This paper proposes a new mechanism for the direct and indirect reduction of Cr(VI) by brown seaweed Ecklonia biomass and also suggests chemical methods to enhance Cr(VI)-removal.

“This paper explains a reduction mechanism of Cr(VI) to Cr(III) by biomaterials and suggests how to enhance Cr(VI) removal performance of it.”

As an alternative to conventional chemical treatment methods—i.e., reduction followed by precipitation—over the last few decades, many researchers have tested various biomaterials to remove toxic Cr(VI) from aqueous solution. Since 1986, to the best of our knowledge, over 200 papers have been published in the research filed of Cr(VI) biosorption.

Most early studies had claimed that Cr(VI) was removed from an aqueous solution through an anionic adsorption, but it has been newly explained that these findings were misinterpreted due to errors in measuring chromium species in an aqueous phase, insufficient contact time required for equilibrium, and the lack of information about the oxidation state of the chromium bound on the biomaterials or activated carbons.

In this paper, a new mechanism was proposed for the Cr(VI) removal by biomaterials: Cr(VI) can be removed from an aqueous solution by biomaterials through the use of a direct and indirect reduction mechanism. The proposed mechanism has now been accepted by other researchers and this paper is therefore highly cited.

Over the last few decades, many researchers have claimed that Cr(VI) was removed by an anionic adsorption mechanism. Although my research team has indicated obvious mistakes in this research, many researchers have still accepted these old/false mechanisms and cited their papers.

Under existing conditions, this paper obviously proves that the mechanism of Cr(VI) removal by biomaterials is not anionic adsorption but adsorption-coupled reduction. I believe firmly that this work will be helpful in understanding the relationship between biomaterials and redox-active metals—such as arsenic, chromium, iron, manganese, selenium, and uranium—within the environment.

Have you seen the 2000 film Erin Brockovich in which an unemployed single mother—played by Julia Roberts—becomes a legal assistant and almost single-handedly brings down a California power company accused of polluting a city’s water supply? In this movie, she finally proved that Cr(VI) actually had a quite harmful impact on residents of the small town of Hinkley, California during the period of the 1960s, ’70s, and ’80s.

Nowadays, Cr(VI) is known to be toxic to both plants and animals, as a strong oxidizing agent and potential carcinogen. Thus, the discharge of Cr(VI) into surface water is regulated to below 0.05 mg/L by the U.S. EPA.

In contrast, Cr(III) is generally only toxic to plants in very high concentrations and is less toxic or nontoxic to animals. As a consequence of these dramatic differences, the environmental reduction of Cr(VI) alone may be considered as a satisfactory solution for treating Cr(VI)-contaminated waters or soils.

As chemical reductants, natural biomaterials such as fallen leaves and tree bark can also reduce toxic Cr(VI) to nontoxic Cr(III). This paper explains a reduction mechanism of Cr(VI) to Cr(III) by biomaterials and suggests how to enhance Cr(VI) removal from aqueous solutions.

I’ve been involved in various research projects related to industrial wastewater treatment since 1990. Chromium is one of the toxic pollutants to be treated before being discharged into the environment. Among various chemical/biological technologies for treating Cr(VI)-contaminated waters, I have focused on the use of abundant and inexpensive biomaterials, instead of conventional ion-exchange resins or activated carbons.

Fortunately, I found a redox reaction between Cr(VI) and biomaterials and could propose a new mechanism for the Cr(VI) biosorption.