# Reduction of Hexavalent Chromium Using _Sorbaria sorbifolia_ Aqueous Leaf Extract
## 1 Introduction  
Chromium is a naturally occurring transition metal, found in contaminated soils, groundwater and drinking water, with the two most common oxidation states of chromium being trivalent, Cr(III) and hexavalent, Cr(VI). Cr(III) is an essential element in human nutrition and plays an important role in carbohydrate, protein and lipid metabolism [1], whereas Cr(VI) is soluble, relatively more mobile than Cr(III) [2] and toxic; Cr(VI) is a known carcinogen and mutagen [3, 4]. The erosion of natural chromium deposits, oxidation of Cr(III), anthropogenic activities (dying, electroplating, leather tanning) and other industrial applications have been responsible for chromium contamination in the environment [5].  
Many remediation strategies have been advanced to deal with Cr(VI) pollution, including adsorption [6], ion exchange [7], chemical precipitation [8], membrane filtration [9] and reduction [10; 11; 12; 13; 14; 15; 16; 17], among others. The reduction of Cr(VI) is among the most widely used strategies in view of the redox activity of chromium. Reductive remediation approaches can address the toxicity of Cr(VI) by transformation into Cr(III), that can be easily removed after precipitation with alkaline substances [8]. Several synthetic and natural electron-rich compounds are capable of reducing Cr(VI): Fe(II) bearing clay minerals, zero-valent aluminum and Mn(II) have been reported as reducing, and being catalyzing agents in the reduction processes of Cr(VI) [10; 11; 12]. Natural organic acids including citric, oxalic, ascorbic, tartaric and formic acid have also been examined for the reduction of Cr(VI)from contaminated aqueous and soil matrices [13; 14; 15; 16]. In general, compounds bearing aliphatic or aromatic hydroxyl groups have been found suitable for the reduction of Cr(VI) [15; 17].  
Most of the remediation-by-reduction strategies described in the literature entail a sharp decrease in reduction rates for a higher pH, but they are effective only at lower pH levels, which thus presents a challenge for more alkaline Cr(VI) contaminated sites [15]. [PERSON] et al. report 100% Cr(VI) reduction efficiency at pH 2.25 that becomes negligible at pH? 7.5 [12]. Similarly, [PERSON] et al. observed that montmorillonite and illite significantly catalyze the reduction of Cr(VI) by citric acid at pH! 5.0; in contrast, their effectiveness are dramatically diminished at pH 5.0 [18].  
Aqueous plant extracts have been extensively utilized for the synthesis of metal nanoparticles via the reduction of noble metal ions [19; 20]; the presence of multiple hydroxy and carboxylic acids groups serve as mild reduction and capping agents [21; 22]. Thus, the use of an aqueous plant extract (PLE) for the reduction of Cr(VI) to Cr(III) is a viable strategy. A local abundant shrub, _Sorbaria sorbifolia_, was selected for this study, since it grows rapidly and contains polyhydroxy constituents in its aerial part i.e., Kaempfer 3-arabofuranoside (C\({}_{20}\)H\({}_{18}\)O\({}_{10}\)), Astragalin (C\({}_{21}\)H\({}_{20}\)O\({}_{11}\)), Quercetin 3-glucuron (C\({}_{21}\)H\({}_{18}\)O\({}_{13}\)), Isorhamene 3-glucoside (C\({}_{22}\)H\({}_{22}\)O\({}_{12}\)), Scutellarein (C\({}_{21}\)H\({}_{18}\)O\({}_{12}\)\(\cdot\)H\({}_{2}\)O), Chlorogenic acid (C\({}_{18}\)H\({}_{18}\)O\({}_{9}\)), Arbutin (C\({}_{12}\)H\({}_{16}\)O\({}_{7}\)) and Sorbifolin (C\({}_{16}\)H\({}_{12}\)O\({}_{6}\)) [23; 24; 25]. Importantly, the reductive capability of its aqueous extract for the conversion of Cr(VI) to Cr(III) has been hitherto unexplored. One of the primary objectives of the present study is to conduct the reduction of Cr(VI) over a wide-range of pH and diverse environmental settings. In this article, aqueous plant extract mediated reduction of Cr(VI) is investigated and the effects of the concentration of plant extract, pH, ionic strength, temperature, initial concentration of Cr(VI) ions and hardness are examined.