Enhancement of auxiliary agent for washing efficiency of diesel contaminated soil with surfactants. Zhaolu Huang a, b, c, d , Daoyuan Wang e , Befkadu Abayneh Ayele a , Juan Zhou a , Indrajit Srivastava b , Dipanjan Pan b, c, d , Zhen Wang b , Quanyuan Chen a, f, * a Department of Environmental Science and Engineering College, Donghua University, Shanghai, 201620, China b Departments of Bioengineering, Materials Science and Engineering, University of Illinois at Urbana Champaign, Urbana, IL, 61801, USA c Carle Foundation Hospital, Urbana, IL, 61801, USA d Illinois Sustainable Technology Center, Prairie Research Institute, University of Illinois at Urbana Champaign, Champaign, IL, 61820, USA e Department of Land, Air and Water Resources, University of California, Davis, CA, 95616, USA f Shanghai Institution of Pollution Control and Ecological Security, Shanghai, 200092, PR China highlights graphical abstract  Surfactant washing enhancement of diesel contaminated soil with salts was studied.  Sodium salts exhibited a higher effi- cacy by changing surface tension and micelle.  Phytotoxicity of biosurfactant on let- tuce, arugula, and cucumber were studied.  Addition of humic acid sodium in soil washing may improve root growth of plants. article info Article history: Received 20 August 2019 Received in revised form 10 March 2020 Accepted 13 March 2020 Available online 18 March 2020 Handling Editor: Prof. X. Cao Keywords: Diesel contaminated soil Soil washing Salt-enhancement Surface tension Phytotoxicity abstract We used five types of surfactants assisted with sodium salts, including sodium tartrate (ST), sodium chloride (SC), and humic acid sodium (HAS) as auxiliary agents for soil washing to remove diesel from contaminated soil. Decontamination enhancement of diesel polluted soil washing with biosurfactant and H 2 O 2 was examined, which showed higher effectiveness for newly contaminated soil. An increase in temperature and sodium salt addition exhibited a profound enhancement in diesel removal from aged contaminated soils. Compared to ST and SC, HAS exhibited a higher removal efficiency with saponin washing for aged diesel contaminated soil by lowering surface tension, shifting zeta potential, and increasing the number of micelles. Phytotoxicity experiments showed no significant inhibition of germination of lettuce, arugula, and cucumber with 0.2 g L 1 saponin incubation. Conversely, there was a promotion on the root extension of lettuce and cucumber except for arugula. Similarly, the addition of 2% HAS (wight of saponin) improved on root growth of lettuce, arugula, and cucumber, increasing by 25%, 5%, and 22% at the period of 14 d, respectively. Because of excellent removal efficiency and non-toxicity, enhanced wash with saponin and HAS might be considered in the future design of full-scale remediation processes of diesel contaminated soil. © 2020 Published by Elsevier Ltd. * Corresponding author. Department of Environmental Science and Engineering College, Donghua University, Shanghai, 201620, China. E-mail address: 1159204@mail.dhu.edu.cn (Q. Chen). Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere https://doi.org/10.1016/j.chemosphere.2020.126494 0045-6535/© 2020 Published by Elsevier Ltd. Chemosphere 252 (2020) 126494