## 4 Discussion
### Bioremediation of Non-Essential Toxic Elements Using Selected Indigenous Microbes  
The four selected indigenous microbes used for bioremediation that are tolerant to non-essential elements were _A. flavus_, _S. aureus_, _P. aeruginosa_, and _E. coli_. Previous reports suggested that _A. flavus_ can be used for the bioremediation of non-essential heavy metals due to its tolerance of Cr (400 mg/kg), Pb (600 mg/kg), and Cd (400 mg/kg) [49,50]. [PERSON] et al. [51] suggested that _S. aureus_, _P. aeruginosa_, and _E. coli_ can be used for the bioremediation of non-essential heavy metals due to their tolerance of Cd (0.8, 0.9, and 0.8 mg/kg), and Cr (5, 4, and 5 mg/kg), respectively. Furthermore, indigenous microbes are better adapted to the prevailing conditions in contaminated soil systems. Soils inoculated with _P. aeruginosa_ had the highest decline in the concentration of Al and As by 45% from 73.7 to 32.07 mg/kg and 20% from 25.57 to 20.53 mg/kg, respectively. Meanwhile soils inoculated with _S. aureus_, _A. flavus_, and _E. coli_ had the highest decline in Cr, Pb, and Cd concentrations by 50% from 13.8 to 6.94 mg/kg, 8% from 6.7 to 6.28 mg/kg, and 17% from 2.39 to 1.99 mg/kg, respectively. The low remediation abilities for Pb and Cd by the inoculants could be attributed to the biogeochemical properties of the environment, accessibility of the pollutant to the microbes, and affinity of these elements to the microbes. Furthermore, the decline in concentrations of non-essential toxic elements in control soils might be due to a consortium of soil microbes, since the soil samples used as controls were not pasteurized. According to [PERSON] et al. [52], Gram-positive bacteria have a higher tolerance of Cr than Gram-negative bacteria; as such, the high bioremediation potential of Cr by _S. aureus_ in this study can be credited to the Gram-positive strain of _S. aureus_ compared to other indigenous bacteria used in the study that are Gram-negative. Gram-positive bacteria contain teichoic acids, which contains a hydroxyl group; thus, Gram-positive bacteria provide a better binding site compared to Gram-negative bacteria [53]. [PERSON] et al. [54] reported that Gram-positive bacteria accumulate higher concentrations of heavy metals than Gram-negative bacteria, and further suggested that Gram-positive bacteria are most capable of bioremediation processes. Additionally, soil without a microbial inoculant (control) demonstrated a decline in Cr by 43% from 13.8 to 6.43 mg/kg, which was below that observed prior to irrigation with TWW, thus suggesting that leaving the soil at rest and irrigating with non-contaminated water might reduce Cr in contaminated soils. The soil microbial consortium might have transformed Cr into a volatile form. The low bioremediation efficiency reported in this study can be credited to the biogeochemical properties of the environment and the accessibility of pollutants to microbes [21]. According to [PERSON] and [PERSON] [55], soil geochemical properties affect microbial growth and activity, and subsequently, the kinetics of biodegradation. An alkaline pH of 8.54 was observed post-irrigation with TWW, which might have reduced the bioavailabilityof non-essential metals, thus reducing the efficacy of selected indigenous microbes in bioremediation. Under alkaline conditions, soil colloids are dominated by negative charges on colloidal surfaces [56]. As such, the alkaline soil pH reported in Table 3 suggests high non-essential metal adsorption on colloidal surfaces, thus reducing their bioavailability to selected indigenous microbes, hence the low percentage loss of non-essential metals since bioremediation is limited to biodegradable metals. High soil pH decreases the mobility of metals due to the precipitation of carbonates and the genesis of inorganic substances [57].