Tannery Sludge Research Articles

Viabilidade do uso de resíduos da agroindústria coureiro-calçadista no solo

As indústrias couro-calçadistas geram grandes quantidades de resíduos que contém cromo, produto utilizado no curtimento do couro. Para avaliar os efeitos de três aplicações de lodo de curtume (LC), resíduo de rebaixadeira (RR) e aparas de couro (AC) sobre o rendimento do sorgo (Sorghum graniferum) e rabanete (Raphanus sativus), foi conduzido um experimento em colunas de PVC, com três tipos de solo, durante o período de 1996 a 1999. Foram determinados no solo os valores de pH e o teor de cromo, bem como o teor de cromo nas partes comestíveis das plantas. Os resíduos não afetaram o crescimento das plantas, quando foram feitas a calagem e a adubação mineral. O LC pode neutralizar a acidez do solo e suprir nitrogênio para as plantas. O teor de cromo nas partes comestíveis das plantas aumentou com a aplicação sucessiva dos resíduos, estando, entretanto, sem exceder os teores aceitáveis para o consumo. Não foi observada a mobilidade de cromo para as camadas subsuperficiais do solo.

Chemical and geochemical characterisation of a disused red brick factory area of central Italy

Characterisation and remedial actions are proposed for a polluted site enclosed within a red brick factory, located in central Italy. Approximately 22,000 t of chromium containing tannery sludge were introduced as ‘secondary raw materials’ for reuse in red brick manufacture. Preliminary geo-electric tomography was carried out on the waste bank, followed by the perforation of 14 boreholes into the bottom geological stratum. Heavy metals [B, Crtot, Cr(VI), Hg, Pb, Cu, Sn, V, Zn] and polycyclic aromatic hydrocarbons [benzo(a)pyrene; phenanthrene; dibenzo(a,h)pyrene; pyrene] were determined on solid samples. Carrots were sliced to give samples from different depths in the waste bank. The following samples were determined in the waste leachate: 1,1,2trichlorethane, phenol, 2,4,6trichlorphenol, hexachlorhexane, dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyldichloroethane (DDD) and dichlorodiphenyldichloroethylene (DDE). During characterisation, laboratory experiments were also carried out on lime-treated and untreated carrot sections to evaluate heavy metal release and potential migration in the subsoil. Pollutant exposure pathways were set up, based on application of the Risk-Based Corrective Action (RBCA) model, followed by the evaluation of potential environmental and sanitary targets for site-specific risk evaluation and the proposition of scientifically based restoration scenarios. Surface capping, coupled with lateral belting of the site, were selected for the site recovery operations after extensive cost–benefit evaluations.

Tannery sludge compost amendment rates on soil microbial biomass of two different soils

Composting is recognized as one of the most cost-effective and environmentally sound alternatives for organic waste recycling. Long-composted wastes have the potential to substitute for inorganic fertilizers. We investigated the effect of tannery sludge compost (TSC) amendment rates on microbial biomass and activity in Brazilian soils. The soils (sandy and clayey soils) were amended with TSC at rates of 0 (control), 7.5, 15, 30 and 60Mgha−1 (equivalent to 0, 0.3, 0.6, 1.2 and 2.4g per 100g of soil, respectively), incubated at 28°C for 60 days. Soil Microbial biomass, soil respiration and enzyme activities were evaluated at 15, 30 and 60 days after incubation. The application of 7.5Mgha−1 TSC significantly increased the microbial biomass and activity. There were no negative or positive effects by the application of 7.5Mgha−1 on soil enzymes. The results of this study suggest that TSC increased soil microbial biomass and activity when it was amended at a low rate. On the other hand, the amendment with the TSC, in all rates, did not negatively affect the soil microorganisms and their activities.

Use of Rice Husk Ash as an Admixture to Remove Chromium from a Tannery Waste

This laboratory-scale study explored the use of rice husk ash (RHA) as an admixture to stabilize and solidify the waste sludge from a chrome-tanning process. Various ratios of RHA and tannery sludge were used, ranging from 0.25 to 1.0 by weight of the cement content, resulting in an experimental matrix of 16 runs. It was observed that although the compressive strength of solidified sludge improved at longer curing times, the increase was moderate after 7 days of curing. In addition, higher RHA and/or sludge ratios progressively reduced the strength of the samples. On the other hand, curing time had a positive effect on chromium leachability since the leaching concentration decreased considerably with an increase in curing time, up to 28 days. Furthermore, both the RHA and/or sludge content affected leachability, producing acceptable results, from a Thai regulatory perspective, at ratios of 0.50 or lower. Finally, the solidified sludge can be disposed of in a secure landfill and/or used as a construction material in a number of applications.

Dosage-dependent shift in the spore community of arbuscular mycorrhizal fungi following application of tannery sludge

The controlled disposal of tannery sludge in agricultural soils is a viable alternative for recycling such waste; however, the impact of this practice on the arbuscular mycorrhizal fungi (AMF) communities is not well understood. We studied the effects of low-chromium tannery sludge amendment in soils on AMF spore density, species richness and diversity, and root colonization levels. Sludge was applied at four doses to an agricultural field in Rolândia, Paraná state, Brazil. The sludge was left undisturbed on the soil surface and then the area was harrowed and planted with corn. The soil was sampled at four intervals and corn roots once within a year (2007/2008). AMF spore density was low (1 to 49 spores per 50cm(3) of soil) and decreased as doses of tannery sludge increased. AMF root colonization was high (64%) and unaffected by tannery sludge. Eighteen AMF species belonging to six genera (Acaulospora, Glomus, Gigaspora, Scutellospora, Paraglomus, and Ambispora) were recorded. At the sludge doses of 9.0 and 22.6Mgha(-1), we observed a decrease in AMF species richness and diversity, and changes in their relative frequencies. Hierarchical grouping analysis showed that adding tannery waste to the soil altered AMF spore community in relation to the control, modifying the mycorrhizal status of soil and selectively favoring the sporulation of certain species.

Environmental Factors Affecting Chromium-Manganese Oxidation-Reduction Reactions in Soil

Disposal of chromium (Cr) hexavalent form, Cr(VI), in soils as additions in organic fertilizers, liming materials or plant nutrient sources can be dangerous since Cr(VI) can be highly toxic to plants, animals, and humans. In order to explore soil conditions that lead to Cr(VI) generation, this study were performed using a Paleudult (Dystic Nitosol) from a region that has a high concentration of tannery operations in the Rio Grande do Sul State, southern Brazil. Three laboratory incubation experiments were carried out to examine the influences of soil moisture content and concentration of cobalt and organic matter additions on soil Cr(VI) formation and release and manganese (Mn) oxide reduction with a salt of chromium chloride (CrCl 3) and tannery sludge as inorganic and organic sources of Cr(III), respectively. The amount of Cr(III) oxidation depended on the concentration of easily reducible Mn oxides and the oxidation was more intense at the soil water contents in which Mn(III/IV) oxides were more stable. Soluble organic compounds in soil decreased Cr(VI) formation due to Cr(III) complexation. This mechanism also resulted in the decrease in the oxidation of Cr(III) due to the tannery sludge additions. Chromium(III) oxidation to Cr(VI) at the solid/solution interface involved the following mechanisms: the formation of a precursor complex on manganese (Mn) oxide surfaces, followed by electron transfer from Cr(III) to Mn(III or IV), the formation of a successor complex with Mn(II) and Cr(VI), and the breakdown of the successor complex and release of Mn(II) and Cr(VI) into the soil solution.

Changes in the genetic structure of Bacteria and microbial activity in an agricultural soil amended with tannery sludge

Abstract The application of tannery sludge to soils is a form of recycling; however, few studies have examined the impacts of this practice on soil microbial properties. We studied effects of two applications (2006 and 2007) of tannery sludge (with a low chromium content) on the structure of the bacterial community and on the microbial activity of soils. We fertilized an agricultural area in Rolândia, Parana state, Brazil with different doses of sludge based on total N content, which ranged from 0 to 1200 kg N ha −1 . Sludge remained on the soil surface for three months before being plowed. Soils were sampled seven times during the experiment. Bacterial community structure, assessed by denaturing gradient gel electrophoresis (DGGE), was modified by the application of tannery sludge. Soon after the first application, there was clear separation between the bacterial communities in different treatments, such that each dose of sludge was associated with a specific community. These differences remained until 300 days after application and also after the second sludge application, but 666 days after the beginning of the experiment no differences were found in the bacterial communities of the lowest doses and the control. The principal response curve (PRC) analysis showed that the first sludge application strongly stimulated biological activity even 300 days after application. The second application also stimulated activity, but at a lower magnitude and for a shorter time, given that 260 days after the second application there was no difference in biological activity among treatments. PRC also showed that the properties most influenced by the application of tannery sludge were enzymatic activities related to N cycling (asparaginase and urease). The redundancy analysis (RDA) showed that tannery sludge’s influence on microbial activity is mainly related to increases in inorganic N and soil pH. Results showed that changes in the structure of the bacterial community in the studied soils were directly related to changes of their biological activity.

Growth responses and metal accumulation capabilities of woody plants during the phytoremediation of tannery sludge

Five woody plants species (i.e. Terminalia arjuna, Prosopis juliflora, Populus alba, Eucalyptus tereticornis and Dendrocalamus strictus) were selected for phytoremediation and grow on tannery sludge dumps of Common Effluent Treatment Plant (CETP), Unnao (Uttar Pradesh), India. Concentration of toxic metals were observed high in the raw tannery sludge i.e. Fe-1667 > Cr-628 > Zn-592 > Pb-427 > Cu-354 > Mn-210 > Cd-125 > Ni-76 mg kg −1 dw, respectively. Besides, physico-chemical properties of the raw sludge represented the toxic nature to human health and may pose numerous risks to local environment. The growth performances of woody plants were assessed in terms of various growth parameters such as height, diameter at breast height (DBH) and canopy area of plants. All the plant species have the capabilities to accumulate substantial amount of toxic metals in their tissues during the remediation. The ratio of accumulated metals in the plants were found in the order Fe > Cr > Mn > Pb > Zn > Cu > Cd > Ni and significant changes in physico-chemical parameters of tannery sludge were observed after treatment. All the woody plants indicated high bioconcentration factor for different metals in the order Fe > Cr > Mn > Ni > Cd > Pb > Zn > Cu. After one year of phytoremediation, the level of toxic metals were removed from tannery sludge up to Cr (70.22)%, Ni (59.21)%, Cd (58.4)%, Fe (49.75)%, Mn (30.95)%, Zn (22.80)%, Cu (20.46)% and Pb (14.05)%, respectively.

Chromium recovery from tannery sludge with saponin and oxidative remediation

Two new methods for treatment of tannery sludge were studied to achieve cost-effective and environmentally acceptable remediation solutions for high chromium containing tannery sludge. Quillaja bark saponin, a plant derived biosurfactant, was applied to dewatered tannery sludge for chromium recovery and a comparative assessment with H 2O 2 oxidative treatment method is presented. Tannery sludge samples were treated on a laboratory scale with saponin in the pH range 2–3. The effects of various factors like time, concentration of saponin, pH, and temperature on the extraction of chromium were studied. The treatment with saponin extracted 24% of Cr from tannery sludge at a pH around2, performing multiple wash of 6 h, at 33 °C. On the other hand, the H 2O 2 treatment, which include Cr(III) oxidation to Cr(VI) and extraction with sulfuric acid solution at pH 2, enabled to extract 70% of chromium within less than 4 h at room temperature (21 °C). The results indicate that the extraction efficiency of saponin was strongly dependent on the organic matter content of the sample, which affects chromium mobility by its high adsorption capacity. On the other hand hydrogen peroxide treatment is effective and the duration of the process is short and requires cheap chemicals and moderate conditions.

Sorção de cromo em solos do cerrado goiano, Brasil

Land application of tannery sludge, which usually contain high levels of chromium, and considerable amounts of organic matter, macronutrients and micronutrients may contribute to the improvement of soil fertility and plant nutrition, and constitutes a form of disposal residue in the environment. The objective of this work was to determine the sorption isotherms of metal chromium (Cr+3) in a Ultisol, Oxisol Typic Acrustox, Quartzipsamment and Kandic Oxisol, identify soil classes that are prone to chromium mobility, and characterize the potential of agricultural soils of Goiás that are subject to groundwater contamination by the potentially toxic metal. For the establishment of sorption isotherms, solutions were prepared at 1:10 in volume. Air dried samples of 5.0 cm3 of each class of soil were placed in triplicates in beakers of 250.0 cm3. A solution containing 50.0 cm3 of the potentially toxic metal was added to solution. The solutions were prepared in CaCl2.(2H2O) (0.01 mol.L-1) as electrolyte support and employing the basic chromium sulphate as a source of metal. Adjustments were made to the polynomial regression between the concentrations of potentially toxic levels of metal contaminants in the solution depending on the concentration of metal in the filtered solution after equilibrium. The Quartzipsamment showed lower retention compared to other classes of soils. Therefore it is more vulnerable to groundwater contamination if industrial wastes containing trivalent chromium are used as fertilizer.

Combustion characteristics of tannery sludge and volatilization of heavy metals in combustion

Incineration is considered one of the most readily available techniques for sewage sludge disposal, including tannery sludge, which often contains significant amounts of volatile heavy metals. The combustion characteristics and kinetic analysis of tannery sludge were investigated using thermogravimetric analysis (TGA) at a heating rate of 30 °C/min in 50–950 °C. In addition to confirming that tannery sludge has a high content of volatile material and ash, it was further discovered that almost all the zinc (Zn) in tannery sludge is volatilized at 900 °C. The degree of volatilization for heavy metals at 900 °C followed the order of Zn>Cd>Cu>Mn>Pb>Cr. Moreover, the volatilization of these heavy metals increased with temperature. It is thus concluded that, to avoid heavy metal volatization during incineration disposal, 800 °C is a reasonable incineration temperature.

Heterotrophic microorganism Rhodotorula mucilaginosa R30 improves tannery sludge bioleaching through elevating dissolved CO 2 and extracellular polymeric substances levels in bioleach solution as well as scavenging toxic DOM to Acidithiobacillus species

The introduction of acid-tolerant heterotrophic microorganisms into sludge bioleaching systems has been proven effective in improving sludge bioleaching processes, and such positive effect is mainly attributed to the biodegradation of low molecular weight organic acids or sludge dissolved organic matter (DOM) toxic to Acidithiobacillus species by the heterotrophic microorganisms introduced. Here we report that elevated dissolved CO 2 concentration and resulting extracellular polymeric substances (EPS) in bioleach solution due to the incorporation of heterotrophic microorganisms also play important roles in improving sludge bioleaching. It was found that in tannery sludge bioleaching system coinoculated with Rhodotorula mucilaginosa R30 and Acidithiobacillus species, dissolved CO 2 concentration in bioleach solution can be elevated from 0.23–0.54 mg/L to 0.76–1.01 mg/L compared to the control inoculated only with Acidithiobacillus species. Correspondingly, the distinct degradation of sludge DOM was also observed in this experiment. It was experimentally demonstrated that the accumulation of CO 2 did greatly enhance the growth of Acidithiobacillus thiooxidans and the decrease rate of pH in the medium. In addition, EPS derived from R. mucilaginosa R30 could bind readily Fe 3+ in bioleach solution with maximum binding capacity (MBC) of 0.82 mg Fe 3+ by per mg DOC of EPS secreted and the oxidization activity of EPS-bound Fe 3+ was decreased but not totally inhibited, indicating that the formation of soluble EPS-Fe(III) complexes enhances, to a certain extent, bioleaching efficiency due to maintaining Fe 3+ level in solution by inhibiting Fe precipitation occurrence.

Growth, metal accumulation and yield performance of Brassica campestris L. (cv. Pusa Jaikisan) grown on soil amended with tannery sludge/fly ash mixture

Application of sewage sludge and fly ash (FA) to soil is gaining practice and has become an alternative to chemical fertilizers in many countries. The present study aimed to assess the potential of fly ash–tannery sludge (TS) mixture in two ratios {4 (TS):1 (FA), denoted as A and 4 (TS):2 (FA) denoted as B} and different amendments were made with soil to study the yield and metal availability and their uptake by Brassica campestris L. (cv. Pusa Jaikisan). Experimental data showed that increase in dry matter yield and oil content of B. campestris was noted in lower amendments of soil with mixture A. However, total chlorophyll content of the leaf increased significantly, whereas carotenoid content showed non-significant increase in all the amendments with mixtures A and B as compared to control. The translocation of most of the tested metals (Pb, Mn, Cd, Ni, and Fe) in the shoot of the plant was found higher except Cr, Cu, and Co. Overall, experimental results showed that mixture A (≥10%) was found to be suitable due to better yield and less accumulation of metal in the seed of B. campestris.

Effect of different tannery sludge compost amendment rates on growth, biomass accumulation and yield responses of Capsicum plants

Composting has been recognized as one of the most cost effective and environmentally sound alternatives for organic wastes recycling from long and composted wastes have a potential to substitute inorganic fertilizers. We investigated the potential of composted tannery sludge for ornamental purposes and to examine the effects of two different composts and concentrations on ornamental Capsicum growth. The two composts were produced with tannery sludge and the composition of each compost was: compost 1 of tannery sludge (C 1TS) – tannery sludge + sugarcane straw and cattle manure mixed in the ratio 1:3:1 (v:v:v); compost 2 of tannery sludge (C 2TS) – tannery sludge + “carnauba” straw and cattle manure in the ratio 1:3:1 (v:v:v). Each compost was amended with soil at rates (% v:v) of 0%, 25%, 50%, 75% and 100% (designation hereafter as T 1– T 5, respectively). The number of leaves and fruits were counted, and the stem length was also measured. Chlorophyll content was recorded on three leaves of each harvested plant prior to harvest. Number of leaves and fruits, stem length, dry weight of shoot and roots did not vary significantly between the plants grown in two tannery composts. All the treatments with composted tannery sludge application ( T 2– T 5) significantly increased the number of leaves and fruits, stem length and chlorophyll content compared with the control ( T 1). The chlorophyll content was higher in plants growing in the C 1TS compared to C 2TS. The results of the present study further suggest that Capsicum may be a good option to be grown on composted tannery amended soil.

Ammonia volatilization in soil treated with tannery sludge

The utilization of tannery sludge in agricultural areas can be an alternative for its disposal and recycling. Despite this procedure may cause the loss of nitrogen by ammonia volatilization, there is no information about this process in tropical soils. For two years a field experiment was carried out in Rolândia (Paraná State, Brazil), to evaluate the amount of NH3 volatilization due to tannery sludge application on agricultural soil. The doses of total N applied varied from zero to 1200kgha−1, maintained at the surface for 89days, as usual in this region. The alkalinity of the tannery sludge used was equivalent to between 262 and 361g CaCO3 per kg. Michaelis–Menten equation was adequate to estimate NH3–N volatilization kinetics. The relation between total nitrogen applied as tannery sludge and the potentially volatilized NH3–N, calculated by the chemical-kinetics equation resulted in an average determination coefficient of 0.87 (P>0.01). In this period, the amount of volatilized NH3 was more intense during the first 30days; the time to reach half of the maximum NH3 volatilization (Km) was 13 an 9days for the first and second experiments, respectively. The total loss as ammonia in the whole period corresponded in average to 17.5% of the total N applied and to 35% of the NH4+–N present in the sludge. If tannery sludge is to be surface applied to supply N for crops, the amounts lost as NH3 must be taken into consideration.

Effect of composting on sludge contaminated with heavy metals from three different tannery streams and their bioavailability to plants

This research was conducted to study the influence of composting on tannery sludge contaminated by Cr, Cd, Zn, Cu, and Ni. Composting of sludge streams was performed in three different piles for 60 days. The results showed that the total concentration of heavy metals was increased with composting, except in the TT-3 sample where the concentrations of Cd, Cu, and Ni did not increase. The composting led to a loss of weight, Composting rendered part of the metal insoluble. Ni was the most easily extracted by H2O or CaCI2 and was therefore the most weakly bound to the organic matrix; in contrast, Zn remained strongly bound.

Metal Accumulation and Growth Response in Vigna radiata L. Inoculated with Chromate Tolerant Rhizobacteria and Grown on Tannery Sludge Amended Soil

The effects of inoculation of four chromate tolerant rhizobacterial strains previously isolated from rhizosphere of plants from chromium contaminated area in mung plant Vigna radiata grown on tannery sludge amended soil were evaluated. An increase of 138%, 88%, 256% and 54.14% in root length, shoot length, biomass and total chlorophyll, respectively was observed after 60 days of treatments by consortium. Similarly, a significant enhancement in Fe, Mn, Zn, Ni, Pb, Cr, Cu and Cd accumulation was observed in consortium inoculated plants as compared to non-inoculated plants. Results showed that rhizobacterial strain helps in ameliorating metal induced phytotoxicity, acquiring higher biomass and metal uptake in the plant may be useful in decontamination of metal from polluted soil.

Distribution and mobility of Cr in tannery waste amended semi-arid soils under simulated rainfall

This study was conducted to evaluate the distribution and mobility of Cr in tannery waste that has been added to semi-arid soils. In addition, the amount of total oxidizable soil Cr (III), Cr (VI), pH and soil microbial activities were determined. Tannery sludge alone or mixed with fleshing waste was added to two types of soils, which were then incubated at 25 °C for 6 months and subsequently subjected to simulated rainfall. The highest total amount of Cr loss occurred due to infiltration, regardless of the treatments. The Cr loss ranged from 0.452 to 0.825 μg g −1 soil for all soils from 1 to 3 months, with the exception of those that were located under the canopy and treated with tannery sludge and fleshing waste, which had the highest runoff (from 1.312 to 1.667 μg Cr g −1 soil). The pH of the soil increased from 1 to 3 months (from 7.35 to 8.46), while the total oxidizable soil Cr (III) (2.12–4.31 mg g −1 soil) peaked after 1 month of treatment. The majority of the fractionated chromium was bound to carbonates at initial time, 3 and 6 months prior to and after application of the simulated rainfall. The microbial activities decreased in each of the treated soils from initial time to 3 months; however, after 6 months bacterial activity increased, while pH decreased (from 8.03 to 7.63). Overall, these data suggest that pH is responsible for Cr loss in response to infiltration and runoff, Cr speciation and the equilibrium of Cr fractionation.

Biolixiviação de metais do lodo de curtume em duas etapas: influência do pH

The present work assessed the effect of the pH in the bioleaching of Cr and other metals from tannery sludge with stage separation of biologic oxidation of elemental sulfur and leaching of solid waste. The concentration of elemental sulfur was 5 g L-1. The final bioleaching pH ranged from 3.0 to 1.0. The concentration of sludge solids was 100 g L-1 and the temperatures were 298.15 and 308.15 K. The leaching extractions of Al, Cr and Zn increased at lower final pH and the leaching extraction of Ca decreased in this condition. The maximum extraction of Cr was 70.11% at sludge solids concentration of 100 g L-1, final pH 1.0 and temperature of 298.15 K. The fraction of Cr bound to Fe and Mn oxides was more affected by changes at final pH than others fractions. The mass ratio of metals and Cr decreased at lower final pH. The concentration of Cr leached from the tannery sludge after the bioleaching at final pH of 1.0 was reduced to satisfactory values.

Occurrence of biogenic schwertmannite in sludge bioleaching environments and its adverse effect on solubilization of sludge-borne metals

Abstract Secondary minerals such as Fe(OH)3 and jarosite are often hypothesized to be formed in sludge during bioleaching processes involving Acidithiobacillus ferrooxidans and be responsible for the lower solubilization efficiencies of sludge-borne metals. Schwertmannite is a ubiquitous mineral formed in acid mine drainage (AMD) impacted environments. However, the authors report for the first time the occurrence of schwertmannite as the sole secondary Fe mineral in a dissolved organic matter (DOM)-rich tannery sludge bioleaching system and its scavenging effect on soluble metals in bioleached sludge. The tannery sludge bioleach solution with pH ∼3 was mainly rich in Fe, Cr(III), SO 4 2 - and Acidithiobacillus ferrooxidans, but also contained high concentrations of DOM (300–400 mg C L−1). The ocherous precipitate that formed rapidly within 40 h in the sludge bioleach solution during incubation was identified to be pure schwertmannite by XRD and FTIR. Results of SEM coupled with EDS analyses showed that the schwertmannite was present as spherical particles with a diameter of about 1 μm and its chemical formula could be expressed as Fe8O8(OH)4.6(SO4)1.7. Newly formed schwertmannite contained about 2.43% Cr(III) derived from already solubilized sludge-borne Cr(III) by bioleaching, suggesting a strong adverse influence of schwertmannite formation on solubilization efficiency of metals from sludge. Biogenic Fe mineral synthesis experiments varying the concentrations of DOM and metals indicated that the occurrence of schwertmannite as a monominerallic phase in the sludge bioleaching environment was predominantly attributed to the presence of insufficient concentrations of monovalent cations and the high content of DOM. Moreover, the short reaction time, high SO 4 2 - concentration and low pH value in this system also facilitated the existence of monominerallic schwertmannite.