The current absence of specific information regarding which materials were adsorbed using which adsorbents under which conditions has led to a potential range of interpretations regarding the underlying cause for the observed negative adsorption capacity. This situation could stem from various possibilities, such as errors in the ICP analysis procedure, experimental process, or in the design of the adsorption experiments.

Several assumptions could be considered to shed light on this issue:

Considering the aforementioned possibilities could contribute to a better understanding of the situation.

There are several possible reasons for higher ICP intensities in the filtrate of samples after adsorption compared to the original solution concentration. Some of them are:

• The adsorbent material may have impurities or contaminants that interfere with the ICP analysis, such as metals, salts, or organic compounds. These impurities may leach out from the adsorbent during the adsorption process and increase the concentration of the analyte or other elements in the filtrate. To avoid this, the adsorbent should be thoroughly washed and dried before use, and its purity should be checked by ICP or other methods1.
• The adsorption process may not reach equilibrium or saturation, meaning that not all the analyte molecules are adsorbed by the adsorbent. This may happen if the contact time, temperature, pH, or agitation of the system are not optimal for the adsorption process. To ensure that the adsorption process is complete, the adsorption isotherm and kinetics should be studied and the optimal conditions should be determined2.
• The filtration process may not be efficient or accurate, meaning that some of the adsorbent particles or analyte molecules may pass through the filter and remain in the filtrate. This may happen if the filter pore size, pressure, or volume are not suitable for the filtration process. To improve the filtration process, the filter type, size, and condition should be selected carefully, and the filtrate should be checked for turbidity or color3.

These are some of the possible causes of higher ICP intensities in the filtrate of samples after adsorption. You may want to check your experimental procedure and data for any errors or inconsistencies that may affect your results. You may also want to consult some references that describe the adsorption and ICP methods in more detail, such as:

: [Adsorption: Principles and Applications] by R. Sips, Elsevier, Amsterdam, Netherlands, 2018.

: [Inductively Coupled Plasma Mass Spectrometry Handbook] by S. Hill, Wiley-Blackwell, Hoboken, NJ, USA, 2019.

: [Adsorption by Powders and Porous Solids: Principles, Methodology and Applications] by F. Rouquerol, J. Rouquerol, K. Sing, and P. Llewellyn, Academic Press, London, UK, 2014.

https://www.academia.edu/33774784/A_Method_For_Evaluating_the_Sorption_Capacity_of_The_Sorbents

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