Lead removal from aqueous solutions by olive mill wastes derived biochar: Batch experiments and geochemical modelling

In this study, lead removal from aqueous solutions using biochar derived from olive mill solid and liquid wastes has been investigated by applying batch experiments and geochemical modelling. The batch adsorption experiments included the assessment of several key parameters such as the contact time (kinetic), initial concentration (isotherm), pH, adsorbent dose, and the presence of competitive cations, whilst the geochemical modelling focused on the involved adsorption mechanisms using the PHREEQC code. The kinetic studies showed that lead adsorption is a relatively fast process, where intraparticle diffusion is the rate-limiting step. Biochar dose, solution pH and the presence of competitive ions significantly affected the Pb adsorption effectiveness by the biochar. Especially the higher Pb removal percentages were observed in mono-elemental solutions with high biochar dose at mildly acidic solution pH values. The maximum Pb adsorption capacity of biochar was estimated as 40.8 mg g^-1 which is higher than various biochars derived from sludge, lignocellulosic and animal biomasses. On the other hand, the geochemical modelling employing the PHREEQC code showed that ion exchange and Pb precipitation are the main reactions controlling its removal from aqueous solutions, whilst surface complexation is insignificant, mainly due to the low surface functional groups on the used biochar.

Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO₄^-, CrO₄^2-, and Cr₂O₇^2- ions in aqueous solutions. At low pH (∼1-4), the availability of HCrO₄^- ions attributes the electrostatic forces of attraction due to the available functional groups such as -NH₄⁺, -COOH, and -OH₂⁺, which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr(VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr(VI) removal and other heavy metals are elegantly discussed.

Sorption of arsenic by composts and biochars derived from the organic fraction of municipal solid wastes: Kinetic, isotherm and oral bioaccessibility study

The historic contamination of water and soils by arsenic (As) is an extremely alarming environmental and public health issue worldwide. This study investigated the relationship between As sorption and physicochemical properties of composts and biochars derived from the organic fraction of municipal solid wastes (OFMSW) towards the development of promising sorbents with value-added solid wastes management solutions. The sorbents were characterized and their effectiveness on the As sorption was tested. Several isothermal and kinetic sorption models were used for the prediction of sorption. Composts did not show promising sorption capacities, and in some cases, the As immobilization was practically null. In contrast, biochars achieved higher sorption performance, and the experimental data fitted well on Dubinin-Rabushkevich and Langmuir models, with higher R² values. The maximum sorption capacities of BC700 estimated by such models were 6.495 and 170.252 mg g^-1, respectively, whereas those of BC500 estimated by D-R and Langmuir models were only 0.066 and 0.070 mg g^-1, respectively. In sorption kinetics, As was retained onto biochars at a faster first stage, reaching equilibrium after approximately 1 h and 2 h for initial concentrations of 10 and 100 mg L^-1. The pseudo-second-order, Ritchie's second-order, Ritchie's, and Elovich models more adequately described the sorption kinetics of As onto biochars with high R² values. Overall, the complexation and precipitation were predominant mechanisms for As sorption by OFMSW-derived biochars. Furthermore, the mathematical models indicated contributions arise from physisorption and external and internal diffusion mechanisms. Although BC700 can immobilize large As amounts, the gastric phase of the oral bioaccessibility test revealed more than 80% of the sorbed As could be released under conditions similar to a human stomach (pH~1.2). Such conclusions have given important insights about the refining of effective and eco-friendly remediation technologies for the management and rehabilitation of As-contaminated soil and water, particularly in developing countries.

Raw data of silver extraction from sodium-silver jarosite using three different lixiviants in alkaline medium

This article presents the raw data of silver concentration ([Ag]) obtained as a function of time (t) from silver leaching experiments, which were conducted using a synthetic sodium-silver jarosite and different complexing agents: thiosulfate, thiocyanate, and cyanide. Leaching experiments were performed under different conditions of temperature, pH and lixiviant concentration. The data refer to the article “Silver leaching from jarosite-type compounds using cyanide and non-cyanide lixiviants: a kinetic approach” (Islas et al., 2021), in which they were used to determine the leaching kinetics of jarosite-type compounds. The datasets were obtained experimentally from batch experiments. Concentration of silver, [Ag], was determined in each experiment as a function of time by atomic absorption spectroscopy. The information presented in this article can be useful for engineering students interested in mineral processing; particularly, for the calculation of kinetic parameters of silver leaching process. The data could also help in the formulation, implementation, or optimization of strategies for extraction of valuable metals from residues generated by the hydrometallurgical industry.

Sulfadiazine, sulfamethazine and sulfachloropyridazine removal using three different porous materials: Pine bark, "oak ash" and mussel shell

This work focuses on studying the efficacy of three different by-products to adsorb three antibiotics (sulfadiazine, SDZ; sulfamethazine, SMT; sulfachloropyridazine, SCP). These antibiotics can be considered pollutants of the environment when they reach water, as well as in cases where they are spread on soils through irrigation or contained in sewage sludge or livestock manure. In this study, batch-type adsorption/desorption experiments were performed for each of the three sulfonamides, adding 7 different concentrations of the antibiotics, going from 1 to 50 μmol L^-1, and with contact time of 24 h. The results indicate that pine bark is the most efficient bioadsorbent among those studied, as it adsorbs up to 95% of the antibiotics added, while desorption is always less than 11%. However, for "oak ash" and mussel shell the adsorption is always lower than 45 and 15%, respectively, and desorption is high, reaching up to 49% from "oak ash" and up to 81% from mussel shell. Adsorption data showed good fitting to the Linear and Freundlich models, with R² values between 0.98 and 1.00 in both cases. K_d and K_F adsorption parameters showed similar values for the same sorbent materials but were much higher for pine bark than for the other two bioadsorbents. The Freundlich's n parameter showed values in the range 0.81-1.28. The highest K_F values (and therefore the highest adsorption capacities) were obtained for the antibiotic SCP in pine bark. Pine bark showed the highest capacity to adsorb each of the antibiotics, increasing as a function of the concentration added. When the concentration of sulfonamide added was 50 μM, the amounts adsorbed were 780 μmol kg^-1 for SDZ, 890 μmol kg^-1 for SMT, and 870 μmol kg^-1 for SCP. "Oak ash" and mussel shell have low adsorption capacity for all three sulfonamides, showing values always lower than 150 μmol kg^-1 (oak ash) and 20 μmol kg^-1 (mussel shell) when a concentration of 50 μmol L^-1 of antibiotic is added. The results of this study could aid to make an appropriate management of the by-products studied, in order to facilitate their valorization and recycling in the treatment of environmental compartments polluted with sulfonamide antibiotics.

Interaction of metal oxide nanoparticles with microplastics: Impact of weathering under riverine conditions

The ubiquitous distribution of microplastics (MPs) leads to inevitable interactions with the toxic pollutants present in the environment including metal-oxide nanoparticles. This study investigates the interaction of CeO₂ nanoparticles (CeNPs) with MPs generated from a disposable plastic container. Further, rough MPs (R-MPs), generated through mechanical abrasion of MPs with sand, were used to probe the impact of roughness. To understand the sorption kinetics and underlying interaction processes, batch experiments were carried out. The results distinctly indicate that CeNPs sorption occurred on MPs surfaces and was consistent with the pseudo-second order kinetics model. For pristine MPs, the sorption capacity was as high as 12.9 mg/g while for R-MPs kinetic equilibrium was achieved faster and an enhanced sorption capacity (13.4 mg/g) was identified. A rise in sorption with an increase in salinity was noted while pH and humic acid exhibited a negative correlation. The observed interactions were attributed to the aggregation profile and surface charge of CeNPs and MPs. Surprisingly, CeNPs also got loaded onto MPs in non-agitated and undisturbed conditions. The sorption process was influenced by the type of aqueous matrix and the sorption capacity at equilibrium followed the trend: distilled water> synthetic freshwater> river water. FTIR spectra, zeta potential, SEM imaging, and elemental mapping revealed electrostatic interaction as the dominant mechanism. This work contributes towards the knowledge gap on the environmental risk of MPs.

Bioreduction of hexavalent chromium on goethite in the presence of Pseudomonas aeruginosa

The effective mineral absorption and bioreduction were considered as two preferred processes to alleviate the bioavailability and toxicity of toxic trace metals. In this study, the bioreduction of hexavalent chromium (Cr(VI)) on goethite (FeOOH) in the presence of Pseudomonas aeruginosa (P. aeruginosa) was investigated with different environmental factors, including carbon source concentrations, pH, temperature and initial Cr(VI) concentrations. The characterization of FeOOH-P. aeruginosa indicated that P. aeruginosa was surrounded by FeOOH, which could provide the essential iron for bacterial growth and reduce Cr(VI) to Cr(III). The optimal experimental conditions for Cr(VI) (initial concentration: 35 mg L^-1) absorption (∼46%) and bioreduction (∼54%) involved a temperature of 45 °C and pH of 5.5. Meanwhile, extracellular polymeric substances (EPS) secreted by P. aeruginosa and its functional groups played important roles in the reduction of Cr(VI). They could reduce Cr(VI) to Cr(III) and transform to Cr(OH)₃ or Fe_x-Cr_(1-x)(OH)₃ precipitation. These results of this study are of significant importance to better understand the environmental geochemical behavior of Cr(VI) with the interactions between soil minerals and microorganisms.

Experimental and model study for fluoride removal by thermally activated sepiolite

The present investigation demonstrates the preparation of thermally activated sepiolite for effective removal of fluoride via adsorption from an aqueous solution. The thermal treatments on sepiolite were conducted at different temperatures (300-950 °C) for 4 h in an N₂ atmosphere, and the thermally activated sepiolite was characterized using a field emission scanning electron microscope (FESEM), X-ray diffractometry (XRD), X-ray fluorescence (XRF), a differential scanning calorimetry-thermogravimetric analyzer (DSC-TGA), and a surface area analyzer. Sepiolite that was treated at 950 °C was shown to have a higher fluoride removal efficiency than other temperatures. The fluoride removal was evaluated under different experimental conditions such as solution pH, adsorbent dose, reaction time, initial concentration, temperature, presence of co-existing ions, and reuses. The kinetic and equilibrium adsorption results were well described by the pseudo-second-order kinetic model and Langmuir isotherm, respectively, and adsorption of fluoride onto thermally activated sepiolite was endothermic and spontaneous in nature. The Langmuir maximum adsorption capacity (169.95 mg/g) was superior to the literature value. The thermally activated sepiolite was also effective in a continuous flow system for treating fluoride. Thus, this thermally activated sepiolite is expected to be used as an effective adsorbent for the removal of fluoride in water.

Thermodynamic and kinetic insights into plant-mediated detoxification of lead, cadmium, and chromium from aqueous solutions by chemically modified Salvia moorcroftiana leaves

Thermodynamic and kinetic aspects for the biosorptive removal of Pb, Cd, and Cr metals from water using Chemically Modified Leaves of Salvia moorcroftiana (CMSML) were determined. Different parameters including pH, temperature, metal's initial concentration, biomass dosage, and contact time were optimized. Optimum biosorptions of Pb, Cd, and Cr were attained at pH values of 6.0, 7.0, and 3.0 respectively. Batch experiments showed maximum removal of both Pb and Cd at 40 °C and that of Cr at 30 °C. Biosorption capability of CMSML was observed to decrease with raising temperature. Optimal equilibrium times for Pb, Cd, and Cr uptake were 120, 60, and 120 min respectively. Based on the values of regression correlation coefficients (R²), the current data is explained better by applying Langmuir isotherms than the Freundlich model. Maximum biosorbent capabilities (q_max) for Pb, Cd, and Cr were approximately 270.27, 100.00, and 93.45 mg/g respectively. Thermodynamically, removal of all the three metal ions was shown to be exothermic and spontaneous.

Assessment of impacts of diphenyl phosphate on groundwater and near-surface environments: Sorption and toxicity

Wastewater recovered from hydraulic fracturing is referred to as flowback and produced water (FPW), and is often saline, contains numerous organic and inorganic constituents, and may pose threats to groundwater resources. Hundreds of spills of FPW have been reported to the Alberta Energy Regulator each year. Recently, samples of FPW derived from hydraulic fracturing of the Duvernay Formation, AB, were found to contain a previously unidentified class of aryl phosphates, including diphenyl phosphate (DPP), triphenyl phosphate (TPP), and others. Aryl phosphates are also used in a variety of other industries and their constituents can be found in flame retardants, plasticizers, lubricants, hydraulic fluids, and oxidizers. Many of these aryl phosphates break down into DPP. Therefore, it is important to determine the environmental fate and potential impact of DPP if spilled in the near-surface, as DPP is an emerging contaminant in soil and groundwater systems. This study was aimed at determining 1) the sorption behavior of DPP onto various surficial sediments collected within the Fox Creek, AB region, and 2) the toxicity of DPP toward aquatic ecosystems. We report that the sorption of DPP onto both clay-rich soils and sandy sediment was low compared to that of other aryl phosphates, with an average log K_OC value of 2.30 ± 0.42 (1σ). Therefore, the transport of DPP in groundwater would be rapid due to its low degree of sorption on surficial materials. We also determined the acute 96 h-LC₅₀ of DPP on zebrafish embryos to be 50.0 ± 7.1 mg/L. Su et al. (2014) studied the toxic effects of DPP and TPP on chicken embryonic hepatocytes and found that DPP had less cytotoxic effects than TPP but altered more gene transcripts. From the results our study, we infer that DPP may pose an environmental risk to aquatic ecosystems if released into the environment.

Activity concentrations of 238U and 226Ra in two European black shales and their experimentally-derived leachates

The production of gas from unconventional resources became an important position in the world energy economics. In 2012, the European Commission's Joint Research Centre estimate 16 trillion cubic meters (Tcm) of technically recoverable shale gas in Europe. Taking into account that the exploitation of unconventional gas can be accompanied by serious health risks due to the release of toxic chemical components and natural occurring radionuclides into the return flow water and their near-surface accumulation in secondary precipitates, we investigated the release of U, Th and Ra from black shales by interaction with drilling fluids containing additives that are commonly employed for shale gas exploitation. We performed leaching tests at elevated temperatures and pressures with an Alum black shale from Bornholm, Denmark and a Posidonia black shale from Lower Saxony, Germany. The Alum shale is a carbonate free black shale with pyrite and barite, containing 74.4 μg/g U. The Posidonia shales is a calcareous shale with pyrite but without detectable amounts of barite containing 3.6 μg/g U. Pyrite oxidized during the tests forming sulfuric acid which lowered the pH on values between 2 and 3 of the extraction fluid from the Alum shale favoring a release of U from the Alum shale to the fluid during the short-term and in the beginning of the long-term experiments. The activity concentration of ²³⁸U is as high as 23.9 mBq/ml in the fluid for those experiments. The release of U and Th into the fluid is almost independent of pressure. The amount of uranium in the European shales is similar to that of the Marcellus Shale in the United States but the daughter product of ²³⁸U, the ²²⁶Ra activity concentrations in the experimentally derived leachates from the European shales are quite low in comparison to that found in industrially derived flowback fluids from the Marcellus shale. This difference could mainly be due to missing Cl in the reaction fluid used in our experiments and a lower fluid to solid ratio in the industrial plays than in the experiments due to subsequent fracking and minute cracks from which Ra can easily be released.

Inhibitory effects of acidic pH and confounding effects of moisture content on methane biofiltration

This study focussed on evaluating the effect of hydrogen sulfide (H₂S) on biological oxidation of waste methane (CH₄) gas in compost biofilters, Batch experiments were conducted to determine the dependency of maximum methane oxidation rate (V_max) on two main factors; pH and moisture content, as well as their interaction effects. The maximum V_max was observed at a pH of 7.2 with decreasing V_max values observed with decreasing pH, irrespective of moisture content. Flow-through columns operated at a pH of 4.5 oxidized CH₄ at a flux rate of 53g/m²/d compared to 146g/m²/d in columns operated at neutral pH. No oxidation activity was observed for columns operated at pH 2.5, and DNA sequencing analysis of samples led to the conclusion that highly acidic conditions were responsible for inhibiting the ability of methanotrophs to oxidize CH₄. Biofilter columns operated at pH 2.5 contained only 2% methanotrophs (type I) out of the total microbial population, compared to 55% in columns operated at pH 7.5. Overall, changes in the population of methanotrophs with acidification within the biofilters compromised its capacity to oxidize CH₄ which demonstrated that a compost biofilter could not operate efficiently in the presence of high levels of H₂S.

Valorization of biosorbent obtained from a forestry waste: Competitive adsorption, desorption and transport of Cd, Cu, Ni, Pb and Zn

Bark from Pinus pinaster is one of the most abundant forestry wastes in Europe, and among the proposed technologies for its reutilization, the removal of heavy metals from wastewater has been gaining increasing attention. In this work, we have studied the performance of pine bark for heavy metal biosorption on competitive systems. Pb, Cu, Ni, Zn and Cd sorption and desorption at equilibrium were studied in batch experiments, whereas transport was studied in column experiments. Batch experiments were performed adding simultaneously different concentrations (0.08-3.15mM) of two or more metals in solution to pine bark samples. Column experiments were performed with 10mM solutions of two metals or a 5mM solution of the five metals. In general, the results under competitive conditions were different to those obtained in monoelemental experiments. The multi-metal batch experiments showed the adsorption sequence Pb≈Cu>Cd>Zn>Ni for lower metal doses, Pb>Cu>Cd>Zn>Ni for intermediate doses, and Pb>Cu>Cd≈Zn≈Ni for high metal doses. Desorption followed the sequence Pb<Cd<Cu<Zn<Ni for the lowest metal doses, and Pb<Cu<Zn<Cd<Ni for the highest ones. The bi-metal batch experiments indicated that Cu and Pb suffered the highest retention, with high capacity to displace Cd, Ni and Zn from adsorption sites on pine bark. The transport experiments produced comparable results to those obtained in the batch experiments, with pine bark retention capacity following the sequence Pb>Cu>Zn>Cd>Ni. The presence of a second metal affected the transport of all the elements studied except Pb, and confirmed the strong influence of Pb and Cu on the retention of the other metals. These results can help to appropriately design decontamination systems using this forestry waste.

Assessment of the zinc diffusion rate in estuarine zones

Industrial pressures suffered by estuarine zones leave a pollution record in their sediment. Thus, high concentrations of many heavy metals and some organic compounds are often found in estuarine sediment. This work aims to contribute to the enhancement of water quality management strategies in these zones by studying in detail the diffusive processes that take place between the water and sediment using a two-pronged approach: experimental practice and numerical simulation. To provide an example of the practical application of the methodologies proposed in this paper, the Suances Estuary (northern Spain) was selected as the study zone. This estuary exhibits significant historical pollution and its sediment acts as a continuous internal source of zinc, mainly due to diffusive processes derived from the concentration gradient between the interstitial water at the solid particles of the sediment and the bottom of the water column. The experimentally obtained results, based on 6 case studies, demonstrated the buffering capacity of the system and allowed the determination of the required time for the mass transfer processes to reach an equilibrium state. Furthermore, the diffusion rate of zinc was approximately modeled taking into consideration the high concentration variability observed in sediment along the entire estuary. The convergence between the modeled and the experimental results indicated the required contact time to reach an equilibrium state in a real field situation.

Low pH anaerobic digestion of waste activated sludge for enhanced phosphorous release

This paper assesses anaerobic digestion of waste activated sludge (WAS) at low pH to enhance phosphorous solubility. Batch biochemical methane potential tests were conducted at a pH range of 5 to 7.2 in two separate sets (two different WAS samples collected from municipal WWTP). Low pH (<5.7) caused a significant (p = 0.004) decrease in methane potential (B0) up to 33% and 3.6 times increase in phosphorus release compared to neutral pH (7-7.7), but with no major change in methane production rate coefficient (khyd). The loss in methane yield was mainly due to decrease in hydrolytic capability rather than inhibition of methanogenesis with volatile fatty acids being <300 mgCOD L(-1) and soluble COD <1300 mgCOD L(-1) even at low pH. While pH did not influence the acetoclastic community (Methanosaeta dominated), it was the primary driver for the remaining community (p = 0.004), and caused a loss of diversity and shift to Clostridia.

Lead and copper removal from aqueous solutions using carbon foam derived from phenol resin

Phenolic resin-based carbon foam was prepared as an adsorbent for removing heavy metals from aqueous solutions. The surface of the produced carbon foam had a well-developed open cell structure and the specific surface area according to the BET model was 458.59m(2)g(-1). Batch experiments showed that removal ratio increased in the order of copper (19.83%), zinc (34.35%), cadmium (59.82%), and lead (73.99%) in mixed solutions with the same initial concentration (50mgL(-1)). The results indicated that the Sips isotherm model was the most suitable for describing the experimental data of lead and copper. The maximum adsorption capacity of lead and copper determined to Sips model were 491mgg(-1) and 247mgg(-1). The obtained pore diffusion coefficients for lead and copper were found to be 1.02×10(-6) and 2.42×10(-7)m(2)s(-1), respectively. Post-sorption characteristics indicated that surface precipitation was the primary mechanism of lead and copper removal by the carbon foam, while the functional groups on the surface of the foam did not affect metal adsorption.

Sorption of norfloxacin in soils: analytical method, kinetics and Freundlich isotherms

Fluoroquinolones are potent antibacterial agents that are active against a wide range of pathogenic organisms and are widely used in veterinary medicine. Fluoroquinolones and their metabolites may reach the soil through animal excreta or manure and may contaminate water and soils. The degree of sorption of these antimicrobials to soils varies widely, as does the mobility of these drugs. In the present study, sorption of norfloxacin in four soils of the state of São Paulo was investigated with batch equilibrium experiments. A strong matrix effect on the chromatographic determination of norfloxacin was verified. Sorption kinetics were best fit by a pseudo second-order model (r>0.99), and sorption/desorption isotherms were well fit by the Freundlich model in log form (r>0.97). Norfloxacin showed high affinity for soil particles, with KF sorption values ranging from 643 to 2410 μg(1-1/n)(cm(3))(1/n)g(-1) and KF desorption values ranging from 686 to 2468 μg(1-1/n)(cm(3))(1/n)g(-1). The high desorption KF values indicate that norfloxacin is highly immobile in the evaluated soils.

Isotopic investigation of the colloidal mobility of depleted uranium in a podzolic soil

The mobility and colloidal migration of uranium were investigated in a soil where limited amounts of anthropogenic uranium (depleted in the 235U isotope) were deposited, adding to the naturally occurring uranium. The colloidal fraction was assumed to correspond to the operational fraction between 10 kDa and 1.2 μm after (ultra)filtration. Experimental leaching tests indicate that approximately 8-15% of uranium is desorbed from the soil. Significant enrichment of the leachate in the depleted uranium (DU) content indicates that uranium from recent anthropogenic DU deposit is weakly bound to soil aggregates and more mobile than geologically occurring natural uranium (NU). Moreover, 80% of uranium in leachates was located in the colloidal fractions. Nevertheless, the percentage of DU in the colloidal and dissolved fractions suggests that NU is mainly associated with the non-mobile coarser fractions of the soil. A field investigation revealed that the calculated percentages of DU in soil and groundwater samples result in the enhanced mobility of uranium downstream from the deposit area. Colloidal uranium represents between 10% and 32% of uranium in surface water and between 68% and 90% of uranium in groundwater where physicochemical parameters are similar to those of the leachates. Finally, as observed in batch leaching tests, the colloidal fractions of groundwater contain slightly less DU than the dissolved fraction, indicating that DU is primarily associated with macromolecules in dissolved fraction.