Low-cost bio-based sustainable removal of lead and cadmium using a polyphenolic bioactive Indian curry leaf (Murraya koengii) powder

Abstract

There is an increasing trend of developing various low-cost biogenic sorbents for the efficient and economical removal of noxious metals . Curry leaf powder (CLP), a promising non-toxic biosorbent containing several bioactive compounds was prepared by the pulverization of the dried leaves for the effective removal of Lead (Pb) and Cadmium (Cd). Various batch sorption experiments were carried out under constant temperature (25 °C), different pH (4.5–10.5), initial concentrations (50–200 mg L⁻¹), adsorbent dosages (0.10–0.40 g) and contact times (0–60 min) to understand the optimum experimental conditions and simultaneously evaluate the adsorption isotherms and removal kinetics of CLP. Adsorption equilibrium was established in less than an hour interval (50 min). The pseudo-equilibrium process was best described by the pseudo-second-order kinetic (R² ≥ 0.99), Freundlich and Langmuir isotherm model (R² ≥ 0.94). The removal rate of Pb and Cd gradually increased (15.7 and 12.7 mg g⁻¹ for Pb and Cd) at 100 mg L⁻¹ of initial concentration till 60 min of contact period in a single contaminant system, the effect was non-significant for multiple adsorbent dosage systems (p > 0.05; t-test) though. The regeneration potential of the exhausted biosorbent was excellent upto 5 cycles with the better efficiency observed for Pb. The obtained results explicitly validated the probable utilization of CLP as a promising green adsorbent for metal removal . Future study may highlight the decontamination aspects of emerging contaminants with such green bio sorbents in large scale as well as mimicing the stomach conditions.

Introduction

The metals and metalloids coming in the environmental compartments can find their way to human food chain via soil-plant-water pathways (Kumar et al., 2017; Shim et al., 2019a). As per USEPA and ATSDR toxicity classification lead (Pb), mercury (Hg), arsenic (As) and cadmium (Cd) can be grouped into first, second, third, and sixth series of metals respectively (Madala et al., 2017; Jain et al., 2017). The potential mobility of heavy metals in water, soil, and plant may engrave potential danger in many urban and industrial areas which may tend to be bio-magnified into the food chain and is further accumulated in living tissues. Among all toxic metals, Pb and Cd may represent a high value of health hazard index (>80–100) and can be considered as potentially toxic and hazardous for environmental risk assessment study (Goswami et al., 2016; De Gisi et al., 2016; El-Naggar et al., 2018). Therefore, our study highlights the removal aspects of Pb and Cd based on their environmental occurrences, public health significance, and prevalence.

Adsorption process by activated carbon is regarded as one of the most effective and commonly applied methods for the removal of toxic metal ions. This process is facilitated through physico-chemical entrapment of sorbates/sorbents, though the involvement of elevated cost, separation problem and regeneration of spent sorbents have restricted its large scale applications (El-Naggar et al., 2018, Wiśniewska et al., 2018; Mukherjee et al., 2016). Biosorption (utilization of biobased waste materials and biological organisms) is considered as a potential environmental friendly technology in adsorption of heavy metals, the major concern with such green materials rely on their potential screening and large scale industrial application (He et al., 2018; Wiśniewska and Nowicki, 2019). The long term usability of adsorbents can be evaluated on the basis of its performance, ecofriendly nature and operational cost.

Curry leaf (Murraya koengii) is an Indian medicinal plant commonly used as spices/food additives for flavoring foods and as a preservative for prevention of biological growth on food materials. Curry leaf contains several bioactive compounds such as flavonoids, polyphenols, terpenes, alcohol, and other essential phytoalkaloids, which are basically plant secondary metabolites commonly used for therapeutic and prophylactic functions (Abbas et al., 2014; Iqbal et al., 2009). Such bioactive phytochemicals have potential free radical scavenging activity and excellent metal sequestration potential (De Gisi et al., 2016; Cheraghi et al., 2015). This process is accomplished through complex metal/biosorbents interaction by electron pair sharing and ion exchange mechanisms (Goswami et al., 2016). The partial delocalization of hard ligands with polyphenolic, hydroxyl, and alcoholic groups leads to the formation of a strong 3 to 5 membered chelate complex that may bind di/tri valent metals.

Though most studies (Abbas et al., 2014; Iqbal et al., 2009; Madala et al., 2017) have highlighted the culinary and biomedical applications of curry leaf powder (CLP) considering their anti-oxidative and anti-inflammatory activities, the biosorption efficiency of such biogenic sorbent is not addressed so far. The interesting and unique structural properties (binding sites and presence of phenolic hydroxyls group) of curry leaf (De Gisi et al., 2016; Jain et al., 2017) compared to other cellulose based biosorbents motivated us to examine the biosorption potential of polyphenols as an eco-efficient metal binding agent. We hypothesized that metal removal properties of such bioactive plant materials advocate the wider application of agricultural/forestry based materials by the formation of coordination bond with target (transition) metals having empty d-orbitals. We assume that the biosorption mechanism will be driven by several molecular interactions such as electrostatic, n-π/π-π, ionic, and dipole-dipole H bonding interactions and will show comparable sorption capacity than conventional functionalized biosorbents. There is a very few number of literature available on such inatomic linkages that can facilitate ionic molecular bridging between charged chemical functional groups of CLP and non-essential heavy metals (Pb, Cd, Ni, Zn, Cu). The present study aims at filling the loopholes of previous research in terms of biosorptive metal removal by replacing high cost conventional materials under various experimental conditions (pH, contact time, initial concentration, adsorbent dose, and composition). The specific objectives were: (i) preparation of curry leaf powder (CLP) and subsequent characterization before and after the removal experiments; (ii) to develop an understanding on the efficiency of CLP for the removal of targeted heavy metal ions (Pb, Cd); (iii) to understand the sorption mechanism; and (iv) to evaluate and mathematically model the sorption isotherm and kinetics.