loss through surface runoff would be a greater concern than leaching following long-term manure amendment.     

Four Swedish long-term field experiments with sewage sludge reveal a limited effect on soil microbes and on metal uptake by crops

Purpose

This study aims to study the effect of sewage sludge amendment on crop yield and on microbial biomass and community structure in Swedish agricultural soils.

Materials and methods

Topsoil samples (0–0.20 m depth) from four sites where sewage sludge had been repeatedly applied during 14–53 years were analysed for total C, total N, pH and phospholipid fatty acids (PLFAs). Heavy metals were analysed in both soil and plant samples, and crop yields were recorded.

Results and discussion

At all four sites, sewage sludge application increased crop yield and soil organic carbon. Sludge addition also resulted in elevated concentrations of some heavy metals (mainly Cu and Zn) in soils, but high concentrations of metals (Ni and Zn) in plant materials were almost exclusively found in the oldest experiment, started in 1956. PLFA analysis showed that the microbial community structure was strongly affected by changes in soil pH. At those sites where sewage sludge had caused low pH, Gram-positive bacteria were more abundant. However, differences in community structure were larger between sites than between the treatments.

Conclusions

At all four sites, long-term sewage sludge application increased the soil organic carbon and nitrogen content, microbial biomass and crop yield. Long-term sewage sludge application led to a decrease in soil pH. Concentrations of some metals had increased significantly with sewage sludge application at all sites, but the amounts of metals added to soil with sewage sludge were found not to be toxic for microbes at any site.     

In situ remediation of cadmium-polluted soil reusing four by-products individually and in combination

Purpose

The aim of this study was to evaluate the effectiveness of mining, industrial and agricultural solid by-products in the in situ immobilisation of soil cadmium (Cd) based on soybean plant Cd content, soil pH, Cd extractability, bioavailability, leachability and Cd distribution in soils.

Materials and methods

The experiment was conducted as a field experiment in Cd-polluted-soil, wherein four by-products, including fly ash, spent mushroom substrate, silkworm excrement and limestone, were tested individually and in combination. The total Cd in soybean and the soil/by-products samples were determined. The Cd contents in the contaminated soil were analysed by the diffusive gradients in thin-film technique, the toxicity characteristic leaching procedure and four chemical methods. Changes in the fractions of Cd were determined following the Tessier method.

Results and discussion

The results showed that all the additions of the by-products increased the soil pH significantly and simultaneously decreased Cd mobility, bioavailability and leachability, particularly weakened the rate of Cd²⁺ ion transport from soil to solution. The by-products caused 23.5–76.4% of the exchangeable (EX) fraction of Cd to immobilised Cd fractions which include carbonates bound (CA), Fe-Mn oxides bound (OX), organic matter bound and residual fractions. The mobile faction of Cd was reduced from 33.7 to 16.8–27.8% for the amendments addition, respectively. Limestone was the most effective in immobilising the soil Cd among all the treatments, followed by fly ash. Soil pH observed significantly negative correlations with the Cd concentration in extractability, bioavailability and leachability. Soil pH had positive correlations with the percentages of CA-Cd and OX-Cd, but negatively correlated with the percentages of EX-Cd and the sum of EX-Cd and CA-Cd.

Conclusions

By-products addition increased the soil pH and decreased Cd mobility, bioavailability and leachability. The addition of limestone and fly ash exhibited higher efficiency than the other five additions. The combined additions had better performance on Cd extractability and soil pH than the corresponding single treatment, which decreased more concentrations of mobile, bioavailable and leachable Cd. This study offered four potentially cost-effective amendments singly or jointly for Cd immobilisation, reducing the potential hazards associated with excess Cd and the waste-disposal pressure and promoting a resource-saving development strategy.