Heavy Metal-contaminated Water Research Articles

Harnessing C@Zn@TiO2 nanocomposites for cadmium adsorption in aquatic media

ABSTRACT In this work, the elimination of Cd+2 ions in aqueous solution using carbon/Zn/TiO2 nanoparticles, synthesised by sol – gel based Pechini method at intermediate temperature, was studied. The synthesised C@Zn@TiO2 was characterised by SEM, TEM, EDX, XRD, BET, and XPS techniques to verify the formation of semi-spherical nanoparticles with dominant anatase TiO2 phase and elemental composition of the expected elements. Concurrently, the impact of the solution pH, pHpzc, contact time, adsorbent dose, and initial ions concentration were discussed. The findings revealed that at pH 7.0, a maximum of 174 mg. g−1 of Cd+2 ions were adsorbed at 120 minutes of equilibrium time. The Langmuir and pseudo-second-order models appropriately fitted the isotherm and kinetics data. The intraparticle model analysis indicated that film diffusion controls the adsorption of metal ions adsorption at the initial stage and, the intraparticle diffusion at the second stage. The FTIR and XPS analysis further proved Cd+2 ions being adsorbed onto the surface via an electrostatic mechanism. The fabricated nanostructure could be recycled and reused four times with a high competence above 71.5%. The facile synthesis, low cost, harmlessness, and remarkable removal efficiency make the C@Zn@TiO2 a promising material for treating heavy metal-contaminated water.

Enhancement of Zn adsorption on coal fly ash-based geopolymer with steel slag incorporation: Leaching behavior and performance insights.

Industrial solid wastes like coal fly ash (CFA) and steel slag pose environmental challenges, while the remediation of heavy metal-contaminated water remains a global priority. This study investigates the impact of incorporating steel slag during the synthesis of CFA-based geopolymers (CFAG) on the leaching characteristics of inherent heavy metals in CFA and the Zn adsorption performance of CFAG. Leaching experiments show geopolymerization effectively immobilizes heavy metals including Fe, Cr, As, Cd, and Ti in CFA while having little effect on Mn, V, and Ni. The incorporation of 10wt% steel slag into CFAG further enhances the stability of As but slightly elevates the release of Fe, Ti, Cu, V, and Cd. Column experiments demonstrate steel slag doped-CFAG (SCG) achieves over a 50% improvement in Zn adsorption performance compared to CFAG. Furthermore, hydrochemical analysis, X-ray absorption spectroscopy, and sequential desorption experiments reveal Zn is removed from solution by SCG primarily through surface complexation and precipitation as ZnSiO4, which have suitable stability and resistance to desorption. These findings suggest doping steel slag into CFAG is a viable strategy to enhance Zn removal in contaminated water, decrease remediation costs, and effectively consume CFA and steel slag. Nevertheless, potential trade-offs concerning heavy metal leaching should be carefully evaluated to balance the performance and environmental impacts of SCG.

Exploration of Cyanobacteria in Heavy Metal-Contaminated Water Bodies with Special Reference to Oil Field

Exploration of Cyanobacteria in Heavy Metal-Contaminated Water Bodies with Special Reference to Oil Field

Evaluating Heavy Metal Contamination in Water and Tissues of Tor putitora: Implications for Human Health Risk Assessment

Evaluating Heavy Metal Contamination in Water and Tissues of Tor putitora: Implications for Human Health Risk Assessment

Sustainable Biopolymer-Based Electrochemical Sensors for Trace Heavy Metal Determination in Water: A Comprehensive Review

The growing concern over heavy metal contamination in environmental and industrial settings has intensified the need for sensitive, selective, and cost-effective detection technologies. Electrochemical sensors, due to their high sensitivity, rapid response, and portability, have emerged as promising tools for detecting heavy metals. Recent years have seen significant progress in utilizing biopolymer-based materials to enhance the performance of these sensors. Biopolymers, derived from renewable raw materials, have garnered considerable interest in both science and industry. These biopolymer-based composites are increasingly recognized as superior alternatives to conventional non-biodegradable materials because of their ability to degrade through environmental exposure. This review provides a comprehensive overview of recent advancements in biopolymer-based electrochemical sensors for heavy metal detection. It discusses various types of biopolymers and bio-sourced polymers, their extraction methods, and chemical properties. Additionally, it highlights the state of the art in applying biopolymers to electrochemical sensor development for heavy metal detection, synthesizing recent advances and offering insights into design principles, fabrication strategies, and analytical performance. This review underscores the potential of biopolymer-based sensors as cost-effective, eco-friendly, and efficient tools for addressing the pressing issue of heavy metal contamination in water and discusses their advantages and limitations. It also outlines future research directions to further enhance the performance and applicability of these sensors.

Characterisation and antibiotic susceptibility pattern of bacterial isolates obtained from some shellfish sold in Lagos-West, Nigeria

Shellfish are rich in essential nutrients and are widely accepted globally. The bacterial contamination, antibiotic susceptibility and heavy metal concentrations on some shellfish muscles above the tolerance permissible limit by WHO/FAO require an in-depth study. An investigative study was carried out on tiger shrimp (Penaeus monodon), pink shrimp (Penaeus notialis) and Lagoon crabs (Callinectes amnicola) obtained from Makoko fish market/landing site on their bacterial quality, antibiotic susceptibility patterns and heavy metal accumulation of bacteria isolated were carried out using standard methods. The highest Total bacterial count (2.48±0.02107cfu/g) was observed in Lagoon crab (C. amnicola) while the lowest count (1.20±0.02107cfu/g) was observed in tiger shrimp (P. monodon). However, the highest Total Faecal count (1.42±0.02104cfu/g) was observed in pink shrimp (P. notialis) while the lowest count was observ ed in P. monodon (1.13±0.03104cfu/g). The bacterial isolates were molecularly identified as (Morganella morganii and Proteus vulgaris) isolated from tiger shrimp, Lagoon crabs had (Proteus mirabilis) while pink shrimp showed (Alcaligenes faecalis). The isolates were 100% susceptible to ciprofloxacin, azithromycin and erythromycin and resistant to cefotaxime, cefuroxime, imipenem/clastatin, augmentin and nitrofurantoin. The mean heavy metals concentration was as follows zincironcoppernickelchromiummanganese Cadmium while Lead and cobalt were not detected in the samples. The study has shown a possible unhygienic environment indicative of the bacteria isolated, a possible environmental spread of antibiotic-resistant bacteria and a heavy metal-contaminated water body. There is a serious need for constant monitoring to lessen future health problems for humans in and around the environment.Keywords: antibiotic-resistant; contamination; Makoko

(Digital Presentation) Advanced Nanomagnetic Adsorbents for Heavy Metal Removal

Heavy metal contamination in water is a significant and persistent environmental challenge, as demonstrated by the 2015 lead contamination crisis in Flint, Michigan. Such contamination primarily arises from industrial activities and improper waste management practices, with key contributors being industries like metalworking, electronics manufacturing, and mining. Heavy metals in water are notorious for their toxic, mutagenic, and carcinogenic effects, posing serious health risks to humans, including potential birth defects in unborn children. The impact of these pollutants is far-reaching, affecting not only human health but also ecosystems and biodiversity.One of the most common methods for remediating heavy metal contamination is adsorption. However, conventional adsorbents often suffer from drawbacks, such as difficulties in separating them from the contaminated water and issues with charge affinity. These limitations underscore the need for the development of novel adsorbents that can efficiently address these challenges. Among the promising candidates are nanomagnetic adsorbents, which offer distinct advantages due to their colloidal stability and minimal internal diffusion resistance.This study focuses on the use of nanoporous magnetic organic-silicate CTAB (NMSC) as a potential solution for the removal of chromium (Cr(VI)) from aqueous media. The adsorption behavior of NMSC was analyzed through isotherm studies, which revealed that the Freundlich model best described the adsorption process, with a high correlation coefficient (R² = 0.988). The model's n value of 0.898 and Kf value of 0.319 mg/g indicate a high affinity and heterogeneity of the adsorbent. Additionally, kinetic studies showed that the adsorption process followed a second-order kinetics model (R² = 0.984), further demonstrating the effectiveness of NMSC. These findings suggest that NMSC holds great potential as an efficient adsorbent for heavy metal removal, offering a viable solution to mitigate the harmful effects of heavy metal contamination in water systems. References A Elmekawy, Q Quach, TM Abdel-Fattah, Nanomaterials 14 (13), 1143 (2024)SELME Mahmoud, TM Abdel-Fattah, ME Mahmoud, E Díaz, Environmental Nanotechnology, Monitoring & Management 22, 100977 (2024)SELME Mahmoud, D Ursueguia, ME Mahmoud, TM Abdel-Fattah, E Díaz, Biomass Conversion and Biorefinery, 1-1 (2023)A Elmekawy, Q Quach, TM Abdel-Fattah, Scientific Reports 13 (1), 12845 (2023)Omar H. Elsayed-Ali, Hani E. Elsayed-Ali and Tarek M. Abdel-Fattah, Journal of Hazardous Materials, 185 (2-3), 1550-1557 (2011)Alya Elsayed-Ali, Tarek Abdel-Fattah, Hani Elsayled-Ali, Hani, Journal of Chemical Education, 88(8), 1126-1129 (2011).TM Abdel-Fattah, ME Mahmoud, Chemical engineering journal 172 (1), 177-183 (2011)TM Abdel-Fattah, ME Mahmoud, MM Osmam, SB Ahmed, Journal of Environmental Science and health, part A 49 (9), 1064-1076 (2014)ME Mahmoud, TM Abdel-Fattah, MM Osman, SB Ahmed, Journal of Environmental Science and Health, Part A 47 (1), 130-141 (2012)TM Abdel-Fattah, B Bishop, Journal of Environmental Science and Health, Part A 39 (11-12), 2855-2866 (2004)ME Mahmoud, MM Osman, SB Ahmed, TM Abdel-Fattah, The Scientific World Journal 2012 (2012)ME Mahmoud, SS Haggag, TM Abdel-Fattah, Polyhedron 26 (14), 3956-3962 (2007)ME Mahmoud, AA Yakout, MT Abed El Aziz, MM Osman, TM Abdel-Fattah, Journal of Environmental Science and Health, Part A 50 (10), 1072-1081 (2015)H Namkoong, E Biehler, G Namkoong, TM Abdel-Fattah, ACS omega 7 (44), 39931-39937 (2022)

Innovations in Phytoremediation: Application of Artificial Wetlands for the Contaminated Water Treatment and Heavy Metals Removal

Objective: The objective of this study was a review to compile research on phytoremediation methods—including phytoextraction, rhizofiltration, phytostabilisation, phytodegradation, and phytovolatilisation—using primary sources to address heavy metal contamination in water. Theoretical Framework: The study is a basic investigation that includes a contextualization regarding the water issue, as well as the treatment of wastewater. A description of the characteristics and quality indicators of water was carried out, along with the importance of the removal of contaminants such as heavy metals using different technologies like phytoextraction, rhizofiltration, phytostabilization, phytodegradation, and phytovolatilization. This led to a detailed description of artificial wetland techniques as a viable and sustainable alternative. Method: The methodology adopted for this research includes data collection and compilation of existing information; regarding the different techniques related to wastewater treatment with artificial wetlands, these systems utilize natural processes involving vegetation, soil, and microbial activity to remove contaminants from water. More than 100 articles have been legally downloaded, and systematised in the database to paraphrase sentences and cite appropriately. Results and Discussion: Phytoremediation is an environmentally friendly technical choice because of its low cost to remediate water and soil contamination through methods of phytoextraction, rhizofiltration, phytostabilisation, phytodegradation and phytovolatilisation for organic contaminants, pesticides, oil waste, heavy metals, antibiotics, etc. It is relevant to choose transgenic plant methods to insert DNA genes and conduct molecular mechanisms through experimental optimisation studies for lightened growth and hyperaccumulation of heavy metals, generating space for metal and biomass storage, and prior knowledge of the physiology and biochemical processes of the plants. Research Implications: The results can be applied or influence practices in environmental protection using Phytoremediation, which is based on a natural technique of plants and their microbiota associated with roots and tissues, endophytic bacteria as Aeromonas sp, Pseudomonas sp, Bacillus sp, Sinorhizobium meliloti, Chryseomonas sp, Curtobacterium sp, Sphingomonas sp, Erwinia sp, Flavimonas sp retain contaminants. On the other hand, aquatic macrophytes such as Lemna spp, Typha spp, Potamogeton spp, Azolla filiculoides, Ceratophyllum demersm, Eichhornia crassipes, and Phragmites australis constitute an effective and feasible complementary alternative for the treatment of contaminated waters. Originality/Value: This study contributes to the literature There are different phytoremediation methods such as phytoextraction, rhizofiltration, phytostabilisation, phytodegradation, and hytovolatilisation for organic contaminants, pesticides, petroleum waste, heavy metals, pharmaceutical antibiotics, etc. Each of the elements has specific applications depending on the conditions of the place and the type of contaminants in an ecosystem.

Terahertz cancer cell sensor based on plasmonic toroidal metasurface

Terahertz cancer cell sensor based on plasmonic toroidal metasurface

Ecological analysis of heavy metals contamination in water from Kaani River, Ogoni, Rivers State, South-South, Nigeria

Ecological evaluation was carried out to study the surface water of the Kaani River, Ogoni axis, Rivers State, South-South, Nigeria, for contamination factor (Cf), degree of contamination (Cd), and modified contamination degree (mCd). Over the course of a year, surface water samples were taken from the Kaani River at four separate locations every two months. The atomic absorption spectrophotometry (AAS) method was used to determine the amounts of heavy metals in the samples after they had been prepared according to normal procedures. The metals analyzed were manganese (Mn), cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), iron (Fe), zinc (Zn), arsenic (As), and nickel (Ni) respectively. The result of the metal concentrations in all stations and months showed the order as As < Cd < Pb < Mn < Cr < Ni < Zn < Fe < Cu. A contamination factor test on the various heavy metals based on their concentrations and the WHO standard revealed that the studied heavy metals were at different levels of contamination and pollution classification in the River at different stations. The values observed for all the trace elements studied, with the exception of Fe, were less than the WHO and NAFDAC requirements for drinking water. The River showed no contamination–moderate pollution with Fe, while that of Mn indicated slight contamination- severe contamination range, Cd fell within no contamination to slight contamination range, slight contamination –slight pollution with Cu, no contamination – slight contamination with Zn, slight contamination – very severe contamination with Cr, moderate contamination- very severe contamination with Pb. As and Ni were at the range of slight contamination – moderate contamination. The analysis results from the River indicated that the water should be treated adequately before consumption. The pollution index analysis showed that the River was not polluted with heavy metals. Contamination degree values showed that the River is at different levels of heavy metal contamination. Modified contamination degree indicated that the River experienced nil to very low degree of contamination by the metals examined.

Heavy metal contamination assessment and source attribution in the Vicinity of an iron slag pile in Hechi, China: Integrating multi-medium analysis

Heavy metals, such as mercury, cadmium, and nickel, may contaminate human inhabited environments, with critical consequences for human health. This study examines the health impacts of heavy metal pollution from an iron slag pile in Hechi, China, by analyzing heavy metal contamination in water, sediment, soil, and crops. Here, the Nemerow pollution index (NI) indicated severe pollution at most sampling sites, the mean NI of groundwater, and surface water had reached 594.13 and 26.79, respectively. Bioaccumulation of mercury (Hg), cadmium (Cd), and nickel (Ni) was noted in crops, cucumbers showed comparatively lower risk levels. Logarithmic surface water-sediment partition coefficient calculations indicated that heavy metals such as chromium (Cr), ferrum (Fe), zinc (Zn), copper (Cu), Ni, arsenic (As), and lead (Pb) tend to accumulate in sediments. There was a high risk in groundwater (67.48-6590.54) and surface water (13.73-2500.85). Variably influenced by rainfall, these metals can be diluted and mobilized from surface water and sediments, thereby changing the contamination levels and ecological risks. Probabilistic health risk assessments indicated that health risks were higher in children than in adults, the mean total carcinogenic risk values of soil, groundwater, and surface water, were 6.79E-04, 4.20E-06, and 1.15E-6 for children, respectively. Moderate soil pollution is the main health hazard. A Positive Matrix Factorization model attributed over 60% of the pollution to slag stacking. Biotechnologies, solidification/stabilization techniques, field management, and institutional controls, driven by principles of green, low-carbon, and economic efficiency may mitigate. These findings contribute to the management of heavy metal pollution in iron slag pile areas.

Assessment of Heavy Metal Contamination in Water, Crab (Scylla senrata) and Sediments in Oil and Non-Oil Producing Communities in Akulga Local Government Area, Rivers State, Nigeria

Assessment of Heavy Metal Contamination in Water, Crab (Scylla senrata) and Sediments in Oil and Non-Oil Producing Communities in Akulga Local Government Area, Rivers State, Nigeria

Harnessing microbial potential: Exploiting heavy oil-laden soil microbiota for sustainable production of high-yield rhamnolipids from waste cooking oil

Harnessing microbial potential: Exploiting heavy oil-laden soil microbiota for sustainable production of high-yield rhamnolipids from waste cooking oil

Enhancing Heavy Metal Detection through Electrochemical Polishing of Carbon Electrodes.

Our research addresses the pressing need for environmental sensors capable of large-scale, on-site detection of a wide array of heavy metals with highly accurate sensor metrics. We present a novel approach using electrochemically polished (ECP) carbon screen-printed electrodes (cSPEs) for high-sensitivity detection of cadmium and lead. By applying a range of techniques, including scanning electron microscopy, energy-dispersive spectroscopy, Raman spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry, we investigated the impact of the electrochemical potential scan range, scan rate, and the number of cycles on electrode response and its ability to detect cadmium and lead. Our findings reveal a 41 ± 1.2% increase in voltammogram currents and a 51 ± 1.6% decrease in potential separations (n = 3), indicating a significantly improved active electrode area and kinetics. The impedance model elucidates the microstructural and electrochemical property changes in the ECP-treated electrodes, showing an 88 ± 2% (n = 3) decrease in the charge transfer resistance, leading to enhanced electrode electrical conductivity. A bismuth-reduced graphene oxide nanocomposite-modified, ECP-treated electrode demonstrated a higher cadmium and lead sensitivity of up to 5 ± 0.1 μAppb-1cm-2 and 2.7 ± 0.1 μAppb-1cm-2 (n = 3), respectively, resulting in sub-ppb limits of detection in spiked deionized water samples. Our study underscores the potential of optimally ECP-activated electrodes as a foundation for designing ultrasensitive heavy metal sensors for a wide range of real-world heavy metal-contaminated waters.

Isolation and characterization of native microalgae with potential for phycoremediation of heavy metal contaminated water

ABSTRACT The capacity of living algal cells to efficiently eliminate metals from water is well known and achieved through the mechanisms of biosorption and bioaccumulation. These investigations highlight the potential of indigenous microalgae for the elimination of toxic elements such as cadmium (Cd), cobalt (Co), chromium (Cr), nickel (Ni), and lead (Pb) from aqueous solutions. To identify algal strains that are effective in removing heavy metals, samples of soil and water were collected from areas contaminated with industrial and municipal effluents. Among the 11 isolates obtained from various sites contaminated with heavy metals, three isolates (designated as A, I and K) were able to tolerate both the threshold and double threshold levels of the tested heavy metals. Subsequent liquid assays confirmed the efficiency of the three selected algal isolates in removing heavy metals, with isolates A and I demonstrating effectiveness in removing all tested heavy metals, exhibiting removal rates of cadmium ranging from 40% to 80%, cobalt ranging from 20% to 60%, chromium up to 90%, nickel ranging from 40% to 90%, and lead ranging from 50% to 70%. Isolate K displayed the highest efficiency in only removing lead, specifically 86.23% at 21 DAI (Days After Inoculation), and therefore was not chosen for further characterization. Through morphological and molecular characterization, isolates A and I were identified as Coelastrella thermophila AAU BR A and Chlorella sp. AAU BR I, respectively. In conclusion, the results demonstrate that the native algal isolates have the potential to serve as eco-friendly agents for remediating heavy metal-contaminated water.

Reduction of Toxic Metal Ions and Production of Bioelectricity through Microbial Fuel Cells Using Bacillus marisflavi as a Biocatalyst.

Industrialization has brought many environmental problems since its expansion, including heavy metal contamination in water used for agricultural irrigation. This research uses microbial fuel cell technology to generate bioelectricity and remove arsenic, copper, and iron, using contaminated agricultural water as a substrate and Bacillus marisflavi as a biocatalyst. The results obtained for electrical potential and current were 0.798 V and 3.519 mA, respectively, on the sixth day of operation and the pH value was 6.54 with an EC equal to 198.72 mS/cm, with a removal of 99.08, 56.08, and 91.39% of the concentrations of As, Cu, and Fe, respectively, obtained in 72 h. Likewise, total nitrogen concentrations, organic carbon, loss on ignition, dissolved organic carbon, and chemical oxygen demand were reduced by 69.047, 86.922, 85.378, 88.458, and 90.771%, respectively. At the same time, the PDMAX shown was 376.20 ± 15.478 mW/cm2, with a calculated internal resistance of 42.550 ± 12.353 Ω. This technique presents an essential advance in overcoming existing technical barriers because the engineered microbial fuel cells are accessible and scalable. It will generate important value by naturally reducing toxic metals and electrical energy, producing electric currents in a sustainable and affordable way.

The Effectiveness of Coontail Plant (Ceratophyllum demersum L.) For Removal of Iron (Fe) Artificial Waste

Industrial wastewater causes pollution to the environment. The content of wastewater contaminated with heavy metal Fe with high levels is toxic which damages the aquatic ecosystem. Thus, an appropriate and environmentally safe method is needed. Phytoremediation is a method of utilizing the ability of plants to reduce to eliminate pollutants in the form of heavy metal contamination in water. The study uses the ability of Coontail Plant (Ceratophyllum demersum L.), the plant is proven to be able to reduce the concentration of polluted heavy metals. Coontail plants are used in the amount of 5 stalks with a plant mass from 40 gr with a plant size length of 10 cm-30 cm in one reactor, containing 5 liters of heavy metal (Fe) artificial wastewater with a concentration of 12 mg/L. The phytoremediation treatment used three reactors where RK1 (control reactor), R2 (5 coontail plants) and R3 (5 coontail plants Aerator). The exposure time is 6 days with variations in testing Day 0, 3, 6 using an Atomic Absorption Spectrophotometer (AAS) with testing methods as SNI 6989-84: 1019. The results of sample analysis for 6 days in RK1, R2, and R3 reactors decreased Fe concentration on day 3rd by 11.6 mg/L; 4.178 mg/L; 0.4848 mg/L. on day 06 the final decrease was 10.87 mg/L; 0.3466 mg/L; 0.3229 mg/L. The percentage effectiveness of Fe concentration removal on day 03 was 3.33%; 64.44%; and 95.96%, on the 6th day it was 9.41%; 97.11%; 97.30%.

Bacterial and microalgal co-fixation for remediation of industrial wastewater contaminated with arsenic, mercury, and other pollutants

Bacterial and microalgal co-fixation for remediation of industrial wastewater contaminated with arsenic, mercury, and other pollutants

Assessing the Lead Removal Potential of Pineapple Waste Hydrochar in Simulated Wastewater Treatment

This study explores the viability of utilizing biochar and hydrochar derived from pineapple waste as adsorbents for removing lead (Pb) from water. Pineapple residues, including stems, leaves, and fruit, were subjected to pyrolysis and Hydrothermal Treatment to produce biochar (PWB) and hydrochar (PWH) respectively. Fourier Transform Infrared (FT-IR) analysis was employed to characterize the surface properties of PWB and PWH, validating their potential as adsorbents. A series of adsorption experiments assessed the impact of pH (ranging from 2 to 6), contact time (ranging from 15 to 90 minutes), and temperature (ranging from 30 to 90°C) on the adsorption efficiency of both materials. Results indicate PWH to be markedly more effective, with an average Pb removal efficiency of 84.07% compared to PWB's 55.68%. The optimal contact time was determined to be 60 minutes for both materials. Moreover, pH 4.0 was identified as the optimal condition for Pb adsorption, showcasing a significant increase in biosorption capacity within this pH range. Additionally, higher temperatures corresponded to enhanced Pb2+ removal efficiency, rising from 75.02% to 87.58% as temperature increased from 30°C to 90°C. Overall, these findings underscore the potential of pineapple waste-derived biochar and hydrochar as promising, environmentally friendly adsorbents for addressing heavy metal contamination in water, particularly in wastewater treatment applications.

A ratiometric electrochemical sensor for simultaneous detection of multiple heavy metal ions in water and herbal medicines based on methylene blue-functionalized metal–organic framework

A ratiometric electrochemical sensor for simultaneous detection of multiple heavy metal ions in water and herbal medicines based on methylene blue-functionalized metal–organic framework