Due to its good performance, low cost and large availability, microorganisms have attracted more and more attention in the removal of heavy metals(Shi et al. 2022a; Zhang et al. 2019). Among various microorganisms, fungi have the stronger ability to remove heavy metal ions, which are capable of producing extracellular enzymes without prior hydrolysis. Fungal cell walls are usually negatively charged due to the presence of carboxyl, amine, sulfhydryl or phosphate groups, positively charged metal ions are easily adsorbed. Fungal cell walls are also rich in polysaccharides and glycoproteins such as chitin, glucan, mannans and phosphomannans, these polymers provide a large number of metal-like ligands. Therefore, they are more resistant than bacteria to metals (Leitão 2009). Among all fungal species, Penicillium spp. is one of the most common and prominent. Penicillium is mostly salt-tolerant, transparent and filamentous, and can be easily filtered (Mishra 2021), Filamentous fungi are less sensitive towards variation in adverse surrounding conditions like nutrient concentration, temperature, pH and aeration, etc. (Mohamed et al. 2019). Penicillium has now been shown to be tolerant to a variety of heavy metals, it is also a good adsorbent for Cd, Fe, Sb, Sn, Ag and Zn (Wang & Chen 2009). It was reported that P. italicum (Durali et al. 2007), P. spinulosum, P. oxalicu (Lenka et al. 2006), P. verrucosu (Omotayo et al. 2006), could adsorb various heavy metals. Mycelium was found to be pH-sensitive to the uptake and selective binding of Ni(II), Zn(II), Cd(II) and Pb(II). Tan and Chen. (2003) reported that the adsorption of metal ions was strongly dependent on pH, early alkaline pretreatment enhanced the adsorption capacity of P. chrysogenum to heavy metals. The ability of P. violetum to adsorb large amounts of Cr was first revealed by Say et al, its adsorption amount increased with time during the first 4 h and then tended to equalize again. The adsorption of Cr(VI) was significantly dependent on pH, and the loading capacity increased with increasing pH (Say et al. 2004). Study also has shown Cu(II) biosorption by living cells of Penicillium, strongly dependent on pH, biomass and initial Cu2+ concentration in solution (Ianis et al. 2006). Sundararaju et al. (2020) found that the adsorption of Ni (II) by the fungus Penicillium sp. MRF1 was influenced by three variables: fungal biomass concentration, pH and contact time. The optimal biosorption conditions were in an aqueous solution with initial Ni (II) concentration of 639 mg L-1, fungal biomass 7.5 g L-1, pH 5.5, contact time (140 min). In this experiment, pH, adsorption time and initial heavy metal solution concentration all had significant effect on XK11 adsorption of Cd, However, the initial pH has little effect on XK11 adsorption of Sb, the adsorption time and initial heavy metal solution concentration had an effect on the adsorption of Sb by XK11.
Different microorganisms have different mechanisms of tolerance to heavy metals, and the same microorganism has different tolerance to different heavy metals (Wen et al. 2020). Wang (2015) found that under the same concentration of U(VI) and Cr(VI) stress, Bacillus atrophy ATCC 9372 strain was more tolerant than B. atrophy Ua strain. Zhou (2022) found that the tolerance of Pseudomonas putida 51 to Cd was 400 mg L− 1, however, the tolerance of Cr was up to 2000 mg L− 1. B. aryabhattai 1010 showed only 200 mg L− 1 tolerance to Cd, but the tolerance to Pb rose to 2000 mg L− 1. In this experiment, the growth of XK11 strain received some inhibition under Cd stress with the increase of Cd concentration. It might be that XK11 is not highly tolerant to Cd, and the growth of the strain was inhibited as the initial Cd2+ concentration increased. However, in the Sb solution, the biomass of strain XK11 increased with the increase of Sb concentration, it might because that XK11 was extremely resistant to strains, it could still grow normally at this experimental concentration.
It was found in this experiment that the initial pH0 adsorption time and initial Cd concentration both have significant effects on the Cd removal rate. With the increase of each single factor variable, the removal rate of Cd showed an increase and then a decrease. Huang et al. (2000) found that Pb2+ and Cd2+ were adsorbed by Aspergillus SPP, when the pH value rose from 4.0 to 5.5, the adsorption capacity increased with the increase of pH value, while when the pH value increased to 6.0, the adsorption capacity gradually decreased. However, the removal rate of Sb remained constant with the increase of initial pH0. the reason for this phenomenon may be mainly related to the chemical properties of Sb, due to its non-polar property, it is difficult for Sb to be adsorbed (Li et al. 2016). Sb was present as SbO+(or Sb(OH)2+) in extremely acidic conditions and as H2SbO3− or Sb(OH)4− in weakly acidic, neutral, and alkaline conditions (Cai et al. 2018). When the initial pH0 was low, the pH of the medium decreased significantly, a large number of H+ and H3O+ plasma occupied most of the biosorption sites on the fungal cell wall surface (Aryal and Liakopoulou-Kyriakides 2015), and only a small number of Sb(OH)2+ ions could be adsorbed by the fungi. When the pH value increased, the negative charge on the surface increased, and positively charged Sb(OH)2+ was adsorbed. Adsorption time was one of the important factors in biological adsorption process (Sedky et al. 2018), Generally, microbial adsorption can be divided into fast adsorption stage and slow adsorption stage. There were studies showed that when L. rhamnosus LC-705 adsorbed Cd2+, the adsorption rate began to rise slowly at 5 min and reached equilibrium at 1 h, then there was also a slight decline in the later stages (Ibrahim et al. 2006). The patterns of adsorption time and adsorption rate in the literature reported by Zhang et al. (2021) and Zhao et al. (2021) were also consistent with the phenomenon in this experiment. Ren (2021) found that increasing the initial Cd2+ concentration, the removal rate of the strain started to increase slowly, continuing to increase the initial Cd2+ concentration to 5 mg L− 1, the removal rate decreased rapidly. The phenomenon of increasing the initial concentration of Cd2+ in this experiment was consistent with its. On the contrary, the change of the removal rate of Sb decreased significantly with the increase of adsorption time and initial Sb concentration. Generally, there were a large number of bioactive adsorption sites on the surface of fungi at the initial stage of adsorption. The growth of the strain stagnated at a later stage and the adsorption active site was gradually saturated at the surface. But in Sb solution, with the increase of adsorption time, the removal rate increased slightly, and the difference was not obvious. Different from all the above experimental phenomena, this may be because with the increase of adsorption time, a small amount of heavy metal ions entered the bacterial body through intracellular accumulation (Ren 2021). The initial concentration(C0) of solution was the key parameter affecting the adsorption capacity and removal rate. With the increase of the initial concentration, the capacity of the adsorbent generally increased rapidly at first, then gradually tended to be flat (Ogbodu et al. 2015; Wan et al. 2009). But when the concentration of heavy metal ions was too high, strong ionic repulsion in solution and saturation of removal sites on the cell membrane, leading to a decrease in subsequent removal rates (Ren 2021). In this experiment, the removal rate of Sb by XK11 decreased significantly with the increase of initial Sb concentration. Li et al. (2021) found that with the initial mass concentration of Pb-Cd-As in solution increased, the sorption of Pb-Cd-As by M. circinelloides also reduced. It could be that the heavy metal disrupts the microbial antioxidant enzyme system and genetic material and inhibits transcription, microbial activity was affected (Deng et al. 2017). It was also possible that with the increase of concentration, the heavy metal binding site tends to saturate. At this time, the fungus was not able to completely adsorb toxic and harmful substances, so the adsorption rate decreased (Zhang and Liu 2018).