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Sources and levels of heavy metals (HMs) in soil and dust of urban and suburban areas in Riyadh (industrial city) and Mahad AD'Dahab (mining area) cities in Saudi Arabia were reported in this study. Additionally, the concentrations of HMs in different soil particle size fractions (> 250, 63–250 and < 63 µm) were reported. Pollution extent, and ecological and human health risks associated with collected soil and dust samples were explored. Contamination levels of HMs were higher in dust as compared to soil samples at all sites. The average integrated potential ecological risk in dust samples of urban area of Mahad AD'Dahab was 139, and thus characterized as a very-high-risk criterion. Enrichment factor (EF), correlation analyses, and principal component analysis showed that aluminum (Al), cobalt (Co), chromium (Cr), iron (Fe), manganese (Mn), nickel (Ni), titanium (Ti), and zinc (Zn) had mainly the lithogenic occurrence (EF < 2). However, Zn, copper (Cu), and lead (Pb) in Riyadh, and cadmium (Cd), Cu, Zn, and Pb in the Mahad AD'Dahab were affected by industrial and mining activities, respectively, that were of anthropogenic origins (EF > 2). The hazard index values of dust and soil (< 63 µm) samples in both urban and suburban areas in Mahad AD'Dahab were > 1, suggesting non-carcinogenic risk. Therefore, the dust and soil samples from the mined area of Mahad AD'Dahab had a higher pollution levels, as well as ecological and human health risks than those from Riyadh. Hence, the pollution of such residential environments with HMs (especially Cd, Cu, Zn, and Pb) needs to be monitored. [ABSTRACT FROM AUTHOR]

Purpose: Tannery waste–contaminated soil has a high amount of several toxic chemicals and heavy metals including chromium (Cr), which makes it unsuitable for agriculture practices. Majority of studies have reported the use of biochar (BC) as an amendment to restore contaminated soil. The efficiency of BC to immobilize Cr depends on its pretreatment and feedstock. This study aimed to investigate the potential of using BC for Cr immobilization and reducing its availability to maize plants in tannery waste–contaminated soil. The effect of BC on plant growth and heavy metal (Cr, Cu, Zn, Pb, Fe, and Mn) contents of maize shoots was also investigated. Materials and methods: The soil was collected from landfill areas of Riyadh city (N 24° 25′, E 46° 34′). BCs were produced by pyrolyzing jujube (Ziziphus spina-christi) leaves (JL) and manure (M) waste at 300 and 700 °C. Collected soil and produced materials were characterized physically and chemically by following standard procedure. A greenhouse pot experiment was conducted with unamended tannery waste–contaminated soil and soil amended with the obtained BCs (JLBC-300, JLBC-700, MBC-300, and MBC-700) at an application rate of 50 g kg−1 and cultivated with maize (Zea mays L.). Before cultivation, soil-soluble Cr was measured in five leachate cycles. Dry matter and heavy metal (Cr, Cu, Zn, Pb, Fe, and Mn) contents of maize shoots were measured after harvesting. Results and discussion: The results show that BCs produced at 700 °C showed the highest decrease in the concentrations of soil-soluble Cr and reduced its cumulative concentrations in soil leachates by about 93% and 59.0% for MBC-700 and JLBC-700 compared with the control soil, respectively. Overall, the highest decrease in cumulative soluble Cr was pronounced for MBC-700 followed by JLBC-700. For heavy metal contents in shoots, Cr concentrations were under detection limit in all treatments. However, BC amendments showed significant differences from the control for Cu, Mn, and Fe. Conclusions: From findings, it could be concluded that application of BCs pyrolyzed at 700 °C (especially for MBC-700) could be used as an amendment for reducing Cr mobility in tannery waste–contaminated soil and may create favorable conditions for crop production. [ABSTRACT FROM AUTHOR]

• Natural clay sediment from Riyadh exhibited highest surface area (288.5 m2 g−1). • Natural clay sediment from Medina and Al-Summan removed highest chlortetracycline. • pH range of 3.5–5.5 was optimum for CTC removal with natural clay sediments. • Interlayer sorption and electrostatic and H–π interactions were the sorption mechanisms. Natural clay sediments were collected from ten different localities in Saudi Arabia (S-1 from eastern, S-2 to S-4 from middle and S-5 to S-10 from western regions), characterized and evaluated for their efficiency towards chlortetracycline (CTC) removal from aqueous solutions. Sediment S-4 exhibited highest surface area (288.5 m2 g−1), followed by S-5, S-9, and S-1 (252.1, 249.6, and 110.4 m2 g−1, respectively). Sediments S-5, S-9, S-2, and S-4 showed the highest cation exchange capacities (CEC) (62.33, 56.54, 52.72, and 46.85 cmol kg−1, respectively). The pH range of 3.5–5.5 was optimum for the highest CTC removal. Freundlich model was best fitted to CTC sorption data (R2 = 0.96–0.99), followed by Dubinin-Radushkevich model (R2 = 0.89–0.97). The sediments S-4, S-5, and S-9 exhibited the highest CTC removal efficiency (98.80–99.05%), which could be due to higher smectite and kaolinite contents, CEC, surface area and layered structure. Post-sorption XRD patterns shown new peaks and peak shifts confirming the sorption of CTC. Electrostatic interactions, interlayer sorption and H–π bonding were the potential CTC sorption mechanisms. Therefore, natural clay sediments with high sorption capacities could efficiently remove CTC from contaminated aqueous media. [ABSTRACT FROM AUTHOR]