The research team, led by Rahat Khan of the Bangladesh Atomic Energy Commission with collaborators in Bangladesh, Saudi Arabia and Australia, collected sixty sediment samples from eighteen sites arranged around the tannery outfall. The design was deliberately three-dimensional: transverse transects crossed the river from the discharge point to the opposite bank at fifty-metre intervals, lateral transects ran along both banks at forty-five-degree angles, and longitudinal sites extended 1,500 to 2,500 metres upstream and downstream. At six key stations the team also extracted sediment cores about twenty-five centimetres long, slicing them into five-centimetre layers to trace how contamination varies with depth. All samples were analysed for fifteen major and trace elements using instrumental neutron activation analysis, a nuclear technique validated against certified reference materials from the International Atomic Energy Agency.

Chromium stood out immediately as the dominant anomaly. Across all sixty samples its concentrations ranged from 63 to 37,846 micrograms per gram, with a coefficient of variation near 280 percent, a statistical signature of a dataset dominated by one extreme location. Chromium exceeded the average upper continental crustal value in 92 percent of samples, while antimony, cobalt and arsenic also surpassed crustal benchmarks in many samples. Yet the spatial structure told the real story. Chromium concentrations declined significantly with distance from the outfall along the river’s flow axis, and a non-parametric comparison across four analytical zones, discharge, opposite bank, upstream and downstream, confirmed that the discharge zone carried far higher levels than all others. The pattern points to a geochemical background that is spatially stable, with tannery influence fading rapidly beyond the immediate discharge area.

The depth profiles added a subtler dimension. Across the six sites with full twenty-five-centimetre cores, including the discharge point itself, chromium showed no systematic trend with depth, suggesting episodic and uneven deposition rather than a clean historical record. Within the nine near-field cores flanking the outfall, however, a significant depth effect emerged, and a paired comparison across all fifteen core sites found surface layers significantly enriched relative to the deepest layers, with a median surface concentration of 1,056 micrograms per gram against 237 micrograms per gram at depth. Near the source, within-core variability reached 50 to 117 percent, reflecting chaotic, source-proximal deposition, while distal cores varied by only 5 to 24 percent, consistent with mixing and depositional averaging. Lateral proximity to the outfall, in other words, exerts stronger control on the sediment burden than vertical stratification.

To separate industrial signals from natural geology, the team applied a battery of geo-environmental indices, including the geo-accumulation index, aluminium-normalised enrichment factors, contamination factors and the pollution load index. Every metric converged on the same conclusion: contamination is strongly element-specific and spatially concentrated, with chromium dominating the discharge zone where the geo-accumulation index climbed as high as 7.2 and mean enrichment factors reached 37.3. The authors caution that aluminium normalisation may be biased at the discharge point itself, because aluminium there is diluted to 1.5 to 1.7 percent by tannery waste compared with roughly 7.5 percent at background sites, so they relied on normalisation-free indices to corroborate the extreme enrichment. Elsewhere in the river, elements such as aluminium, potassium, titanium, rubidium, caesium, thorium and uranium showed low enrichment, reflecting mineralogical composition and Himalayan sediment provenance rather than industrial loading.

The centrepiece of the study was source apportionment using Positive Matrix Factorisation, an Environmental Protection Agency receptor model that decomposes measured concentrations into source fingerprints and their contributions. Guided by principal component analysis, the team settled on a three-factor solution whose stability was confirmed through twenty base runs and twenty bootstrap runs, with all elements classified as strong variables and predicted-observed correlations above 0.75. The first factor, carrying caesium, cobalt, titanium, rubidium, aluminium, arsenic, uranium, thorium and potassium, accounted for 65.5 percent of elemental variance and was attributed to natural weathering of parent rock. The second factor, dominated by chromium at 46.06 percent with supporting contributions from barium and zinc, explained 15.5 percent and was interpreted as the anthropogenic tannery signature. The third, led by antimony and sodium, contributed 19.0 percent and was conservatively labelled a mixed natural-industrial source.

That arithmetic delivers the study’s most striking insight: a comparatively small anthropogenic fraction of the total elemental budget governs the river’s principal ecological concern. Screening against consensus-based sediment quality guidelines showed that half of all sixty samples exceeded the chromium Effects Range Median threshold, the concentration above which adverse biological effects are frequently expected, and 46.7 percent fell between the Effects Range Low and Medium values. Manganese averaged 613 micrograms per gram, above its Lowest Effect Level but below the Severe Effect Level, while zinc sat marginally above its LEL and arsenic remained below it. Station-level toxic unit sums ranged from 1.07 to 154.23, and mean ERM quotients reached 22.3 at the discharge station, corresponding to a 76 percent probability of toxicity under published interpretation frameworks, compared with probabilities of 21 to 49 percent at most other sites.

The authors are careful to frame these results as screening-level evidence rather than proof of biological harm. The measurements capture total chromium, not its speciation, and the difference matters enormously: trivalent chromium, Cr(III), is comparatively immobile, whereas hexavalent chromium, Cr(VI), is far more toxic and bioavailable. Sediment binding phases, redox conditions and organic matter content, all of which control whether chromium can actually enter food webs, were not measured here. Elevated total chromium has been linked elsewhere to biological effects such as bacterial DNA damage, but confirming such consequences in the Dhaleshwari would require dedicated speciation work, sediment bioassays and benthic community surveys. The Th/U ratio of 4.52, above the crustal value of 3.89, does suggest oxidising sediments, a condition under which uranium is mobilised while thorium stays put, but the redox chemistry of chromium itself remains unresolved.

The mechanistic picture that emerges is one of particle-reactive chromium being rapidly scavenged into bed sediments near its release point, producing a hotspot rather than basin-wide dispersion. Chromium released from leather processing travels largely in particulate and reactive phases that settle quickly, while zinc associates with multiple sediment fractions, clays, iron-manganese oxides, organic matter and carbonates, allowing broader redistribution, and sodium disperses with solution chemistry. Hydrodynamic sorting compounds these differences, as fine contaminated particles stay in suspension longer and travel farther than coarse grains. For managers, the implication is direct and potentially cost-saving: targeted intervention at the discharge zone, backed by rigorous enforcement of treatment performance, will do more good than basin-wide remediation. That message is urgent because the treatment plant’s capacity of roughly 14,000 cubic metres per day falls far short of peak effluent generation exceeding 35,000 cubic metres per day. The team calls for chromium speciation, seasonal monitoring, bioaccumulation studies in locally consumed fish and mineralogical characterisation to convert this spatial and source-apportionment framework into a fully management-relevant risk assessment for tannery-impacted rivers everywhere.

Subject of Research: Chromium contamination and source apportionment in river sediments near a relocated tannery discharge in Bangladesh

Article Title: Chromium as a severe anthropogenic geochemical anomaly in river sediments near a tannery outfall: spatial attenuation, source apportionment and ecological risk screening

Article References: Khan, R., Das Shuvo, S., Basir, M. S., Siddique, M. A. B., Mallick, J., Hang, H. T., Khan, M. H. R., Aldawood, S., Roy, D. K., & Saha, N. (2026). Chromium as a severe anthropogenic geochemical anomaly in river sediments near a tannery outfall: spatial attenuation, source apportionment and ecological risk screening. Environmental Geochemistry and Health, 48(14), Article 570. https://doi.org/10.1007/s10653-026-03457-5

Image Credits: AI Generated

DOI: 10.1007/s10653-026-03457-5

Keywords: chromium, river sediments, tannery effluent, Dhaleshwari River, Bangladesh, source apportionment, positive matrix factorisation, ecological risk, sediment quality guidelines, heavy metal contamination, industrial pollution, neutron activation analysis

The scale of the problem the method addresses is enormous. Global plastic production has climbed from roughly two million metric tons in 1950 to nearly 391 million metric tons in 2021, and because most plastics do not biodegrade, discarded items persist and fragment. Macroplastics larger than 25 millimeters break down under sunlight and physical weathering into mesoplastics and then microplastics, defined as solid polymer-containing particles between one micrometer and five millimeters. These particles are classified as primary, when manufactured at microscopic sizes, or secondary, when produced by the fragmentation of larger debris, and they display enormous diversity in polymer type, shape, color and size.

Aquatic systems bear the brunt of this pollution. An estimated 4.8 to 12.7 million metric tons of plastic waste entered the oceans from land-based sources in 2015 alone, and rivers act both as sinks where particles settle and as major highways transporting them to marine environments. Once in the water, microplastics harm organisms that ingest them, disrupting feeding, energy metabolism and reproduction, and they can act as vectors for persistent organic pollutants and heavy metals that bioaccumulate up the food chain. Understanding where and when microplastics accumulate in river sediments is therefore essential for environmental risk assessment, yet no standardized, robust protocol for sampling and analysis in complex matrices has existed, meaning field studies often produce results that cannot be compared.

The difficulty lies in the sediment itself. Riverbed material is a mixture of light organic matter, such as plant detritus and locally produced biomass, and fine inorganic particles like silt and clay, all of which can trap microplastics and interfere with their chemical identification. Traditional protocols rely on sieving, density separation and filtration, often supplemented by chemical oxidation or enzymatic digestion. Some multi-step procedures applied to organic-rich sediments have required as long as 24 days per batch. Faster alternatives exist, including dye staining with Nile Red, electrostatic or magnetic separation, and oil-assisted extraction, but each carries drawbacks: staining produces false positives in organic-rich samples and adds fluorescence that overwhelms Raman signals, while olive oil residues left on filters proved fluorescent and blocked spectroscopic identification in the Florence team’s own tests.

To overcome these limitations, the researchers combined several established techniques into a streamlined workflow tailored for Raman analysis. Sediment collected from the Arno River in Florence and Pisa was dried, sieved, and sonicated to expose embedded particles. Fenton oxidation, using hydrogen peroxide with an iron catalyst at 35 degrees Celsius, then digested the organic matter over about a week. A density separation in saturated sodium chloride solution floated the low-density fraction, and finally a custom-made glass separator performed an affinity extraction in which microplastics preferentially migrate into cyclohexane, a non-polar solvent, while residual vegetable matter stays in the water phase. A small amount of sodium dodecyl sulfate was added to weaken the adhesion between particles and the sediment matrix, and the extraction was repeated three times to maximize recovery.

The choice of cyclohexane was the key innovation. Unlike olive oil, it does not stick to glassware, evaporates easily, and is optically transparent in the visible range because it is a saturated, symmetrical hydrocarbon with no conjugated electrons or heteroatoms. That means it generates no fluorescence background in Raman measurements, preserving the sensitivity needed to identify even weakly scattering environmental particles. It is also chemically inert, so free radicals left over from the Fenton step are unlikely to degrade it into fluorescing residues. Although cyclohexane is ranked as problematic from a safety and environmental standpoint, the protocol uses only about six milliliters per sample, and the custom glassware was designed to minimize solvent waste.

Validation was carried out on real Arno sediments spanning a wide range of conditions, from fine and medium silts to coarse gravel, with organic carbon contents between 0.14 and 2.2 percent. Samples were spiked with reference particles of five polymers covering a range of densities: polyethylene, acrylonitrile butadiene styrene, polyvinyl chloride, polyamide and polyethylene terephthalate. The overall recovery was 0.65, rising to 0.82 for low-density polyethylene and 0.72 for medium-density PVC, but dropping to 0.42 for high-density PET, which barely floats in the 1.2 grams per cubic centimeter sodium chloride solution. The team accepted this trade-off deliberately, since heavier salts such as sodium iodide or zinc chloride are costly, toxic or energy-intensive to recycle, and the vast majority of environmental microplastics have densities below the threshold. Only about 13 percent of global plastic production exceeds it.

The tests also revealed how strongly the sediment matrix itself influences recovery. Recovery was highest in medium and coarse sands but fell sharply in fine sands and coarse silts, where the greater surface area, cohesiveness and aggregation of small particles entrap microplastics and prevent them from detaching and floating. The cyclohexane extraction step alone achieved an overall efficiency of 0.89, and recovery correlated with polymer wettability: particles with a water contact angle above roughly 75 to 80 degrees were recovered almost completely, while more hydrophilic surfaces were harder to extract. Fibers proved slightly harder to recover than spheres or fragments. Crucially, a stability test confirmed that the Fenton oxidation did not alter the Raman spectra, dimensions or morphology of the reference particles.

When applied to unspiked Arno River samples, the workflow identified five polymers: polypropylene, polyester, polyethylene terephthalate, polycarbonate and polyethylene. Concentrations ranged from 0.030 items per gram of dry sediment in fine sand to 0.653 items per gram in coarse gravel, with a median of 0.151 and a mean of 0.217 items per gram. These values are lower than figures previously reported for emerged sediment bars on the Arno, which trap particles during floods and were measured with the hot needle method known to overestimate counts through false positives, but they align well with studies of smaller Tuscan rivers. Blank samples contained no microplastics above 20 micrometers, so no contamination correction was needed, thanks to plastic-free equipment, cotton clothing, filtered compressed air for the filters and rigorous glassware cleaning.

The researchers position the protocol as a practical compromise for large-scale monitoring, where throughput and cost matter more than perfect recovery of every high-density polymer. Compared with enzymatic digestion methods that offer high accuracy at the price of weeks of processing, or instrument-heavy approaches requiring expensive equipment and expert operators, the new workflow is fast, inexpensive and compatible with standard microRaman instruments using a 785-nanometer diode laser and the open-source OpenSpecy library for spectral identification. The authors suggest that testing alternative solvents could further improve effectiveness and eco-compatibility, and that a denser flotation salt could extend the method to specific high-density polymers when needed. As European regulators move toward harmonized basin-scale microplastic monitoring under the Water Framework Directive, tools like this one may determine whether such ambitions become routine practice.

Subject of Research: Optimization of sample pretreatment for Raman spectroscopic determination of microplastics in riverbed sediments

Article Title: Advances in the sample pretreatment for determination of microplastics in riverbed sediments by Raman spectroscopy

Article References: Dali, A., Mancini, M., Santianni, D., Solari, L., & Becucci, M. (2026). Advances in the sample pretreatment for determination of microplastics in riverbed sediments by Raman spectroscopy. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38231-8

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38231-8

Keywords: microplastics, Raman spectroscopy, river sediments, sample pretreatment, Fenton oxidation, cyclohexane extraction, Arno River, density separation, plastic pollution, environmental monitoring, polymer identification, water quality