📄 chemistry

Solid Phase Extraction for Oil Analysis in Beach Sand

This study developed and validated a rapid, solvent-efficient solid-phase extraction method for quantifying oil and grease in Copacabana beach sand, which yielded results statistically comparable to the traditional Soxhlet extraction technique.

Original authors: Rodrigo F. Leal, Ana Clara S. Coelho, Gabriel L. Mendes, Antônio P. M. Fonseca, Glaucio G. Honorio, Ana G. H. P. van Elk

Published 2026-09-22

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CC BY 4.0

Original authors: Rodrigo F. Leal, Ana Clara S. Coelho, Gabriel L. Mendes, Antônio P. M. Fonseca, Glaucio G. Honorio, Ana G. H. P. van Elk

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Coastal zones are among the most economically vital and heavily visited places on Earth, supporting industries from fishing to tourism while serving as the backdrop for daily life in many cities. Yet, these vibrant shorelines face a quiet, persistent threat: the accumulation of oil and grease. While large oil spills make headlines, a more common form of pollution comes from the daily discard of used cooking oil by restaurants, street vendors, and beachgoers. Unlike water, which can dilute contaminants, sand acts as a sponge, trapping these oily residues deep within its grains. Once buried, this oil can block air and water from reaching the soil, harm plant life, and slowly release methane, a potent greenhouse gas. Understanding how much oil is hiding in the sand, and where it is hiding, is essential for managing the health of these ecosystems, but measuring it has traditionally been a slow, labor-intensive process that requires large amounts of chemical solvents.

In a study focused on Copacabana Beach in Rio de Janeiro, researchers set out to find a faster, cleaner way to measure this hidden pollution. They developed a new method using solid-phase extraction, a technique that works like a specialized filter. Instead of soaking a large amount of sand in a chemical bath for hours, the team packed a small amount of sand into a cartridge and passed a solvent through it to wash out the oil. To make this process as efficient as possible, they tested different amounts of sand and different volumes of solvent, looking for the combination that pulled out the most oil in the shortest time. They discovered that using just two grams of sand and fifteen milliliters of solvent yielded the best results, a setup that took significantly less time and used far less chemical than the standard methods currently in use.

When the researchers compared their new, streamlined approach against the traditional method, which involves boiling sand in a solvent for several hours, the results were nearly identical. Statistical analysis showed no meaningful difference between the two, proving that the new technique could replace the older, more cumbersome one without sacrificing accuracy. With a reliable method in hand, the team turned their attention to the sand itself. They collected samples from various points along the beach, from the area near the boardwalk where food is sold, to the middle of the beach, and down to the water's edge where waves constantly crash.

The findings revealed a clear pattern in how the pollution is distributed. The sand closest to the commercial establishments and the boardwalk contained the highest levels of oil, with some samples showing nearly four grams of oil for every kilogram of sand. As the researchers moved toward the water, the oil content dropped significantly. The areas constantly washed by waves had the lowest concentrations, suggesting that the natural action of the tides helps wash the oil out of the sand and back into the ocean. To confirm what kind of oil they were finding, the team analyzed the chemical signature of the extracted residue. The results strongly matched the profile of used cooking oil, indicating that the pollution is primarily driven by food-related activities rather than industrial spills or maritime accidents.

This study confirms that used cooking oil is a significant contaminant in the sand of busy urban beaches, accumulating most heavily near where people gather to eat. By demonstrating a quicker and more efficient way to measure this pollution, the researchers have provided a practical tool for environmental managers to monitor beach health. The work highlights that while the ocean's waves can help clean the shoreline, the sand near the city's edge remains a reservoir for waste, requiring better management of how food establishments and visitors handle their used oils.

Technical Summary: Solid Phase Extraction for Oil Analysis in Beach Sand

Problem Statement
Coastal areas, particularly those reliant on tourism like Copacabana Beach in Rio de Janeiro, face significant environmental challenges due to the generation of liquid and solid waste. Among these, oil and grease residues—primarily from used cooking oil discarded by beachfront commercial establishments and street vendors—pose a severe threat. These residues can contaminate vast volumes of water, alter sand permeability, reduce oxygen levels, and release greenhouse gases. While monitoring is essential, the paper notes a lack of standardized technical procedures for quantifying oil content in solid matrices like beach sand. Existing methods, such as EPA Method 9071B and AWWA Method 5520D (Soxhlet extraction), are characterized by long extraction times (approximately 5–6 hours), high solvent consumption, and multiple procedural steps, which can lead to imprecise results and operational inefficiencies.

Methodology
The study aimed to develop and optimize a methodology for quantifying oil and grease in beach sand using Solid Phase Extraction (SPE) coupled with gravimetric analysis. The experimental approach involved the following key components:

Sample Collection: Sand samples were collected from Copacabana Beach at three distinct zones relative to commercial activity and wave action: near the boardwalk (supratidal), the intermediate zone, and the constantly wet area (intertidal). Samples were taken at depths of 5–20 cm to avoid UV degradation of hydrocarbons.
SPE Setup: Custom cartridges were prepared by packing sand into polypropylene syringes with filters at both ends. Hexane was used as the elution solvent.
Experimental Design: A $3^2$ factorial design was employed to optimize two variables: solvent volume (10, 15, and 20 mL) and sand mass (2, 3.5, and 5 g). The data was modeled using a second-order polynomial to generate a response surface.
Quantification: Oil content was determined gravimetrically by evaporating the hexane eluate at 70°C until constant weight. The oil content was calculated using the mass difference of the test tube before and after extraction relative to the sample mass.
Validation: The proposed SPE method was validated against the conventional Soxhlet extraction method (adapted from AWWA Method 5520D) using Student's t-test. Recovery tests were conducted at two spiking levels. Additionally, Fourier Transform Infrared Spectroscopy (FTIR) was used to characterize the chemical nature of the extracted residues.

Key Results

Optimization: The factorial design analysis revealed that increasing solvent volume generally increased oil extraction, but volumes above 15 mL offered diminishing returns. Conversely, increasing the sand mass beyond 2 g hindered the extraction process, likely due to increased path length for the solvent. The optimal conditions were determined to be 2 g of sand and 15 mL of hexane.
Method Comparison: The Student's t-test (95% confidence level) showed no statistically significant difference between the proposed SPE method and the conventional Soxhlet extraction ( $t_{calc} < t_{crit}$ ). The relative error between the two methods ranged from 1.0% to 6.4%.
Recovery: The method demonstrated high accuracy with recovery rates of 100% at the lower spiking level (0.007 g) and 96% at the upper level (0.068 g).
Field Findings: Oil content in Copacabana sand ranged from 0.50 g/kg to 3.83 g/kg. Higher concentrations were consistently found in the supratidal and middle beach zones (closer to commercial establishments), while the intertidal zone showed lower levels, likely due to wave action washing residues into the sea.
Chemical Characterization: FTIR analysis confirmed that the extracted residues were predominantly composed of triglyceride-based compounds (cooking oil), evidenced by characteristic C–H stretching and ester carbonyl vibrations. Spectral similarity with standard cooking oil ranged from 72.1% to 82.5%.

Significance and Claims
The paper asserts that the proposed SPE-gravimetric methodology offers a viable alternative to traditional Soxhlet extraction for analyzing oil in beach sand. Its primary significance lies in being a faster and less solvent-intensive approach while maintaining analytical accuracy comparable to the standard method. The study successfully demonstrates that this technique can effectively monitor the accumulation of hydrophobic organic compounds in coastal environments.

The authors conclude that the contamination observed in Copacabana is primarily driven by anthropogenic inputs related to food services and tourism, rather than petroleum spills. By providing a more efficient analytical tool, the study supports the need for sustainable waste management practices in high-traffic coastal areas to mitigate environmental damage. The paper does not claim to solve the pollution issue itself but highlights the importance of effective monitoring to understand the extent of contamination caused by improper disposal of used cooking oil.

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