Decontamination of Arsenic and Manganese in Polluted Water by Canary Seed Plant (Phalaris arundinacea L.) in Saturated Sand Culture

Water contamination by heavy metals is an environmental threat, endangering human health. Phytoremediation, through artificial wetlands, emerges as an eco-friendly method for heavy metals decontamination. This study aimed to assess the efficiency of Phalaris arundinacea L. (canary seed plant) in removal of arsenic and manganese from a saturated sand culture under different hydraulic retention times (HRT). The research consisted of two distinct experiments utilizing a completely randomized factorial design, encompassing three factors: two bed lengths (BL) of 50 and 100 cm, four inflow concentrations (Ci) of heavy metals (arsenic at 0, 2, 4, and 6 mg/L, and manganese at 0, 10, 20, and 30 mg/L), and five HRT (3, 6, 9, 12 and 15 days), in three replications. Results revealed that the maximum arsenic absorption (Absmax) (5.88 mg/L) occurred at Ci of 6 mg/L, HRT of 6 days and BL of 50 cm. However, the manganese Absmax (29.25 mg/L) was observed at Ci of 30 mg/L, HRT of 9 days and BL of 50 cm. Phalaris bio-concentration factors (BCF) were 2.29 and 0.15 for arsenic and manganese, respectively, highlighting superior arsenic absorption by 15.26 times. The surface absorption values of arsenic and manganese in the sand bed (qeAs=0.005 mg/g and qeMn=0.004 mg/g) confirmed the insignificant effect of this process in this experiment. The average maximum removal efficiency (RE) for manganese (99.61 %) attained at Ci of 20 mg/L, BL of 100 cm, on the fifteenth day, however that of arsenic (98.11 %) reached at Ci of 6 mg/L, BL of 50 cm, on the sixth day. These findings underscore the Phalaris exceptional capacity to efficiently remove dissolved manganese and arsenic under saturated conditions in the sand culture.

Phytoremediation through constructed wetlands stands out as environmentally friendly method for eliminating heavy metals from aquatic environments (Priyanka et al., 2017). The removal efficiency (RE) in these systems highly depends on the culture media, dissolved oxygen (DO), type of flow, hydraulic retention time (HRT) and plant species (Roy et al., 2022). The time to reach maximum absorption (Absmax) and maximum RE are the two basic criteria in the evaluation of phytoremediation plans. In some cases, a heavy metal utilizes the same absorption mechanism as a vital plant element. Despite heavy metal toxicity, it gets absorbed in quantities exceeding the plant's tolerance level, ultimately leading to the demise of the plant. Conversely, a heavy metal may be on the list of essential plant nutrients, but when present in excessive concentrations in the root environment, it is regarded as a toxic element. The plant utilized in this study, Phalaris arundinacea L., is locally known as canary seed plant in some regions of Iran (Mehrnia and Jalili, 2022). This research aimed to assess the removal capabilities of Phalaris arundinacea L. in absorbing arsenic and manganese from saturated sand culture under different HRTs.

Our research consisted of two distinct experiments utilizing a completely randomized factorial design, encompassing three factors: two BLs (50 and 100 cm), four Ci (arsenic at 0, 2, 4 and 6 mg/L, and manganese at 0, 10, 20 and 30 mg/L), and five HRT (3, 6, 9, 12 and 15 days), in three replications. For this purpose, 240 PVC pots were filled with washed sand (< 2 mm). The number of 5 and 10 Phalaris seedlings were planted in 50 and 100 cm pots, respectively. The average concentrations of DO in the inflow and outflow solutions were 13.7 and 1.2 mg/L respectively. Dissolved manganese and arsenic were measured by atomic absorption and a hybrid method, respectively. The manganese and arsenic concentrations in plant tissues were measured using standard methods followed by ash dissolution. Factors related to each pot were analyzed by SAS 9.3 software. The means were compared by LSD test at 5% probability level. The MS Excel was used for drawing the graphs.

Based on the analysis of variance, all the main and interactive effects of the treatments on the manganese and arsenic retention were significant (p < 0.01). The results also revealed that the Absmax of arsenic (5.88 mg/L) occurred at Ci of 6 mg/L and HRT of 6 days. However, the Absmax of manganese (29.25 mg/L) observed at Ci of 30 mg/L and HRT of 9 days. The Phalaris’ BCF values for arsenic and manganese were 2.29 and 0.15, respectively, highlighting its 15.26-fold greater effectiveness in accumulating arsenic compared to manganese. The average maximum RE for manganese (99.61 %) was attained at Ci of 20 mg/L on the fifteenth day, however, that of arsenic (98.11 %) was reached at Ci of 6 mg/L on the sixth day. It seems, at the commencement of the experiment, with DO=13.7 mg/L, arsenate (As V) was the prevalent form of arsenic in the root environment. Given its structural resemblance to phosphate, it was largely taken up by Phalaris. However, as the experiment approached its end, with lower DO (2.1mg/L), the remaining arsenic, in the form of arsenite (As III), exerted persistent toxicity, leading to the gradual deterioration of the plants. Similarly, the process of manganese absorption exhibited a declining trend starting from the ninth day onward. This decrease was attributed to the significant reduction in DO, disrupting the biological processes within the plant. In the context of this research, the calculated BCF (2) for Phalaris in the treatments with the highest concentrations of arsenic and manganese were determined to be 2.29 and 0.15, respectively.

The results of this research underscore Phalaris arundinacea L. remarkable ability to accumulate arsenic in comparison to manganese by a factor of 15.26, in saturated sand culture. As a native species, Phalaris exhibits significant promise as a phytoremediation agent in addressing heavy metals contamination.