Immobilization of selected heavy metals in sewage sludge by natural zeolites

The effect of leaching and bulking agents on the quality of municipal solid waste compost

Compost quality varies greatly depending on the feedstocks used and the composting process. Thus, improving the quality of compost is highly important for producing a high-quality agricultural product. Accordingly, the effect of leaching and addition of wheat straw (WS) and wood shaving (WSH) as bulking agents (BAs) on the organic fraction of municipal solid waste (OFMSW) was evaluated through assessing their effects on composting process and final product quality. Two pilot-scale experiments were prepared, each consisting of 3 piles of 1.2 m3. The first experiment (A) has three plies: A1 =  (100% OFMSW), A2 =  (88.5% OFMSW + 11.5% WS) and A3 =  (90.1% OFMSW + 9.9% WSH). The second one (B), where OFMSW was subjected to leaching, has also three piles: B1 =  (100% OFMSW), B2 =  (96.2% OFMSW + 3.8% WS) and B3 =  (97.1% OFMSW + 2.9% WSH). The results showed that the addition of BAs and leaching treatments accelerated the starting step, raised the temperature and decomposing process, and reduced the duration of the thermophilic phase. The addition of BAs, primarily WS, increased compost stability and maturity in terms of greater total organic carbon (TOC) 22.8-25.5%, total kjeldahl nitrogen (TKN) 1.73-2.37%, germination index (GI) 84.6-107.2%, and reduced C/N 10.8-14 ratio and CO2 evolution rate 2.67-3.69 mg CO2 - C/g OM/d. The use of the leaching process and BAa decreased EC values and heavy metal content in the final products. The implementation of these experimental approaches can be used successfully to reduce the undesired high salt and heavy metal content in the final composts.

Stabilization of heavy metals in sewage sludge by attapulgite

The mobility and bioavailability of heavy metals in sewage sludge are the major risks to utilize for agricultural application. In this study, the chemical speciation of Cu, Ni, Cr, and Zn in the Lanzhou municipal sewage sludge were investigated with the addition of a natural attapulgite. The influences of attapulgite amendment in sewage sludge on heavy metals stabilization were evaluated by investigating the leaching procedure and sequential chemical extraction experiments. The sequential extraction procedure described by European Community Bureau of Reference was used in sludge to determine the distribution of heavy metal species. The addition of attapulgite accelerated more reducible speciation of all metals transformed into residual speciation evidently. It reduced the leaching content of metals significantly and decreased the ecotoxicity accounted for the germination index values climbed rapidly with increased attapulgite addition.Implications: This research developed a method to stabilize heavy metals in municipal sewage sludge with clay. The activated attapulgite improved the treatment of sewage sludge containing heavy metals, and reduced the environmental risk of heavy metals.

Sorption Behaviors of Light Lanthanides(III) (La(III), Ce(III), Pr(III), Nd(III)) and Cr(III) Using Nitrolite

The sorption of light lanthanides(III) (La(III), Ce(III), Pr(III), Nd(III)) and chromium(III) ions from acidic solutions on Nitrolite was studied at varying ions concentrations, pH, contact time and temperatures. The sorption capacity of lanthanides(III) and chromium(III) ions were examined in the ranges 2–9 and 2–5, respectively. The adsorption capacities of all metals are increase with the increasing pH (up to initial pH 9), despite the potential precipitation of metals at higher pH values. Therefore, an initial pH 9 of lanthanides gives the highest adsorption capacities. The kinetics of sorption chromium(III) and light lanthanides(III) were investigated. The experimental data were analyzed using the pseudo-first-order, pseudo-second-order forms, Elovich, and intra-particle diffusion models. The sorption kinetics of investigated ions was described by pseudo-second-order model the best. The results indicate the endothermic process of Cr(III), La(III), Ce(III), Pr(III) and Nd(III) ions sorption. The sorption capacities of La(III) 4.77 mg/g, Ce(III) 4.45 mg/g, Pr(III) 4.30 mg/g, Nd(III) 4.13 mg/g and Cr(III) 2.39 mg/g were calculated from the Langmiur model, which describes adsorption better than Freundlich and Dubinin–Radushkevich.

Use of clay minerals for sewage sludge stabilization and a preliminary assessment of the treated sludge's fertilization capacity

Preserving sewage sludge's N is important for its agronomic use and this could possibly be achieved by treating sludge with certain clay minerals. Nine clay minerals and additionally Ca(OH)₂ were added to dewatered sewage sludge at 0-30 % rates (wet weight basis) (treatments). After 70 days of equilibration, all mixtures were analyzed for certain properties and the mineral-sludge mixtures which showed the highest microbial load reduction were further assayed, along with the limed and untreated sludge. From all minerals' treatments, the fecal indicators of sludge treated with 30% of two bentonites, attapulgite, saponite-attapulgite, and zeolite decreased considerably compared to the control. These treatments were performed also well regarding sludge's retention capacity of available inorganic N, with the attapulgite and zeolite treatments containing the significantly highest amounts of NO₃-N and NH₄-N, respectively. For the water-soluble inorganic N, similar results were obtained for the zeolite treatment, whereas the treatments with the two bentonites had the significantly highest NO₃-N content. Also, considerable amounts of water-soluble P were obtained in all cases of the treated sludge with minerals. Limed sludge had the lowest content of the water-soluble inorganic N and P. As far as the micronutrients are concerned, only Zn and B were detectable in the water-soluble fraction of all five minerals' treatments. The heavy metals, which regulate sludge's agronomic use, were far below the respective permissible limits and lower than the untreated sludge, except for Ni and Cr in the attapulgite and saponite-attapulgite treatments. In conclusion, certain clay minerals, i.e., bentonite, attapulgite, mixed clay of saponite and attapulgite, and zeolite, seem promising materials for the stabilization of sewage sludge in the perspective of using them as a fertilizer. In addition, they seem to have higher fertilizing value than limed sludge. However, environmental (in respect of Ni and Cr) and agricultural (in respect of Zn and B) impacts must be considered.

Immobilization of cadmium and lead in contaminated paddy field using inorganic and organic additives

Heavy metal contamination of agricultural soils has posed a risk to environment and human health. The present study was conducted to assess the effectiveness of soil amendments for reducing cadmium (Cd) and lead (Pb) uptake by rice (Oryza sativa L) in a contaminated field. The soil amendments used include lime, DaSan Yuan (DASY), DiKang No.1 (DEK1), biochar, Fe-biochar, Yirang, phosphorus fertilizer, (Green Stabilizing Agent) GSA-1, GSA-2, GSA-3, and GSA-4, applied at 1% rate in a field experiment. The results exposed that GSA-4 treatment showed best effects on reducing Cd and Pb phytoavailability in soil and uptake by early rice. Linear increase in pH (i.e. 5.69 to 6.75) was recorded in GSA-4 amended soil from sowing to the 3rd month of growth season. GSA-4 decreased DTPA extractable contents of cadmium (Cd) from 0.324 to 0.136 mg kg⁻¹ soil and lead (Pb) from 53.21 to 24.68 mg kg⁻¹ soil at 90 days of amendment. Treatment with GSA-4 improved rice growth (56%) and grains yield (42%). The enhancement effects on grain yield may be result from the positive effects of GSA-4 application on increasing photosynthesis (116%) and transpiration rate (152%) as compared to the control. Significant reduction in Cd and Pb uptake in shoot (42% and 44%) and in grains (77 and 88%), was observed, respectively in GSA-4 treatment as compared with the control. Moreover, negative correlation was recorded between DTPA extractable Cd/Pb and soil pH that directly depended on applied amendments. In short, use of combined amendment (GSA-4) was more effective for immobilizing heavy metals in contaminated paddy field, and secures rice safe production, as compared other tested amendment products.

Potential of Zeolite and Algae in Biomass Immobilization

The interest in utilizing algae for wastewater treatment has been increased due to many advantages. Algae-wastewater treatment system offers a cost-efficient and environmentally friendly alternative to conventional treatment processes such as electrocoagulation and flocculation. In this biosystem, algae can assimilate nutrients in the wastewater for their growth and simultaneously capture the carbon dioxide from the atmosphere during photosynthesis resulting in a decrease in the greenhouse gaseousness. Furthermore, the algal biomass obtained from the treatment process could be further converted to produce high value-added products. However, the recovery of free suspended algae from the treated effluent is one of the most important challenges during the treatment process as the current methods such as centrifugation and filtration are faced with the high cost. Immobilization of algae is a suitable approach to overcome the harvesting issue. However, there are some drawbacks with the common immobilization carriers such as alginate and polyacrylamide related to low stability and toxicity, respectively. Hence, it is necessary to apply a new carrier without the mentioned problems. One of the carriers that can be a suitable candidate for the immobilization is zeolite. To date, various types of zeolite have been used for the immobilization of cells of bacteria and yeast. If there is any possibility to apply them for the immobilization of algae, it needs to be considered in further studies. This article reviews cell immobilization technique, biomass immobilization onto zeolites, and algal immobilization with their applications. Furthermore, the potential application of zeolite as an ideal carrier for algal immobilization has been discussed.

Functionality of surfactants in waste-activated sludge treatment: A review

Proper treatment of waste-activated sludge (WAS) involves three pivotal processes, dewatering, anaerobic digestion, and pollutants removal, which need to be re-assessed urgently. Although many traditional sludge treatments have been developed, it is prudent to enhance the efficiency of sludge treatment using multifunctional, flexible, and environmentally friendly surfactants. With regard to sludge dewatering, surfactants can weaken the binding interaction between sludge flocs and promote the dissolution of extracellular polymeric substances (EPSs), resulting in the release of bound water. Using surfactants in anaerobic digestion promotes the release of enzymes trapped in sludge and improves the activity of enzymes during hydrolysis. Owing to their characteristic encapsulation of hydrophobes into self-assembled aggregates (micelles), surfactants can form host-guest complexes with polycyclic aromatic hydrocarbons (PAHs). Additionally, surfactants can enhance the desorption of heavy metals and prevent the emergence of heavy metal residue. This review summarizes the current surfactant-based sludge treatment technologies according to their roles in sludge disposal solutions. Then, possible mechanisms of surfactants in sludge dewatering, anaerobic digestion, and the removal of organic pollutants and heavy metals are analysed systemically. Finally, changes to sludge treatment via the aid of surfactants are highlighted. This review presents the comprehensive advances in the use of surfactants in WAS reduction, recycling, and risk relief, underscoring their roles in increasing economic efficiency and ensuring environmental quality.

Immobilization of Heavy Metals in Sewage Sludge during Land Application Process in China: A Review

The safe disposal of sewage sludge during the process of municipal wastewater treatment has become one of the major concerns of increased production. Land application was thought of as a more economical method for sewage sludge disposal than landfill and incineration. However, the presence of heavy metals in sewage sludge restricted the use of land application. The environmental risk of heavy metals was dependent on their contents, chemical speciations, and soil characteristics. Composting and chemical immobilization were the commonly used methods to immobilize the heavy metals in sewage sludge. The immobilization mechanism and speciation transformation of heavy metals during the composting process were presented. Aluminosilicate, phosphorus-bearing materials, basic compounds, and sulfides were reviewed as the commonly used chemical immobilizing agents. The problems that occur during the immobilization process were also discussed. The combination of different methods and the modification of chemical immobilizing agents both improved the fixation effect on heavy metals.

Compost maturity and nitrogen availability by co-composting of paddy husk and chicken manure amended with clinoptilolite zeolite

The availability of paddy husk from rice processing plants remains high owing to increase in the worldwide rice consumption. Increasing demand for chicken products leads to poultry wastes production. Co-composting of the aforementioned wastes could solve the indiscriminate disposal of these wastes. Thus, co-composting of paddy husk and chicken slurry with clinoptilolite zeolite and urea as additive was carried out. Clinoptilolite zeolite was used to enhance ammonium and nitrate retention in the compost. Temperature of the compost was monitored three times daily for 55 days. Cation exchange capacity, organic matter, ash, humic acids, pH, total C, N, C/N ratio; total P, exchangeable Ca, Mg, K, NH4+, NO3-, and heavy metals contents were determined using standard procedures. pH, total N, humic acids, ash, NH4+, NO3-, P, Ca, Mg, and K contents increased but the salinity, heavy metals contents, and microbial population were low after the co-composting process. Zea mays L. (test crop) seed germination rate in distilled water and the compost were not significantly different. Growth of Spinach oleracea (test crop) on a peat-based growing medium and the compost was also not significantly different. These findings were possible because the clinoptilolite zeolite used in co-composting reduced accumulation of heavy metals that may have damage effects on the test crops. Mature compost with good agronomic properties can be produced by co-composting chicken slurry and paddy husk using clinoptilolite zeolite and urea as additives.

Phytoremediation of sewage sludge and use of its leachate for crop production

The land application of sewage sludge has the potential risk of transferring heavy metals to soil or groundwater. The agricultural reuse of sludge leachate could be a cost-effective way to decrease metal contamination. Sludge leachate collected during the phytoremediation of sludge by co-cropping with Sedum alfredii and Zea mays was used for irrigating vegetables in a field experiment. Results indicate that the concentrations of Cu, Zn, Pb, and Cd in sludge leachates complied with the National Standards for agricultural irrigation water in China. For the vegetable crop Ipomoea aquatica, nutrients obtained only from the sludge leachate were not sufficient to support growth. For the second crop, Brassica parachinensis, no differences in biomass were observed between the treatment with leachate plus a half dose of inorganic fertilizer and the treatment with a full dose of inorganic fertilizers. The concentrations of heavy metals in I. aquatica and B. parachinensis were not significantly affected by the application of sludge leachates. Compared with initial values, there were no significant differences in Zn, Cd, Cu, and Pb concentrations in soil following treatment with sludge leachate. This study indicates that on range lands, sludge phytoremediation can be conducted at the upper level, and the generated sludge leachate can be safely and easily used in crop production at the lower level.

Immobilization of heavy metals in sewage sludge by using subcritical water technology

Heavy metals (HMs) immobilization in sewage sludge was investigated by using subcritical water technology (SCWT) in this study. The characteristics of sludge and toxicity of HMs were analyzed after SCWT process. The results showed that besides large reduction in sludge volume, SCWT had some positive effect on HMs dissolution into liquid phase, while the majority of HMs was still accumulated in solid phase. The direct toxicity and bioavailability of HMs in sludge was greatly decreased with no toxicity fractions of HMs highly increased. Pb was always at low risk level and the risk of other HMs was greatly reduced from low risk to no risk after SCWT treatment. Moreover, the leaching toxicity of HMs declined after SCWT and the best result was obtained at 280°C with the metal concentrations in leachate decreased by 97.46%, 93.91%, 86.14%, 73.67%, 71.93% and 10.71% for Cu, Cd, Zn, Cr, Ni and Pb, respectively.

Competitive adsorption of dyes and heavy metals on zeolitic structures

The adsorption of Acid blue 25, basic blue 9, basic violet 3, Pb(2+), Ni(2+), Zn(2+) and Cd(2+) ions has been studied in single and dye-metal binary solutions using two mineral materials: Clinoptilolite (CL) and ER (Erionite). These zeolites were characterized by FT-IR spectroscopy; potentiometric titration and nitrogen adsorption isotherms at 77 K to obtain their textural parameters. Results indicated that ER has an acidic character and a high specific surface (401 m(2) g(-1)) in contrast with the zeolite CL (21 m(2) g(-1)). Surprisingly, the removal of dyes was very similar for the two zeolites and they showed a considerable selectivity by the basic dyes in comparison with the acid dyes. In the case of heavy metals, ER was more effective in the adsorption process showing a selectivity of: Pb(2+) > Ni(2+) > Zn(2+) > Cd(2+). In the multicomponent adsorption experiments an antagonistic effect was observed in the removal of basic dyes and heavy metals. Particularly, the adsorbed amount of basic violet 3 decreased more significantly when the heavy metals are presents in contrast with the basic blue 9.

Immobilizer-assisted management of metal-contaminated agricultural soils for safer food production

Production of food crops on metal contaminated agricultural soils is of concern because consumers are potentially exposed to hazardous metals via dietary intake of such crops or crop derived products. Therefore, the current study was conducted to develop management protocols for crop cultivation to allow safer food production. Metal uptake, as influenced by pH change-induced immobilizing agents (dolomite, steel slag, and agricultural lime) and sorption agents (zeolite and compost), was monitored in three common plants representative of leafy (Chinese cabbage), root (spring onion) and fruit (red pepper) vegetables, in a field experiment. The efficiency of the immobilizing agents was assessed by their ability to decrease the phytoavailability of metals (Cd, Pb, and Zn). The fruit vegetable (red pepper) showed the least accumulation of Cd (0.16-0.29 mgkg(-1) DW) and Pb (0.2-0.9 mgkg(-1) DW) in edible parts regardless of treatment, indicating selection of low metal accumulating crops was a reasonable strategy for safer food production. However, safer food production was more likely to be achievable by combining crop selection with immobilizing agent amendment of soils. Among the immobilizing agents, pH change-induced immobilizers were more effective than sorption agents, showing decreases in Cd and Pb concentrations in each plant well below standard limits. The efficiency of pH change-induced immobilizers was also comparable to reductions obtained by 'clean soil cover' where the total metal concentrations of the plow layer was reduced via capping the surface with uncontaminated soil, implying that pH change-induced immobilizers can be practically applied to metal contaminated agricultural soils for safer food production.

Modified natural clinoptilolite detoxifies small mammal's organism loaded with lead I. Lead disposition and kinetic model for lead bioaccumulation

Zeolites, especially clinoptilolites, have wide application in removing heavy metals from different solutions and wastewater. The detoxification capacity of the clinoptilolite sorbent KLS-10-MA, a modified natural Bulgarian zeolite, applied as a food supplement in conditions of an ecotoxicological experiment with conventional food and lead was demonstrated for the first time. Laboratory mice, inbred imprinting control region strain, were used in a 90-day ecotoxicological experiment. Animals were divided into four experimental groups. Lead bioaccumulations in exposed and non-supplemented/supplemented with KLS-10-MA animals were compared. As additional control, healthy animals non-exposed to Pb were fed with conventional forage mixed with 12.5% KLS-10-MA. The dietary inclusion of the sorbent reduced Pb concentrations in exposed and supplemented mice by 84%, 89%, 91%, 77%, and 88% in carcass, liver, kidneys, bones, and feces, respectively. A mathematical model was proposed to outline the common trends of bone Pb bioaccumulation in exposed and non-supplemented/supplemented animals. Characteristic parameters of the kinetics of Pb concentrations were determined. Based on the model, the coefficient of absorption of Pb by gastrointestinal mucosa in the supplemented mice was found-η = 3.53% (versus η = 15% in non-supplemented ones). The present study clearly indicates that there is a realistic perspective to create a new drug based on modified natural clinoptilolites in cases of chronic heavy metal intoxication, without negatively affecting the environment.

Application of genetic algorithms to determine heavy metal ions sorption dynamics on clinoptilolite bed

In the last decade a growing interest was observed in low-cost adsorbents for heavy metal ions. Clinoptilolite is a mineral sorbent extracted in Poland that is used to remove heavy metal ions from diluted solutions. The experiments in this study were carried out in a laboratory column for multicomponent water solutions of heavy metal ions, i.e. Cu(II), Zn(II) and Ni(II). A mathematical model to calculate the metals' concentration of water solution at the column outlet and the concentration of adsorbed substances in the adsorbent was proposed. It enables determination of breakthrough curves for different process conditions and column dimensions. The model of process dynamics in the column took into account the specificity of sorption described by the Elovich equation (for chemical sorption and ion exchange). Identification of the column dynamics consisted in finding model coefficients β, KE and Deff and comparing the calculated values with experimental data. Searching for coefficients which identify the column operation can involve the use of optimisation methods to find the area of feasible solutions in order to obtain a global extremum. For that purpose our own procedure of genetic algorithm is applied in the study.

Quantifying the environmental impact of As and Cr in stabilized/solidified materials

The assessment of the quality of sediment from the Great Backi Canal (Serbia) based on the pseudo-total As and Cr content according to the corresponding Dutch standards and Canadian guidelines showed its severe contamination with these two elements. Microwave assisted BCR sequential extraction procedure was employed to assess their potential mobility and risk to the aquatic environment. Comparison of the results of sequential extraction and different criteria for sediment quality assessment has led to somewhat contradictory conclusions. While the results of sequential extraction showed that Cr comes under the medium risk category, As shows no risk to the environment, despite of its high pseudo-total content. The contaminated sediment, irrespective of the different distribution of As and Cr, was subjected to the same immobilization, stabilization/solidification (S/S) treatment. Semi-dynamic leaching test was conducted for As and Cr contaminated sediment in order to assess the long-term leaching behavior of these elements. In order to simulate "worst case" leaching conditions, the test was modified using acetic acid and humic acid solution as leachants instead of deionized water. The effectiveness of S/S treatment was evaluated by determining diffusion coefficients. Four different single-step leaching tests were applied to evaluate the extraction potential of As and Cr. A diffusion-based model was used to elucidate the controlling leaching mechanisms. The test results indicated that all applied S/S treatments were effective in immobilizing As and Cr, irrespective of their different availabilities in the untreated samples. In most treated samples, the controlling leaching mechanism appeared to be diffusion, which indicates that a slow leaching of As and Cr could be expected.

Biosorption of Cu(II) and Pb(II) ions from aqueous solution by natural spider silk

Aside from its excellent mechanical properties, spider silk (SS) would offer an active surface for heavy metal interaction due to its rich protein structure. The present study describes the potential use of natural (SS) as a sorbent of heavy metals from aqueous solutions. Single and multi-species biosorption experiments of heavy metals by natural SS were conducted using batch and column experiments. The biosorption kinetics, in general, was found to follow the second-order rate expression, and the experimental equilibrium biosorption data fitted reasonably well to Freundlich isotherm. From the Freundlich isotherm, the biosorption capacities of Cu(II) and Pb(II) ions onto SS were found as 0.20 and 0.007 mmol g⁻¹, respectively. The results showed a decrease in the extent of metal ion uptake with lowering the pH.

Utilization of natural chabazite and mordenite as a reactive barrier for immobilization of hazardous heavy metals

INTRODUCTION

Understanding the sorption process in natural geomedia is necessary for effective utilization of these materials as low-cost adsorbents and consequently as controlled release hazardous elements. This research was oriented to investigate the affinity of two natural zeolite minerals towards cobalt, zinc, and nickel mixture as an important industrial and radioactive waste.

METHOD

The uptake of metal ions as a function of different parameters has been studied using a batch equilibrium technique.

RESULTS

The results revealed that the affinity and adsorption capacity of chabazite and mordenite follow the order: Zn(2+) > Co(2+) > Ni(2+), with good fits being obtained using Langmuir and Freundlich adsorption isotherms. The metal uptake was found to be concentration-dependent and independent of the pH over 3.0 to 8.0 range; this reveals that the adsorption mechanism is controlled mainly by a pure ion-exchange reaction at the experimental conditions used. Kinetic curves showed a rather fast exchange reaction for three cations, as equilibrium was mostly reached within 20 min.

CONCLUSION

These materials especially chabazite are recommended to be used as a reactive barrier for hazardous heavy metals control.

Composting domestic sewage sludge with natural zeolites in a rotary drum reactor

This work aimed the influence of zeolites addition on a sludge-straw composting process using a pilot-scale rotary drum reactor. The type and concentration of three commercial natural zeolites were considered: a mordenite and two clinoptilolites (Klinolith and Zeocat). Mordenite caused the greatest carbon removal (58%), while the clinoptilolites halved losses of ammonium. All zeolites removed 100% of Ni, Cr, Pb, and significant amounts (more than 60%) of Cu, Zn and Hg. Zeocat displayed the greatest retention of ammonium and metals, and retention efficiencies increased as Zeocat concentration increased. The addition of 10% Zeocat produced compost compliant with Spanish regulations. Zeolites were separated from the final compost, and leaching studies suggested that zeolites leachates contained very low metals concentrations (<1 mg/kg). Thus, the final compost could be applied directly to soil, or metal-polluted zeolites could be separated from the compost prior to application. The different options have been discussed.

Adsorption of Ba2+ by Ca-exchange clinoptilolite tuff and montmorillonite clay

The adsorption of barium by Ca-exchanged clinoptilolite and montmorillonite is presented. The kinetics of adsorption of Ba(2+) were evaluated contacting 1g portion of each adsorber with 100mL 0.1N BaCl(2) for 200 h. Adsorption by Ca-clinoptilolite is defined by second-order kinetics of rate constant K(v) 8.232 x 10(-2) g mg(-1)h(-1) and maximum removal of 71.885 mg g(-1). It is a two-stage process initiated by a rapid uptake of Ba(2+) followed by more moderate kinetics. The adsorption isotherms were determined contacting 0.2g of each adsorber with 10 mL 0.1-0.005N BaCl(2)+CaCl(2) solution, Ba(2+)/Ca(2+) ratio 1, for periods of 7 days for the tuff and 2 days for the clay. The equilibrium adsorption is described by the Langmuir model, of equilibrium constant K 0.0151 L mg(-1) and maximum adsorption of 15.29 mg g(-1). The adsorption of Ba(2+) by Ca-exchanged montmorillonite also follows a second-order reaction of rate constant K(v) 3.179 x 10(-2) g mg(-1)h(-1), and calculated separation of 36.74 mg g(-1); the Langmuir isotherm is defined by the constant K 0.034 L mg(-1) and maximum adsorption of 15.29 mg g(-1). X-ray diffraction shows that the exchange of Ba(2+) modifies the d(001) of Ca-montmorillonite from 15.4 to 12.4A.

Sewage sludge bioleaching by indigenous sulfur-oxidizing bacteria: effects of ratio of substrate dosage to solid content

The aim of this work was to study the effect of ratio of substrate dosage to solid content (Sd/SC) on sewage sludge bioleaching. The inocula--indigenous sulfur-oxidizing bacteria were enriched and cultured from the fresh activated sludge to a wastewater treatment plant. The results showed that Sd/SC significantly influenced the sludge bioleaching process. With increase in Sd/SC the sludge bioleaching was enhanced, which was represented by the acceleration of sludge acidification, oxidizing environment formation, and substrate (sulfur) utilization. Higher Sd/SC was more efficient to solubilize the heavy metals and total phosphorus (TP) than lower Sd/SC, while total nitrogen (TN) release was not influenced by Sd/SC. Zinc and copper were efficiently bioleached because of sludge acidification and sludge oxidation, but lead was bioleached with a low efficiency because of the formation of low soluble PbSO(4) precipitates. After bioleaching the biotoxicity of sewage sludge greatly reduced.