Biosorption of chromium species by aquatic weeds: Kinetics and mechanism studies

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Abstract

In this paper, we have presented the results of Cr(VI) and Cr(III) removal from aqueous phase by different aquatic weeds as biosorbents. Batch kinetic and equilibrium experiments were conducted to determine the adsorption kinetic rate constants and maximum adsorption capacities of selected biosorbents. In most of the cases, adsorption followed a second-order kinetics. For Cr(III), maximum adsorption capacity was exhibited by reed mat (7.18 mg/g). In case of Cr(VI), mangrove leaves showed maximum removal/reduction capacity (8.87 mg/g) followed by water lily (8.44 mg/g). There was a significant difference in the concentrations of Cr(VI) and total chromium removed by the biosorbents. In case of Cr(VI) removal, first it was reduced to Cr(III) with the help of tannin, phenolic compounds and other functional groups on the biosorbent and subsequently adsorbed. Acid treatment significantly increased Cr(VI) removal capacity of the biosorbents whereas, alkali treatment reduced the Cr(VI) removal capacities of the biosorbents. FTIR spectrum showed the changes in functional groups during acid treatment and biosorption of Cr(VI) and Cr(III). Aquatic weeds seem to be a promising biosorbent for the removal of chromium ions from water environment.

Introduction

Chromium is released into the environment through a large number of industrial operations, which include manufacturing of alloys, dyes and pigments, electroplating, metal finishing, petroleum refining, leather tanning, wood preservation, and as corrosion inhibitors in conventional and nuclear power plants [1]. Chromate is toxic, mutagenic and carcinogenic in nature. Hence, removal of Cr(VI) from industrial effluents is essential.

Conventional methods for removing dissolved heavy metal ions include chemical precipitation, chemical oxidation and reduction, ion exchange, filtration, electrochemical treatment and evaporative recovery. However, these high-end processes have significant disadvantages, including incomplete metal removal, requirement of expensive equipment and monitoring systems, high energy requirements and/or generation of toxic sludge or other waste products that require disposal [2]. This has led to the development of alternative low-cost technologies for the removal of Cr(VI) and other heavy metals from industrial effluents.

Several recent publications reported the utilization of locally available adsorbents like fly ash, peat, microbial biomass [3], [4], [5], [6], [7] and agricultural byproducts [4] for heavy metal removal. The sorption capacity of different biosorbents like dried mycelium of some species of fungi, baggase, rice husk and fermented baggase were examined for cyanide and heavy metal removal from industrial effluents [8]. Adsorption of Cd(II) and Ni(II) was found to be higher than Cr(VI) and Zn(II) when phosphate treated rice husk was used as an adsorbent [9]. Adsorption of Cr(VI) with activated rice husk carbon, activated alumina [10], formaldehyde and sulphuric acid treated sawdust carbons [11] were investigated. The amount of adsorbed Cr(VI) increased with increase in dose of these adsorbents and the contact time. Many biosorbents such as fungus, seaweed, micro-algae and other plant-derived material have been studied for their metal uptake. Agarwal et al. [12] evaluated the effectiveness of low-cost agro-based materials, namely, Tamarindus indica seed (TS), crushed coconut shell (CS), almond shell (AS), ground nut shell (GS) and walnut shell (WS) for Cr(VI) removal. The state-of-the art information regarding the remediation of Cr(III) and Cr(VI) by activated carbon and other low-cost adsorbents is available elsewhere [13].

Aquatic weeds are available in abundance and therefore attempts have been made to use them for heavy metal removal. It is reported that the roots of water hyacinth posses more metal uptake capacity compared to leaves and stem [14]. Dried Chinese Reed (Miscanthus sinensis) was investigated as adsorbent by Namasivayam and Holl [15] for the removal of Cr (III) from tannery wastewater. Most of the studies dealing with Cr(VI) biosorption described the process as anionic adsorption. There are a few reports on partial reduction along with anionic adsorption of hexavalent chromium by chemically modified Saccharomytces cerevisiae [16] and dead fungal biomass Aspergillus niger [2].

It is reported that in case of Cr(VI) biosorption in the pH range of 1–5, Cr(VI) was completely reduced to Cr(III), which appeared in the solution phase, or partly bound to the biomass [17]. Recently, the occurrence of the non-enzymatic reduction of Cr(VI) to Cr(III) by biomaterials or biomaterial-based activated carbons under acidic conditions were also reported [18], [19], [20]. When Cr(VI) comes in contact with organic substances or reducing agents, especially in an acidic medium, it spontaneously reduced to Cr(III), because Cr(VI) has high redox potential [17], [21], [22], [23], [24]. Therefore, it is very important to quantify the reduction of Cr(VI) by organic materials and the effects of various parameters on Cr(VI) reduction while using biosorbents for removal of Cr(VI). However, not many studies have been carried out to understand the kinetics and mechanism of Cr(VI) reduction and Cr(VI) and Cr(III) biosorption by aquatic weeds.

This study aims at comparative evaluation of various low-cost adsorbents derived from aquatic weeds for removal of Cr(VI) and Cr(III) from simulated wastewater. An attempt was also made to elucidate the mechanism of biosorption by the selected sorbents.

Section snippets

Biosorbent

The biosorbents used in the present study include reed mat (Cannomois Vvirgata), water lettuce (Pistia stratiotes), arrow-leaved tear thumb (Polygonum sagittatum), lotus flower (Nelumbo nucifera), green taro (Colocasia esculenta), water lily flower (Nymphaea sp.), water hyacinth (Eichornia crassipes), and mangrove leaves (Rhizophora mangle L). All these biosorbents were collected from contaminated water bodies in and around Chennai, India.

Preparation of raw biosorbents

Collected water weeds (plants) were rinsed with

Screening of biosorbents

The selected adsorbents were screened for their biosorption potential using adsorption kinetic studies. The results are summarized in Table 1. With an adsorbent dose of 10 g/L and an initial Cr(III) concentration of 25 mg/L, the removal of Cr(III) was between 78.55% (lotus) and 63.5% (green taro) for a contact time of 3 h. Under similar conditions, Cr(VI) removal varied from 99.3% (mangrove leaves) to 31.79% (water lettuce). It is clear from the results that different biosorbents have different

Conclusions

In the present study, various aquatic weeds were screened for Cr(VI) and Cr(III) biosorption potential. Batch kinetic and equilibrium experiments were conducted to determine the adsorption rate and adsorption capacities of various biosorbents. Among the biosorbents screened, reed mat has shown maximum Cr(III) adsorption capacity whereas, mangrove leaves performed better in case of Cr(VI) removal. In most of the cases, adsorption followed second-order kinetics. In case of Cr(VI), first it was

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