Clay-Rich Alluvial Soils Show Strongest Grip on Arsenic, but Phosphate Fights Back

Arsenic in groundwater is one of the most quietly devastating public health crises of our time, and in the alluvial plains of South Asia it is woven directly into the landscape. Now, a team of soil scientists at Banaras Hindu University in Varanasi has taken a close look at an underappreciated line of defense: the soils themselves. In a study published in Discover Soil, Arghya Chattopadhyay, Amitava Rakshit and colleagues examined how three naturally occurring alluvial soils of different textures bind arsenate, the oxidized and highly toxic form of arsenic, and how common dissolved ions such as phosphate, silicate and sulphate interfere with that binding. Their findings carry real consequences for how arsenic contamination is managed across the Ganga basin and beyond.

The stakes could hardly be higher. Inorganic arsenate, or As(V), is a class-one carcinogen, and prolonged exposure through contaminated drinking water and food chains is linked to cancers and a range of other serious illnesses. Across India, the sand, silt and clay deposits laid down by the Ganga and its tributaries are a major reservoir of the element, which can shift from inert mineral phases into soil solution and groundwater whenever chemistry and redox conditions favor mobilization. Whether arsenic stays locked in place or leaks into aquifers depends largely on adsorption, the process by which dissolved ions attach themselves to reactive mineral surfaces. Soils that adsorb arsenate strongly act as natural buffers; soils that adsorb it weakly let the poison travel.

Most laboratory research on arsenic removal has focused on pure mineral phases, especially iron oxides, which are celebrated sorbents of arsenate. But natural soils are messy composites of clay minerals, oxides, organic matter and variable surface chemistry, and the team wanted to know how the whole package performs. They collected three alluvial soils from Varanasi spanning a textural gradient: a sandy loam, a sandy clay loam and a clay loam. Each was characterized in detail using a battery of techniques, including nitrogen gas adsorption to measure surface area, scanning electron microscopy with energy-dispersive X-ray spectroscopy to image surface morphology, Fourier-transform infrared spectroscopy to identify reactive functional groups, and X-ray diffraction to pin down the mineral assemblage.

The characterization told a coherent story. The clay loam, the finest-textured of the three, had the largest BET surface area at 11.08 square meters per gram, the highest cation exchange capacity at 12.4 centimoles of charge per kilogram, and the most organic carbon at 0.42 percent. X-ray diffraction revealed quartz, feldspar and aluminosilicate phases in all three soils, along with clay minerals such as illite and kaolinite. Infrared spectroscopy detected broad hydroxyl bands and metal-oxygen vibrations, the very groups known to participate in ligand-exchange reactions with arsenate. Electron microscopy of the clay loam showed a heterogeneous, highly aggregated mineral matrix with platy particles and abundant inter-particle voids, exactly the kind of architecture that exposes reactive surfaces to dissolved contaminants.

With the soils characterized, the team ran batch adsorption experiments at 25 degrees Celsius and neutral pH. Kinetics experiments showed rapid initial uptake, with the clay loam adsorbing 0.296 milligrams of arsenate per gram within the first thirty minutes, rising to 0.494 milligrams per gram by twenty-four hours, after which adsorption plateaued. The researchers fitted the data to pseudo-first-order, pseudo-second-order, intra-particle diffusion and Elovich models. The pseudo-second-order model performed best, pointing to chemisorption, meaning arsenate forms actual chemical bonds with surface functional groups rather than merely sticking by weak physical forces. The strong performance of the Elovich model, meanwhile, indicated that the adsorption sites are energetically heterogeneous, a signature of natural soils with mixed mineralogy.

The pH dependence of adsorption proved dramatic. All three soils grabbed the most arsenate in the acidic range between pH 4 and pH 6, and adsorption fell sharply as conditions turned alkaline. At pH 7, the sandy loam, sandy clay loam and clay loam adsorbed 0.350, 0.420 and 0.490 milligrams per gram respectively, but at pH 10 those figures collapsed to just 0.100, 0.178 and 0.248. The explanation lies in surface charge. Under acidic conditions, protonated soil surfaces carry positive charge that electrostatically attracts the negatively charged arsenate species H2AsO4− and HAsO4²−. As pH rises, hydroxide ions compete for binding sites and clay surfaces become increasingly negative, repelling arsenate and driving it back into solution. This is bad news for alkaline soils and groundwater systems, where arsenate mobility is inherently higher.

Equilibrium isotherm experiments, spanning arsenate concentrations from 2.5 to 50 milligrams per liter, quantified the capacity of each soil. The Langmuir model estimated maximum monolayer adsorption capacities of 0.531, 0.612 and 0.662 milligrams per gram for the sandy loam, sandy clay loam and clay loam respectively, a clear texture-driven hierarchy. Yet the Langmuir model, which assumes uniform binding sites, actually fit the data relatively poorly. The Freundlich and Sips models, which explicitly account for surface heterogeneity, performed better, confirming that arsenate retention in these soils is distributed across a mosaic of sites with different affinities: clay mineral edges, hydroxyl-bearing surfaces, and reactive Al-OH groups. The Temkin model further revealed a higher heat of adsorption in the clay loam, signaling stronger overall interactions.

The most consequential part of the study came when the researchers introduced competition. Groundwater is never a pure arsenate solution; it carries phosphate from fertilizers, silicate from mineral weathering, and sulphate from a host of natural and anthropogenic sources. All three are oxyanions that can occupy the same inner-sphere binding sites arsenate uses. When the team added 10, 20 and 30 milligrams per liter of each competitor alongside 20 milligrams per liter of arsenate, adsorption dropped significantly in every soil. The competitive strength followed a clear order: phosphate was the fiercest rival, silicate came second, and sulphate was the weakest. At 30 milligrams per liter, phosphate slashed arsenate adsorption to 0.092, 0.082 and 0.130 milligrams per gram in the three soils, cutting retention by more than half in the sandy loam and sandy clay loam.

Phosphate’s dominance is chemically logical. Arsenate and phosphate are structural near-twins, with almost identical acid dissociation constants, so they compete for precisely the same hydroxyl-bearing mineral sites through ligand exchange and inner-sphere complexation. Silicate binds similarly and can even polymerize on surfaces, gradually clogging adsorption sites, while sulphate’s weaker affinity leaves more room for arsenate. Intriguingly, the inhibitory effect of every competitor weakened progressively from the sandy loam to the clay loam. The clay-rich soil, with its larger and more diverse pool of reactive sites, simply had more binding capacity to spare, buffering it against competition. The authors conclude that competitive adsorption is governed not only by the chemical affinity of each anion for mineral surfaces but also by the abundance and heterogeneity of reactive sites within the soil itself.

The implications ripple outward in two directions. For arsenic mitigation, the study suggests that clay-rich alluvial soils are naturally better at immobilizing arsenate, and that soil texture and mineralogy deserve more attention when mapping contamination risk across the Indo-Gangetic plain. For agriculture, the phosphate result is a caution: heavy phosphorus fertilization on arsenic-affected soils could displace arsenate from grain surfaces and push it into soil solution and crops, a trade-off farmers and land managers rarely see. The authors are careful to note that their batch experiments, conducted at fixed pH and single-anion conditions, simplify the far messier reality of field soils, where redox fluctuations, dissolved organic matter, seasonal water flow and multiple competing ions interact. Still, by connecting spectroscopic mineral characterization to quantitative adsorption behavior under realistic competitive pressure, the work delivers a practical message: the ground beneath arsenic-affected regions is not just a passive contaminant source but an active, texture-dependent filter, and understanding its chemistry may be one of the cheapest tools available for protecting water and food supplies.

Subject of Research: Competitive adsorption of arsenate on alluvial soils under phosphate, silicate and sulphate conditions

Article Title: Assessment of adsorption potential of alluvial soils for arsenic removal under competing anion conditions

Article References: Chattopadhyay, A., Singh, A. P., Rakshit, A., Barman, A., Chourasia, A., & Mondal, T. (2026). Assessment of adsorption potential of alluvial soils for arsenic removal under competing anion conditions. Discover Soil, 3(1), Article 177. https://doi.org/10.1007/s44378-026-00332-8

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00332-8

Keywords: arsenic, arsenate, alluvial soil, adsorption, phosphate, silicate, sulphate, clay minerals, groundwater contamination, soil chemistry, Ganga basin, chemisorption